Publications by authors named "Detlef Gabel"

44 Publications

Versatile, one-pot introduction of nonahalogenated 2-ammonio-decaborate ions as boron cluster scaffolds into organic molecules; host-guest complexation with γ-cyclodextrin.

Chem Commun (Camb) 2019 Nov 30;55(91):13669-13672. Epub 2019 Oct 30.

Institute of Inorganic Chemistry, Czech Academy of Sciences, 25068 ŘeŽ, Czech Republic.

We report the modification of the 2-ammonio group at halogenated decaborate ions with 2,3-epoxypropane, the product of which reacts readily with nucleophiles to form previously inaccessible coupling of polyhedra with organic molecules and materials. We demonstrate that these ions present a good binding motif in supramolecular chemistry.
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http://dx.doi.org/10.1039/c9cc07678fDOI Listing
November 2019

High-Affinity Binding of Metallacarborane Cobalt Bis(dicarbollide) Anions to Cyclodextrins and Application to Membrane Translocation.

J Org Chem 2019 09 18;84(18):11790-11798. Epub 2019 Jul 18.

Department of Life Sciences and Chemistry , Jacobs University Bremen , Campus Ring 1 , D-28759 Bremen , Germany.

Metallacarboranes are a class of inorganic boron clusters that have recently been recognized as biologically active compounds. Herein, we report on the host-guest complexation of several cobalt bis(1,2-dicarbollide) anions (COSANs) with cyclodextrins (CDs) in aqueous solution. The binding affinities reach micromolar values, which are among the highest known values for native CDs, and exceed those for neutral hydrophobic organic guest molecules. The entrapment of the COSANs inside the cavity of CDs was confirmed using NMR and UV-visible spectroscopy, mass spectrometry, cyclic voltammetry, and isothermal titration calorimetry. Complexation by CDs greatly influences the photophysical and electrochemical properties of COSANs. In combination with indicator displacement assays, a label-free fluorescence-based method was developed to allow real-time monitoring of the translocation of COSANs through lipid bilayer membranes.
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http://dx.doi.org/10.1021/acs.joc.9b01688DOI Listing
September 2019

New stains for anterior capsule surgery.

J Cataract Refract Surg 2019 02 17;45(2):213-218. Epub 2018 Nov 17.

Department of Life Sciences and Chemistry, Jacobs University Bremen, Bremen, Germany. Electronic address:

Purpose: To investigate whether new dyes and dye combinations can give equivalent or better staining in anterior capsule surgery than existing dyes with a low degree of toxicity on relevant cells.

Setting: University laboratory of Jacobs University Bremen, Germany.

Design: Laboratory experimental study.

Methods: Pig eyes were collected post mortem. Cataract was induced by microwave irradiation. Access to the lens capsule was through open-sky surgery. Staining was performed and results were documented by photography. The toxicity of the dyes was evaluated in 3 different cell lines immediately after exposure and with a delay of 24 hours, with exposure in the dark or subsequent strong illumination.

Results: A new cyanine dye, BIP (2-[5-[3,3-dimethyl-1-(4-sulfobutyl)-1,3-dihydro-indol-2-ylidene]-penta-1,3-dienyl]-3,3-dimethyl-1-(4-sulfobutyl)-3H-indolium sodium), was found to lead to green staining, with reduced toxicity on corneal endothelial cells. Staining could be further enhanced by combining it with trypan blue. Methylene blue was very toxic, whereas its combination with trypan blue was much less toxic.

Conclusions: With BIP alone or in combination with trypan blue, safe staining of the capsule can be achieved, resulting in a green color.
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http://dx.doi.org/10.1016/j.jcrs.2018.09.016DOI Listing
February 2019

Host-Guest Chemistry of Carboranes: Synthesis of Carboxylate Derivatives and Their Binding to Cyclodextrins.

Chemistry 2018 Sep 26;24(49):12970-12975. Epub 2018 Jul 26.

Jacobs University Bremen, Department of Life Sciences and Chemistry, Campus Ring 1, 28759, Bremen, Germany.

Polyhedral carboxymethyl carborane (C B H ) derivatives, including mono- and disubstituted o-, m-, and p-isomers, have been synthesized. Supramolecular host-guest complexation of these derivatives with cyclodextrins (CDs; namely, α-, β-, and γ-CD) has been investigated in water. The globular structure of the carborane binding moiety and its hydrophobic character qualify it as an ideal recognition site to form stable inclusion complexes with macrocyclic host molecules in aqueous solution. The measured binding affinities for the carborane derivatives were in the millimolar range (K =10 -10  m ) with differently sized CDs, and preferential binding to β-CD.
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http://dx.doi.org/10.1002/chem.201802134DOI Listing
September 2018

Toxicity and phototoxicity in human ARPE-19 retinal pigment epithelium cells of dyes commonly used in retinal surgery.

Eur J Ophthalmol 2018 Jul 1;28(4):433-440. Epub 2018 Apr 1.

1 Department of Life Sciences and Chemistry, Jacobs University Bremen, Bremen, Germany.

Purpose: To compare, for the first time, systematically the toxicity and phototoxicity of dyes and dye combinations used in vitreoretinal surgery. The dyes were trypan blue, brilliant blue G, trypan blue + brilliant blue G, indocyanine green, bromophenol blue, bromophenol blue + brilliant blue G, and acid violet 17, in clinically used concentrations.

Methods: Human ARPE retinal pigment epithelium cells were exposed to the dyes for 30 min. For phototoxicity, the cells were exposed for 15 min to high-intensity light from a light emitting diode source with an intensity similar to surgical conditions. Toxicity was assayed either directly after exposure to either dye alone or dye and light, or with a delay of 24 h.

Results: None of the dyes or their combinations was toxic when cells were exposed to them at ambient light. Acid violet led to a reduction viability by 90% already immediately after light exposure. Bromophenol blue and its combination with brilliant blue G showed strong phototoxicity (reduction of viability by 83%) when assayed with delay. Indocyanine green with different agents to adjust osmolarity (balanced salt solution, glucose, and mannitol) was not found to be toxic.

Conclusion: The strong immediate phototoxicity of acid violet reflects its clinical toxicity. Bromophenol blue might also be disadvantageous for patient outcome because of its delayed phototoxicity. The other dyes (trypan blue, brilliant blue g, and indocyanine green) were not found to be toxic neither with exposure to ambient light nor after exposure to light of intensities used in surgery.
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http://dx.doi.org/10.1177/1120672118766446DOI Listing
July 2018

The chaotropic effect as an orthogonal assembly motif for multi-responsive dodecaborate-cucurbituril supramolecular networks.

Chem Commun (Camb) 2018 Feb;54(17):2098-2101

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China.

In aqueous solution, host-guest complexation is frequently driven by the hydrophobic effect, which also constitutes a popular approach in the design of supramolecular assemblies. Herein, we report an orthogonal assembly motif, the chaotropic effect, which exploits the tendency of chaotropic anions to associate with hydrophobic surfaces.
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http://dx.doi.org/10.1039/c7cc08078fDOI Listing
February 2018

From boron clusters to gold clusters: new label-free colorimetric sensors.

Chem Commun (Camb) 2017 Aug;53(86):11790-11793

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China.

A fundamental boron cluster, [closo-BH], mainly known as an inert agent for boron neutron capture therapy, can also serve as a bi-functional reductant and a capping agent for the formation of monodispersed size-controlled gold nanoparticles (AuNPs) under mild reaction conditions. The obtained AuNPs remained thermo-dynamically stable over 3 months, and exhibited high performance in phase transfer and sensing of heavy metal ions.
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http://dx.doi.org/10.1039/c7cc06607dDOI Listing
August 2017

Hierarchical host-guest assemblies formed on dodecaborate-coated gold nanoparticles.

Chem Commun (Camb) 2017 Apr;53(33):4616-4619

Department of Life Sciences and Chemistry, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Bremen, Germany.

Undecahydro-mercapto-closo-dodecaborate (BSH), a purely inorganic cluster anion, serves as an unconventional stabilizing ligand for the preparation of gold nanoparticles (AuNPs). The BSH-capped AuNPs serve as nano-scaffolds, allowing for the creation of supramolecular architectures by using polycationic macrocyclic host molecules, through strong host-guest complexation. An amphiphilic calixarene (CX) forms first mono- and subsequently bi-layer assemblies on the BSH-AuNP surfaces depending on the CX concentration. The inorganic-organic hybrid materials, combined with host-dye displacement assays, serve as a chemosensing ensemble for negatively charged molecules, including DNA.
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http://dx.doi.org/10.1039/c7cc01507kDOI Listing
April 2017

High-affinity host-guest chemistry of large-ring cyclodextrins.

Org Biomol Chem 2016 Aug;14(32):7702-6

Department of Life Sciences and Chemistry, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.

The host-guest chemistry of large-ring cyclodextrins (LRCDs) has been largely unexplored due to the lack of suitable guest molecules that bind with significant affinities to enable potential applications. Herein, we report their complexation with dodecaborate anions (B12X12(2-)), a novel class of guest molecules. The binding constants of the inorganic guests (10(4)-10(6) M(-1)) allow their classification as the first tight binders for LRCDs.
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http://dx.doi.org/10.1039/c6ob01161fDOI Listing
August 2016

Dodecaborate-Functionalized Anchor Dyes for Cyclodextrin-Based Indicator Displacement Applications.

Org Lett 2016 Mar 22;18(5):932-5. Epub 2016 Feb 22.

Department of Life Sciences and Chemistry, Jacobs University Bremen , Campus Ring 1, 28759 Bremen, Germany.

A new type of water-soluble anchor dyes, that is, dyes which carry an auxiliary unit for strong binding to macrocyclic host molecules, has been synthesized. It consists of 7-nitrobenzofurazan (NBD) as a dye and the dodecaborate cluster (B12H11R) as a dianionic, globular, and purely inorganic anchoring group for cyclodextrins (Ka > 10(5) M(-1)). The synthesized dodecaborate-substituted dyes show marked changes in their photophysical properties (UV-vis and fluorescence) upon complexation with cyclodextrins (β-CD and γ-CD), such that the resulting host·dye complexes (1:1 stoichiometry) present sensitive reporter pairs for indicator displacement applications.
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http://dx.doi.org/10.1021/acs.orglett.5b03611DOI Listing
March 2016

Reduction of cytotoxicity of benzalkonium chloride and octenidine by Brilliant Blue G.

EXCLI J 2015 21;14:123-32. Epub 2015 Jan 21.

Molecular Life Science Research Center, Jacobs University Bremen, Campus Ring 1, 28759, Bremen, Germany.

The irritative effects of preservatives found in ophthalmologic solution, or of antiseptics used for skin disinfection is a consistent problem for the patients. The reduction of the toxic effects of these compounds is desired. Brilliant Blue G (BBG) has shown to meet the expected effect in presence of benzalkonium chloride (BAK), a well known preservative in ophthalmic solutions, and octenidine dihydrochloride (Oct), used as antiseptic in skin and wound disinfection. BBG shows a significant protective effect on human corneal epithelial (HCE) cells against BAK and Oct toxicity, increasing the cell survival up to 51 % at the highest BAK or Oct concentration tested, which is 0.01 %, both at 30 min incubation. Although BBG is described as a P2x7 receptor antagonist, other selective P2x7 receptor antagonists, OxATP (adenosine 5'-triphosphate-2',3'-dialdehyde) and DPPH (N'-(3,5-dichloropyridin-4-yl)-3-phenylpropanehydrazide), did not reduce the cytotoxicity of neither BAK nor Oct. Therefore we assume that the protective effect of BBG is not due to its action on the P2x7 receptor. Brilliant Blue R (BBR), a dye similar to BBG, was also tested for protective effect on BAK and Oct toxicity. In presence of BAK no significant protective effect was observed. Instead, with Oct a comparable protective effect was seen with that of BBG. To assure that the bacteriostatic effect is not affected by the combinations of BAK/BBG, Oct/BBG and Oct/BBR, bacterial growth inhibition was analyzed on different Gram-negative and Gram-positive bacteria. All combinations of BAK or Oct with BBG hinder growth of Gram-positive bacteria. The combinations of 0.001 % Oct and BBR above 0.025 % do not hinder the growth of B. subtilis. For Gram-negative bacteria, BBG and BBR reduce, but do not abolish, the antimicrobial effect of BAK nor of Oct. In conclusion, the addition of BBG at bacterial inhibitory concentrations is suggested in the ready-to-use ophthalmic preparations and antiseptic solutions.
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http://dx.doi.org/10.17179/excli2014-556DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4553885PMC
September 2015

Water Structure Recovery in Chaotropic Anion Recognition: High-Affinity Binding of Dodecaborate Clusters to γ-Cyclodextrin.

Angew Chem Int Ed Engl 2015 Jun 7;54(23):6852-6. Epub 2015 May 7.

Department of Life Sciences and Chemistry, Jacobs University Bremen, Campus Ring 1, 28759 Bremen (Germany).

Dodecaborate anions of the type B12X12(2-) and B12X11Y(2-) (X=H, Cl, Br, I and Y=OH, SH, NH3(+), NR3(+)) form strong (K(a) up to 10(6) L mol(-1), for B12Br12(2-)) inclusion complexes with γ-cyclodextrin (γ-CD). The micromolar affinities reached are the highest known for this native CD. The complexation exhibits highly negative enthalpies (up to -25 kcal mol(-1)) and entropies (TΔS up to -18.4 kcal mol(-1), both for B12I12(2-)), which position these guests at the bottom end of the well-known enthalpy-entropy correlation for CDs. The high driving force can be traced back to a chaotropic effect, according to which chaotropic anions have an intrinsic affinity to hydrophobic cavities in aqueous solution. In line with this argument, salting-in effects revealed dodecaborates as superchaotropic dianions.
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http://dx.doi.org/10.1002/anie.201412485DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4510780PMC
June 2015

Halogenated Dodecaborate Clusters as Agents to Trigger Release of Liposomal Contents.

Chempluschem 2015 Apr 12;80(4):656-664. Epub 2015 Jan 12.

Department of Chemistry, University of Bremen, PO Box 330440, 28334 Bremen (Germany).

Halogenated dodecaborates, and especially dodecaiodododecaborate(2-), are found to trigger effectively the release of the contents of phospholipid liposomes, including liposomes containing distearoylphosphatidylcholine and cholesterol, which are used clinically in cancer therapy. The basis of the release is studied through differential scanning calorimetry, cryo-transmission electron microscopy, and atomic force microscopy. Upon administration at high concentrations, drastic morphological changes are induced by the dodecaborates. Their possible use in triggered release is suggested.
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http://dx.doi.org/10.1002/cplu.201402286DOI Listing
April 2015

Hemorrhage in mouse tumors induced by dodecaborate cluster lipids intended for boron neutron capture therapy.

Int J Nanomedicine 2014 29;9:3583-90. Epub 2014 Jul 29.

Department of Chemistry, University of Bremen, Germany ; Institute of Neuropathology, Klinikum Bremen-Mitte, Germany ; School of Engineering and Science, Jacobs University Bremen, Bremen, Germany.

The potential of boron-containing lipids with three different structures, which were intended for use in boron neutron capture therapy, was investigated. All three types of boron lipids contained the anionic dodecaborate cluster as the headgroup. Their effects on two different tumor models in mice following intravenous injection were tested; for this, liposomes with boron lipid, distearoyl phosphatidylcholine, and cholesterol as helper lipids, and containing a polyethylene glycol lipid for steric protection, were administered intravenously into tumor-bearing mice (C3H mice for SCCVII squamous cell carcinoma and BALB/c mice for CT26/WT colon carcinoma). With the exception of one lipid (B-THF-14), the lipids were well tolerated, and no other animal was lost due to systemic toxicity. The lipid which led to death was not found to be much more toxic in cell culture than the other boron lipids. All of the lipids that were well tolerated showed hemorrhage in both tumor models within a few hours after administration. The hemorrhage could be seen by in vivo magnetic resonance and histology, and was found to occur within a few hours. The degree of hemorrhage depended on the amount of boron administered and on the tumor model. The observed unwanted effect of the lipids precludes their use in boron neutron capture therapy.
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http://dx.doi.org/10.2147/IJN.S65166DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4122578PMC
March 2015

Case numbers for a randomized clinical trial of boron neutron capture therapy for Glioblastoma multiforme.

Appl Radiat Isot 2014 Jun 4;88:16-9. Epub 2013 Dec 4.

Department of Chemistry, University of Bremen, Bremen, Germany; Cooperative Center Medicine, University of Bremen, Bremen, Germany; School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, D-28759 Bremen, Germany. Electronic address:

Boron neutron capture therapy (BNCT) with Na2B12H11SH (BSH) or p-dihydroxyborylphenylalanine (BPA), and with a combination of both, was compared to radiotherapy with temozolomide, and the number of patients required to show statistically significant differences between the treatments was calculated. Whereas arms using BPA require excessive number of patients in each arm, a two-armed clinical trial with BSH and radiotherapy plus temozolomide is feasible.
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http://dx.doi.org/10.1016/j.apradiso.2013.11.092DOI Listing
June 2014

Dyes for Eyes™: hydrodynamics, biocompatibility and efficacy of 'heavy' (dual) dyes for chromovitrectomy.

Ophthalmologica 2013 6;230 Suppl 2:51-8. Epub 2013 Sep 6.

Eye Hospital, St.-Joseph-Stift, Bremen, Germany.

As epiretinal membranes (ERMs), the internal limiting membrane (ILM) and the vitreous cortex are essentially transparent tissues, or translucent structures, nontraumatic removal may be challenging in various types of macular surgery. Vital dyes stain these thin tissues, thus allowing for better visualization of these structures during vitrectomy and selective 'membrane peeling' from the underlying retina. To avoid swirling of the dye within the fluid-filled vitreous cavity, and to better target the dye onto the macula, a fluid-air exchange is commonly performed. However, this may jeopardize visualization of the macula during peeling due to clouding of the posterior lens capsule, and may lead to postoperative visual field defects. Recently, a new dye solution for staining the ERM and ILM simultaneously has been developed that circumvents the need for fluid-air exchange, i.e. MembraneBlue-Dual™. This paper will focus on the hydrodynamics and biocompatibility of this 'heavy' dual dye and its efficacy for staining of the ILM and/or ERMs during posterior segment surgery in a multicenter clinical setting.
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http://dx.doi.org/10.1159/000353870DOI Listing
April 2014

[FeFe]-hydrogenase models assembled into vesicular structures.

J Liposome Res 2014 Mar 5;24(1):59-68. Epub 2013 Sep 5.

Department of Pharmaceutical Technology, Friedrich-Schiller-University Jena , Jena , Germany .

Compartmentalization is a major prerequisite for the origin of life on earth according to Wächtershäuser "Iron-Sulfur-World". The hypothesis is mainly based on an autocatalytic inorganic energy reproducing redox system consisting of iron and sulfur as requirement for the subsequent synthesis of complex organic structures. Here, we modified [FeFe]-hydrogenase models by means of covalent coupling to either oleic acid or the amphiphilic block copolymer polybutadiene-polyethyleneoxide (PB-PEO) and incorporated those into the membranes of vesicles composed of phospholipids (liposomes) or the unmodified amphiphilic polymer (polymersomes). We employed a [2Fe-2S] cluster as a hydrogenase model, since these structures are known to be suitable catalysts for the generation of H2 in the presence of weak acids. Successful incorporation was confirmed by spectrophotometric iron quantification and the vesicles formed were characterized by size determination (photon correlation spectroscopy (PCS)), and zeta potential as well as by cryo-transmission electron microscopy (Cryo-TEM). The modified models could be incorporated into liposomes or polymersomes up to molar proportions of 3.15% and 28%, respectively. Due to the immobilization in vesicular bilayers the [FeFe]-hydrogenase models can even exhibit catalytic action under the particular conditions of the intravesicular microenvironment. Our results suggest that the vesicular systems described may be applied as a nanoreactor for the reduction of encapsulated substances by generating hydrogen and thus as a minimal cell model.
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http://dx.doi.org/10.3109/08982104.2013.833225DOI Listing
March 2014

On the gas-phase dimerization of negatively charged closo-dodecaborates: a theoretical study.

Phys Chem Chem Phys 2013 Jul 15;15(25):10358-66. Epub 2013 May 15.

School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.

We have studied the intriguing gas-phase dimerization of the B12In(-) (n = 9, 8) anions to B24I2n(2-) dianions by means of density functional theory calculations. The dimerization of B12I9(-) to B24I18(2-) has been detected experimentally in a previous study (Phys. Chem. Chem. Phys., 2011, 13, 5712) utilizing electrospray ionization ion trap mass spectrometry (ESI-IT-MS), whereas the formation of B24I16(2-) from B12I8(-) is modeled here prior to experiment. Calculations are carried out to determine the molecular and electronic structures, the bonding situation and the stability of the dimers relative to the respective monomers. The dimerization process is found to be thermodynamically favorable, and the stability of the lowest-energy structures is substantiated by molecular dynamics simulations. The calculations imply that the experimentally observed B24I18(2-) and the hypothetical B24I16(2-) species are formed through dimerization of the respective B12In(-) (n = 9, 8) monomers, rather than through loss of two I radicals from B24I2n+2(2-) dimers. Electronic properties such as natural charges, vertical detachment energies (VDEs), frontier molecular orbitals (FMOs), and HOMO-LUMO gaps are computed and analyzed in detail for all monomers and dimers. The analysis shows that the most stable B24I2n(2-) dimers are formed through two 2c-2e B-B and two 3c-2e B-I-B bridges between the parent B12In(-) (n = 9, 8) monomers. These new bridging bonds engage the deiodinated (bare) faces of the two B12 icosahedra, as well as one (per monomer) of the nearest boron neighbors and its iodine substituent.
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http://dx.doi.org/10.1039/c3cp50722jDOI Listing
July 2013

Brilliant Blue G as protective agent against trypan blue toxicity in human retinal pigment epithelial cells in vitro.

Graefes Arch Clin Exp Ophthalmol 2013 Jul 14;251(7):1735-40. Epub 2013 Apr 14.

Department of Biochemistry, Faculty of Science, Alexandria University, Moharam Bek, 21511, Alexandria, Egypt.

Background: Combinations of trypan blue (TB), Brilliant Blue G (BBG) and polyethyleneglycol had been shown before to be less toxic to ARPE retinal pigment epithelial cells than TB alone. We studied systematically the influence of combinations of dyes on cell damage.

Methods: ARPE cells were exposed to TB (concentration range 0.025 to 1 %), BBG (0.0025 to 0.5 %), and combinations of the two dyes, dissolved in phosphate buffered saline (PBS), for periods between 5 and 60 min. Cell damage was monitored with the WST-1 assay. The effect of different salt concentration was measured in the same way.

Results: TB in concentrations of 0.075 % and higher was toxic to the cells already after 30 min incubation. BBG was toxic after 30 min in concentration of 0.1 % and higher, but had a protective effect on cells with incubation time of 5 min and concentrations up to 0.1 %. BBG at concentrations of 0.025 % protected against TB-induced damage at 5 min and 30 min incubation. Salt concentrations between 113 and 225 mM did not influence cell survival even after 30 min. In the presence of TB, propidium iodide bound strongly to the cells.

Conclusions: BBG acts as a protecting agent against TB toxicity.
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http://dx.doi.org/10.1007/s00417-013-2342-3DOI Listing
July 2013

From an icosahedron to a plane: flattening dodecaiodo-dodecaborate by successive stripping of iodine.

Chemistry 2012 Oct 7;18(41):13208-12. Epub 2012 Sep 7.

Institut de Ciència de Materials de Barcelona ICMAB-CSIC, 08193 Bellaterra, Spain.

It has been shown by electrospray ionization-ion-trap mass spectrometry that B(12)I(12)(2-) converts to an intact B(12) cluster as a result of successive stripping of single iodine radicals or ions. Herein, the structure and stability of all intermediate B(12)I(n)(-) species (n=11 to 1) determined by means of first-principles calculations are reported. The initial predominant loss of an iodine radical occurs most probably via the triplet state of B(12)I(12)(2-), and the reaction path for loss of an iodide ion from the singlet state crosses that from the triplet state. Experimentally, the boron clusters resulting from B(12)I(12)(2-) through loss of either iodide or iodine occur at the same excitation energy in the ion trap. It is shown that the icosahedral B(12) unit commonly observed in dodecaborate compounds is destabilized while losing iodine. The boron framework opens to nonicosahedral structures with five to seven iodine atoms left. The temperature of the ions has a considerable influence on the relative stability near the opening of the clusters. The most stable structures with five to seven iodine atoms are neither planar nor icosahedral.
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http://dx.doi.org/10.1002/chem.201200828DOI Listing
October 2012

Interaction of dodecaborate cluster compounds on hydrophilic column materials in water.

J Chromatogr A 2012 Sep 24;1256:98-104. Epub 2012 Jul 24.

Department of Chemistry, University of Bremen, Germany.

The interaction of a series of dodecaborate cluster compounds B(12)X(12)(2-) and B(12)X(11)Y(2-) (X=H, Cl, Br, I and Y=SH, OH, NR(3)) with hydrophilic column materials (Superdex 200, Sepharose 4B, Sephadex G-50, Sephadex G-100, alumina, silica gel and anion exchange material) was studied. Almost all the dodecaborate cluster compounds were retained strongly on Superdex 200. The halogenated cluster compounds interacted with Sepharose 4B, Sephadex G-50, Sephadex G-100 and alumina; on alumina, also the non-halogenated clusters were retained. Silica gel showed the least interaction with all compounds. The thermodynamic parameters were investigated for a selection of compounds on Superdex 200 and Sephadex G-100. Values for ΔH° were found to be negative on both gels. As the change in entropy ΔS° was also negative, it compensated ΔH° to a large extent. The clusters interacted also strongly with anion exchange material in ion chromatography; the interaction decreased with increasing acetonitrile concentration, implying a large contribution from solvent effects.
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http://dx.doi.org/10.1016/j.chroma.2012.07.055DOI Listing
September 2012

Structure and dynamics of dodecaborate clusters in water.

Inorg Chem 2012 May 9;51(9):4894-6. Epub 2012 Apr 9.

School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, D-28759 Bremen, Germany.

We have studied using molecular dynamics simulations the interaction of the dodecaborate anion, B(12)H(12)(2-), and its amino, trimethyl, and triethyl derivatives with water molecules. We found peculiar organization of the water molecules in the first solvation shell with the formation of a dihydrogen bond between the hydrogen atoms of the anions and the hydrogen atoms of the water molecules. The simulations also show that the organization of the hydration shell is strongly influenced by the substituents in the anions. These differences are likely to play an important role in understanding the interaction of the anions with biological systems like membranes and proteins in aqueous environments.
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http://dx.doi.org/10.1021/ic300223zDOI Listing
May 2012

Comparative toxicology of trypan blue, brilliant blue G, and their combination together with polyethylene glycol on human pigment epithelial cells.

Invest Ophthalmol Vis Sci 2011 Jun 9;52(7):4085-90. Epub 2011 Jun 9.

Department of Chemistry, University of Bremen, Bremen, Germany.

Purpose: To determine the toxicity in ARPE-19 human retinal pigment epithelium cells of trypan blue (TB) at 0.15% and 0.25% concentration, brilliant blue G (BBG) at 0.025% and 0.05%, their combination, and the effect of the addition of 4% polyethyleneglycol (PEG), as an additive for increasing the density and thus improving the staining in internal limiting membrane removal, on the individual dyes and their combinations, and compare the toxicity of the dyes to that of clinically used preparations.

Methods: Cells were exposed for 5 and for 30 minutes to the different preparations. Cell viability was measured with the WST-1 assay measuring intracellular dehydrogenase activity.

Results: Solutions containing PEG with BBG (0.025%), TB (0.15%), and mixtures of BBG (0.025%) with TB (0.15% and 0.25%) were the least toxic of the preparations as well as preparations of BBG at 0.025% in phosphate-buffered saline solution, while TB at 0.25% in phosphate-buffered saline solution was the most toxic. The addition of PEG reduced the toxicity of preparations containing TB either alone or in combination with BBG. These results were seen only after an incubation for 30 minutes; for a 5-minute incubation, no toxicity was seen for any of the preparations.

Conclusions: For short incubation times, all dyes appear equally safe. For longer incubation times, TB preparations were more toxic than BBG preparations. The toxicity of TB was reduced by the addition of PEG. Further studies are required to determine the clinical impact of this finding.
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http://dx.doi.org/10.1167/iovs.10-6336DOI Listing
June 2011

Collision-induced gas-phase reactions of perhalogenated closo-dodecaborate clusters--a comparative study.

Phys Chem Chem Phys 2011 Apr 10;13(13):5712-21. Epub 2011 Feb 10.

Fachbereich 2 (Chemie/Biologie), Universität Bremen, Bremen, Germany.

The gas phase reactivity of perhalogenated closo-dodecaborate clusters [B(12)X(12)](2-) (X = F, Cl, Br, I) with N-tetraalkylated ammonium counter ions was investigated by electrospray ionization ion trap mass spectrometry (ESI-IT-MS). Collisions with the background gases introduced a broad variety of gas phase reactions. This study represents the first experimental approach to a new class of boron-rich boron clusters that are not accessible in the condensed phase. The anionic ion pair [B(12)X(12) + N(C(n)H(2n+1))(4)](-) is generally found as the ion of highest mass. Its reaction sequence starts with an alkyl transfer from the ammonium ion to the dodecaborate cluster. Subsequently, the alkylated intermediate [B(12)X(12) + C(n)H(2n+1)](-) decomposes to give very reactive ions of the general formula [B(12)X(11)](-). These ions possess a free boron vertex and immediately bind to the residual gases N(2) and H(2)O in the ion trap by formation of the corresponding adducts [B(12)X(11) + N(2)](-) and [B(12)X(11) + H(2)O](-). Subsequent fragmentations of the water adduct repetitively substitute halogen atoms by hydroxyl groups. The fragmentation process of the free anion [B(12)X(12)](2-) depends on the applied excitation energy and on the halogen substituent X. A radical dehalogenation of the B(12) unit is observed for X = I, whereas for X = Cl or F the loss of small molecules (mainly BX(3)) dominates. The different reaction behavior is explained by the different electron affinity of the halogens and the strength of the boron-halogen-bonds. Surprisingly, isolation of the fragment ion [B(12)I(9)](-) in the ion trap yields the highly stable [B(24)I(18)](2-) dianion. This observation suggests a reaction between two negative ions in the gas phase.
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http://dx.doi.org/10.1039/c0cp02386hDOI Listing
April 2011

Charge transfer via the dative N-B bond and dihydrogen contacts. Experimental and theoretical electron density studies of four deltahedral boranes.

J Phys Chem A 2011 Mar 9;115(8):1385-95. Epub 2011 Feb 9.

Institut für Chemie und Biochemie/Anorganische Chemie, Freie Universität Berlin, Fabeckstrasse 34-36 and 36a, 14195 Berlin, Germany.

In an approach combining high resolution X-ray diffraction at low temperatures with density functional calculations, two closo-borates, B12H12(2-) (1) and B10H10(2-) (2), and two arachno-boranes, B10H12L2 (L = amine (3) or acetonitrile (4)), are studied by means of Atoms In Molecules (AIM) theory and Electron Localizability Indicator (ELI-D). The charge transfer via the dative N-B bonds in the arachno-boranes and via dihydrogen contacts in the closo-borates is quantified. The dative N-B bond in 4 is significantly shorter and stronger than that in 3 and in small N-B Lewis acid base adducts from the literature. It is even shorter in the gas phase than in the crystal environment in contrast to the bond shortening in the crystal generally found for N-B Lewis acid-base adducts. Furthermore, the calculated charge transfer in terms of AIM charges is opposite to the expected N → B direction but still weak as found for all other N-B bonds. The intramolecular charge redistributions due to intermolecular interactions are quantified by the AIM and ELI-D analysis of contact ion pairs. The latter method gives a deeper understanding of delocalization effects in the borane cages as well as in the counterions. Since dihydrogen bonds are rarely found in crystal structures, one focus was directed to the topologies of the large number of 58 experimentally found contacts of this type. The analysis reveals that the electron density at the bond critical point, the corresponding Laplace function, and the curvature along the bond path (λ3) show a behavior that clearly discriminates these interactions from classical hydrogen bonds, confirming earlier theoretical findings.
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http://dx.doi.org/10.1021/jp109576aDOI Listing
March 2011

Real-space indicators for chemical bonding. Experimental and theoretical electron density studies of four deltahedral boranes.

Inorg Chem 2011 Jan 29;50(1):90-103. Epub 2010 Nov 29.

Institut für Chemie und Biochemie/Anorganische Chemie, Freie Universität Berlin, Fabeckstrasse 34-36 and 36a, 14195 Berlin, Germany.

In an approach combining high-resolution X-ray diffraction at low temperatures with density functional theory calculations, two closo-borates, B(12)H(12)(2-) (1) and B(10)H(10)(2-) (2), and two arachno-boranes, B(10)H(12)L(2) [L = amine (3) or acetonitrile (4)], were analyzed by means of the atoms-in-molecules (AIM) theory and electron localizability indicator (ELI-D). The two-electron three-center (2e3c) bonds of the borane cages are investigated with the focus on real-space indicators for chemical bonding and electron delocalization. In compound 2, only two of the three expected bond critical points (bcp's) are found. However, a weakly populated ELI-D basin is found for this pair of adjacent B atoms and the delocalization index and the Source contributions are on the same order of magnitude as those for the other pairs. The opposite situation is found in the arachno-boranes, where no ELI-D basins are found for two types of B-B pairs, which, in turn, exhibit a bcp. However, again the delocalization index is on the same order of magnitude for this bonding interaction. The results show that an unambiguous real-space criterion for chemical bonding is not given yet for this class of compounds. The arachno-boranes carry a special B-B bond, which is the edge of the crown-shaped molecule. This bond is very long and extremely curved inward the B-B-B ring. Nevertheless, the corresponding bond ellipticity is quite small and the ELI-D value at the attractor position of the disynaptic valence basin is remarkably larger than those for all other B-B valence basins. Furthermore, the value of the ED is large in relation to the B-B bond length, so that only this bond type does not follow a linear relationship of the ED value at the bcp versus B-B bond distances, which is found for all other B-B bcp's. The results indicate that both 2e2c and 2e3c bonding play a distinct role in borane chemistry.
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http://dx.doi.org/10.1021/ic1013158DOI Listing
January 2011

Pyridinium lipids with the dodecaborate cluster as polar headgroup: synthesis, characterization of the physical-chemical behavior, and toxicity in cell culture.

Bioconjug Chem 2009 Nov;20(11):2190-8

Department of Chemistry, University of Bremen, P.O. Box 330440, D-28357 Bremen.

We have prepared nine new dodecaborate cluster lipids with potential use in boron neutron capture therapy of tumors. This new generation of boron lipids is only singly negatively charged and consists of a pyridinium core with C(12), C(14), and C(16) chains as lipid backbone, connected through the nitrogen atom via a butylene, pentylene, or ethyleneoxyethylene linker to the oxygen atom on the dodecaborate cluster as headgroup. The lipids were obtained by nucleophilic attack of 4-(bisalkylmethyl)pyridine on the tetrahydrofurane, the dioxane, and a newly prepared tetrahydropyrane derivative, respectively, of closo-dodecaborate. All of these boron lipids are able to form closed vesicles in addition to some bilayers in the pure state and in the presence of helper lipids. The thermotropic behavior was found to be increasingly complex and polymorphic with increasing alkyl chain length. Except for two lipids, all lipids have low in vitro toxicity, and longer alkyl chains lead to a significant decrease in toxicity. The choice of the linker plays no major role with respect to their ability to form liposomes and their thermotropic properties, but the toxicity is influenced by the linkers in the case of short alkyl chains.
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http://dx.doi.org/10.1021/bc900147wDOI Listing
November 2009

Interaction of N,N,N-trialkylammonioundecahydro-closo-dodecaborates with dipalmitoyl phosphatidylcholine liposomes.

Chem Phys Lipids 2010 Jan;163(1):64-73

Department of Chemistry, University of Bremen, PO Box 330440, D-28334 Bremen, Germany.

N,N,N-Trialkylammonioundecahydrododecaborates (1-), a novel class of compounds of interest for use as anions in ionic liquids, interact with DPPC liposomes. Increasing compound concentration causes an increasing negative zeta potential. Dissociation constants demonstrate that the binding capacity increases strongly with longer chain length. N,N,N-Trialkylammonioundecahydrododecaborates with longer alkyl chains show a detergent-like behavior: the compounds incorporate into the liposome membrane and differential scanning calorimetric experiment show already low concentrations cause a complete disappearance of the peak representing the gel-to-liquid crystalline phase transition. In contrast, compounds with shorter alkyl chains only interact with the headgroups of the lipids. Investigations by means of cryo-TEM reveal that all derivatives induce significant morphological changes of the liposomes. N,N,N-Trialkylammonioundecahydrododecaborates with short alkyl chains produce large bilayer sheets, whereas those with longer alkyl chains tend to induce the formation of open or multi-layered liposomes. We propose that the binding of N,N,N-trialkylammonioundecahydrododecaborates is mainly due to electrostatic interactions between the doubly negatively charged cluster unit and the positively charged choline headgroup; the positively charged ammonium group might be in contact with the deeper-lying negatively charged phosphate. For N,N,N-trialkylammonioundecahydrododecaborates with longer alkyl chains hydrophobic interactions with the non-polar hydrocarbon part of the membrane constitute an additional important driving force for the association of the compounds to the lipid bilayer.
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http://dx.doi.org/10.1016/j.chemphyslip.2009.09.004DOI Listing
January 2010

Interaction of Na2B12H11SH with dimyristoyl phosphatidylcholine liposomes.

Chem Phys Lipids 2009 Feb 30;157(2):78-85. Epub 2008 Nov 30.

Department of Biochemistry, Faculty of Science, Alexandria University, Moharam Bek, 21511 Alexandria, Egypt.

Previous investigations have revealed that the boron cluster compound Na2B12H11SH (BSH) is very potent in causing major structural rearrangements of and leakage from phosphatidylcholine liposomes. This somewhat unexpected finding is interesting from a fundamental point of view and may also constitute the basis of future important pharmaceutical/medical applications of BSH. In order to further explore the BSH-lipid interaction, we have studied the effects caused by BSH on dimyristoyl phosphatidylcholine (DMPC) liposomes. Cryo-transmission electron microscopy showed that BSH induces aggregation, membrane rupture and increasing wall thickness of the liposomes. Differential scanning calorimetry revealed a BSH dependent shift of the gel to liquid crystalline phase transition temperature of DMPC. The zeta potential of the liposomes decreases with increasing BSH concentrations, and an apparent dissociation constant of 0.23 mM was found. BSH caused leakage of liposome-encapsulated carboxyfluorescein; leakage was higher at 23 degrees C (near the phase transition temperature) than at 15 degrees C and 37 degrees C. It induced lipid mixing only at very high concentrations.
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http://dx.doi.org/10.1016/j.chemphyslip.2008.11.006DOI Listing
February 2009

Trialkylammoniododecaborates: anions for ionic liquids with potassium, lithium and protons as cations.

Chemistry 2008 ;14(6):1918-23

Department of Chemistry, University of Bremen, P.O. Box 330 440, 28334 Bremen, Germany.

Herein we report a new class of low-melting ionic liquids (IL) that consist of N,N,N-trialkylammonioundecahydrododecaborates(1-) as the anion and a range of cations. The cations include the common cations of conventional ILs such as tetraalkylammonium, N-alkylpyridinium, and N-methyl-N'-alkylimidazolium. In addition, their salts with lithium, potassium, and proton cations also exist as ILs. Pulse radiolysis studies indicate that the anions do not react with solvated electrons.
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http://dx.doi.org/10.1002/chem.200701427DOI Listing
June 2008