Publications by authors named "Kiminori Maeda"

29 Publications

  • Page 1 of 1

Quantum control of radical pair reactions by local optimization theory.

J Chem Phys 2020 Jan;152(1):014301

Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura Ward, 338-8570 Saitama, Japan.

Recently, AWG (arbitrary waveform generator) based pulse electron paramagnetic resonance and nuclear magnetic resonance have been developed in a high field regime for the improvement of sensitivity and selectivity and quantum information processing. Here, we propose the application of AWG based reaction control of radical pairs in a rather low magnetic field regime. We calculated the locally optimized radio frequency (RF) field with the control theory by Sugawara [J. Chem. Phys. 118(15), 6784-6800 (2003)]. The calculation results manifest the applicability of AWG-RF fields to reaction control (reaction yield detected magnetic resonance), stimulated nuclear polarization, magnetic isotope selection, and coherent control of the spin dynamics.
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http://dx.doi.org/10.1063/1.5131557DOI Listing
January 2020

Gigantic Magnetic Field Effect on the Long-Lived Intermolecular Charge-Separated State Created at the Nonionic Bilayer Membrane.

J Phys Chem B 2018 12 30;122(50):12173-12183. Epub 2018 Nov 30.

Department of Chemistry , Niigata University , 2-8050 Ikarashi, Nishi-ku , Niigata 950-2181 , Japan.

For realization of low-cost organic photon-energy conversion, the supramolecular approach has been a focus of attention as a counter approach to precise synthesis of covalently linked donor (D)-acceptor (A) molecules. Here we report photogeneration of a long-lived (∼3 μs) intermolecular charge-separated (CS) state of metal porphyrins (D) and an alkyl viologen (A) at an interface of a vesicle membrane formed by self-assembly of nonionic surfactant and cholesterol molecules. The yield of escaped free radicals is negligibly low as in the case of CS states in covalently linked D-A systems. Furthermore, the transient concentration of the CS state dramatically increases by ∼100% upon application of a magnetic field of 250 mT at room temperature. The simulation of the spin dynamics of the CS state indicates that fast (∼10 s) spin-selective recombination and slow (10-10 s) dissociation-re-encounter dynamics are the key processes for the long CS-state lifetime and the gigantic magnetic field effect. It has turned out that such dynamics are sharply dependent on temperature and alkyl chain length of the viologen. The present results would lead to the development of future materials for light energy conversion, drug delivery, and microscopic bioprobes.
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http://dx.doi.org/10.1021/acs.jpcb.8b08389DOI Listing
December 2018

Millitesla magnetic field effects on the photocycle of an animal cryptochrome.

Sci Rep 2017 02 8;7:42228. Epub 2017 Feb 8.

Department of Chemistry, University of Oxford, Physical &Theoretical Chemistry Laboratory, Oxford OX1 3QZ, United Kingdom.

Drosophila have been used as model organisms to explore both the biophysical mechanisms of animal magnetoreception and the possibility that weak, low-frequency anthropogenic electromagnetic fields may have biological consequences. In both cases, the presumed receptor is cryptochrome, a protein thought to be responsible for magnetic compass sensing in migratory birds and a variety of magnetic behavioural responses in insects. Here, we demonstrate that photo-induced electron transfer reactions in Drosophila melanogaster cryptochrome are indeed influenced by magnetic fields of a few millitesla. The form of the protein containing flavin and tryptophan radicals shows kinetics that differ markedly from those of closely related members of the cryptochrome-photolyase family. These differences and the magnetic sensitivity of Drosophila cryptochrome are interpreted in terms of the radical pair mechanism and a photocycle involving the recently discovered fourth tryptophan electron donor.
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http://dx.doi.org/10.1038/srep42228DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5296725PMC
February 2017

Long-Distance Sequential Charge Separation at Micellar Interface Mediated by Dynamic Charge Transporter: A Magnetic Field Effect Study.

J Phys Chem Lett 2015 Jan 6;6(2):267-71. Epub 2015 Jan 6.

†Department of Chemistry, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan.

Construction of photogenerated long-lived charge-separated states is crucial for light-energy conversion using organic molecules. For realization of cheap and easy-to-make long-distance electron transfer (ET) systems, we have developed a supramolecular donor(D)-chromophore(C)-acceptor(A) triad utilizing a micellar interface. Alkyl viologen (A(2+)) is adsorbed on the hydrophilic interface of Triton X-100 micelle, which bears D units in the hydrophobic core. Excited triplet state of a hydrophobic flavin C entrapped in the supercage gives rise to primary ET from D, which is followed by the secondary ET from C(-•) to A(2+) to give the long-lived (>10 μs) charge-separated state with negligible yield of escaped C(-•). Analysis of magnetic field effect reveals that diffusion of C(-•) from the core to the hydrophilic interface leads to long-distance ET with a low charge recombination yield of ∼20%. This novel concept of "dynamic charge transporter" has important implications for development of photon-energy conversion systems in solution phase.
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http://dx.doi.org/10.1021/jz502495uDOI Listing
January 2015

Sensitive fluorescence-based detection of magnetic field effects in photoreactions of flavins.

Phys Chem Chem Phys 2015 Jul;17(28):18456-63

Department of Chemistry, University of Oxford, Centre for Advanced Electron Spin Resonance, Oxford, OX1 3QR, UK.

Magnetic field effect studies have been conducted on a variety of flavin-based radical pair systems chosen to model the magnetosensitivity of the photoinduced radical pairs found in cryptochrome flavoproteins. Cryptochromes are blue-light photoreceptor proteins which are thought to mediate avian magnetoreception, an hypothesis supported by recent in vitro observations of magnetic field-dependent reaction kinetics for a light-induced radical pair in a cryptochrome from the plant Arabidopsis thaliana. Many cryptochromes are difficult to express in large quantities or high concentrations and are easily photodegraded. Magnetic field effects are typically measured by spectroscopic detection of the transient radical (pair) concentrations. Due to its low sensitivity, single-pass transient absorption spectroscopy can be of limited use in such experiments and much recent work has involved development of other methodologies offering improved sensitivity. Here we explore the use of flavin fluorescence as the magnetosensitive probe and demonstrate the exceptional sensitivity of this technique which allows the detection of magnetic field effects in flavin samples at sub-nanomolar concentrations and in cryptochromes.
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http://dx.doi.org/10.1039/c5cp00723bDOI Listing
July 2015

Probing a chemical compass: novel variants of low-frequency reaction yield detected magnetic resonance.

Phys Chem Chem Phys 2015 Feb 24;17(5):3550-9. Epub 2014 Dec 24.

Department of Chemistry, University of Oxford, Centre for Advanced Electron Spin Resonance, Inorganic Chemistry Laboratory, Oxford, OX1 3QR, UK.

We present a study of a carotenoid-porphyrin-fullerene triad previously shown to function as a chemical compass: the photogenerated carotenoid-fullerene radical pair recombines at a rate sensitive to the orientation of an applied magnetic field. To characterize the system we develop a time-resolved Low-Frequency Reaction Yield Detected Magnetic Resonance (tr-LF-RYDMR) technique; the effect of varying the relative orientation of applied static and 36 MHz oscillating magnetic fields is shown to be strongly dependent on the strength of the oscillating magnetic field. RYDMR is a diagnostic test for involvement of the radical pair mechanism in the magnetic field sensitivity of reaction rates or yields, and has previously been applied in animal behavioural experiments to verify the involvement of radical-pair-based intermediates in the magnetic compass sense of migratory birds. The spectroscopic selection rules governing RYDMR are well understood at microwave frequencies for which the so-called 'high-field approximation' is valid, but at lower frequencies different models are required. For example, the breakdown of the rotating frame approximation has recently been investigated, but less attention has so far been given to orientation effects. Here we gain physical insights into the interplay of the different magnetic interactions affecting low-frequency RYDMR experiments performed in the challenging regime in which static and oscillating applied magnetic fields as well as internal electron-nuclear hyperfine interactions are of comparable magnitude. Our observations aid the interpretation of existing RYDMR-based animal behavioural studies and will inform future applications of the technique to verify and characterize further the biological receptors involved in avian magnetoreception.
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http://dx.doi.org/10.1039/c4cp04095cDOI Listing
February 2015

Broadband cavity-enhanced detection of magnetic field effects in chemical models of a cryptochrome magnetoreceptor.

J Phys Chem B 2014 Apr 4;118(15):4177-84. Epub 2014 Apr 4.

Department of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford , Oxford, OX1 3QZ, United Kingdom.

Broadband cavity-enhanced absorption spectroscopy (BBCEAS) is shown to be a sensitive method for the detection of magnetic field effects (MFEs) in two flavin-based chemical reactions which are simple models for cryptochrome magnetoreceptors. The advantages of optical cavity-based detection and (pseudo-white-light) supercontinuum radiation have been combined to provide full spectral coverage across the whole of the visible spectrum (425 < λ < 700 nm). This region covers the absorbance spectra of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) as well as their photogenerated radicals. To illustrate the power of this technique, BBCEAS has been used to record the spectral dependence of MFEs for photoinduced radical pairs formed in the intermolecular reaction of FMN with lysozyme and the intramolecular photochemistry of FAD. These reactions have been chosen for their photochemical similarities to cryptochrome proteins which have been proposed as key to the magnetic compass sense of many animals including birds. In experiments performed using low protein concentrations (10 μM) and 1 mm optical path-lengths, absorbance changes as small as 1 × 10(-7) (representing <0.1% MFEs) have been detected with good signal-to-noise offering the prospect of sensitive MFE detection in cryptochrome.
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http://dx.doi.org/10.1021/jp500732uDOI Listing
April 2014

Magnetic field effects in flavoproteins and related systems.

Interface Focus 2013 Oct;3(5):20130037

Centre for Advanced Electron Spin Resonance, Department of Chemistry , University of Oxford , South Parks Road, Oxford OX1 3QR , UK.

Within the framework of the radical pair mechanism, magnetic fields may alter the rate and yields of chemical reactions involving spin-correlated radical pairs as intermediates. Such effects have been studied in detail in a variety of chemical systems both experimentally and theoretically. In recent years, there has been growing interest in whether such magnetic field effects (MFEs) also occur in biological systems, a question driven most notably by the increasing body of evidence for the involvement of such effects in the magnetic compass sense of animals. The blue-light photoreceptor cryptochrome is placed at the centre of this debate and photoexcitation of its bound flavin cofactor has indeed been shown to result in the formation of radical pairs. Here, we review studies of MFEs on free flavins in model systems as well as in blue-light photoreceptor proteins and discuss the properties that are crucial in determining the magnetosensitivity of these systems.
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http://dx.doi.org/10.1098/rsfs.2013.0037DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3915827PMC
October 2013

Spin-selective recombination reactions of radical pairs: experimental test of validity of reaction operators.

J Chem Phys 2013 Dec;139(23):234309

Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, Oxford, United Kingdom.

Spin-selective reactions of radical pairs are conventionally modelled using an approach that dates back to the 1970s [R. Haberkorn, Mol. Phys. 32, 1491 (1976)]. An alternative approach based on the theory of quantum measurements has recently been suggested [J. A. Jones and P. J. Hore, Chem. Phys. Lett. 488, 90 (2010)]. We present here the first experimental attempt to discriminate between the two models. Pulsed electron paramagnetic resonance spectroscopy has been used to investigate intramolecular electron transfer in the radical pair form of a carotenoid-porphyrin-fullerene molecular triad. The rate of spin-spin relaxation of the fullerene radical in the triad was found to be inconsistent with the quantum measurement description of the spin-selective kinetics, and in accord with the conventional model when combined with spin-dephasing caused by rotational modulation of the anisotropic g-tensor of the fullerene radical.
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http://dx.doi.org/10.1063/1.4844355DOI Listing
December 2013

Multielement NMR studies of the liquid-liquid phase separation and the metal-to-nonmetal transition in fluid lithium- and sodium-ammonia solutions.

J Phys Chem B 2013 Oct 13;117(42):13322-34. Epub 2013 Sep 13.

Department of Chemistry, Centre for Advanced Electron Spin Resonance (CAESR), Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford , Oxford OX1 3QR, U.K.

(1)H, (7)Li, (14)N, and (23)Na high resolution nuclear magnetic resonance (NMR) measurements are reported for fluid solutions of lithium and sodium in anhydrous liquid ammonia across the metal-to-nonmetal transition (MNM transition), paying particular attention to the phenomenon of liquid-liquid phase separation which occurs in the composition/temperature region close to the MNM transition. Our results are discussed in terms of the electronic structure of fluid metal-ammonia solutions at low temperatures (ca. 240 K). We find that the electronic phase transition to the metallic state in these solutions, especially at temperatures close to the liquid-liquid critical consolute temperature, occurs from a nonmetallic, electrolytic solution containing a predominance of electron spin-paired, (diamagnetic) charged bosonic states. The possible implications of these observations to the nature of the liquid-liquid phase separation are discussed, both from the views of N. F. Mott, regarding the MNM transition in sodium-ammonia solutions, and those of R. A. Ogg, regarding the possibility of high-temperature superconductivity in these solutions. Similarities between the electronic structure of metal-ammonia solutions and the high-temperature cuprate superconductors are also briefly emphasized.
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http://dx.doi.org/10.1021/jp404023jDOI Listing
October 2013

Electron tunneling in lithium-ammonia solutions probed by frequency-dependent electron spin relaxation studies.

J Am Chem Soc 2012 Jun 24;134(22):9209-18. Epub 2012 May 24.

Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, United Kingdom.

Electron transfer or quantum tunneling dynamics for excess or solvated electrons in dilute lithium-ammonia solutions have been studied by pulse electron paramagnetic resonance (EPR) spectroscopy at both X- (9.7 GHz) and W-band (94 GHz) frequencies. The electron spin-lattice (T(1)) and spin-spin (T(2)) relaxation data indicate an extremely fast transfer or quantum tunneling rate of the solvated electron in these solutions which serves to modulate the hyperfine (Fermi-contact) interaction with nitrogen nuclei in the solvation shells of ammonia molecules surrounding the localized, solvated electron. The donor and acceptor states of the solvated electron in these solutions are the initial and final electron solvation sites found before, and after, the transfer or tunneling process. To interpret and model our electron spin relaxation data from the two observation EPR frequencies requires a consideration of a multiexponential correlation function. The electron transfer or tunneling process that we monitor through the correlation time of the nitrogen Fermi-contact interaction has a time scale of (1-10) × 10(-12) s over a temperature range 230-290 K in our most dilute solution of lithium in ammonia. Two types of electron-solvent interaction mechanisms are proposed to account for our experimental findings. The dominant electron spin relaxation mechanism results from an electron tunneling process characterized by a variable donor-acceptor distance or range (consistent with such a rapidly fluctuating liquid structure) in which the solvent shell that ultimately accepts the transferring electron is formed from random, thermal fluctuations of the liquid structure in, and around, a natural hole or Bjerrum-like defect vacancy in the liquid. Following transfer and capture of the tunneling electron, further solvent-cage relaxation with a time scale of ∼10(-13) s results in a minor contribution to the electron spin relaxation times. This investigation illustrates the great potential of multifrequency EPR measurements to interrogate the microscopic nature and dynamics of ultrafast electron transfer or quantum-tunneling processes in liquids. Our results also impact on the universal issue of the role of a host solvent (or host matrix, e.g. a semiconductor) in mediating long-range electron transfer processes and we discuss the implications of our results with a range of other materials and systems exhibiting the phenomenon of electron transfer.
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http://dx.doi.org/10.1021/ja212015bDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3415590PMC
June 2012

Magnetically sensitive light-induced reactions in cryptochrome are consistent with its proposed role as a magnetoreceptor.

Proc Natl Acad Sci U S A 2012 Mar 14;109(13):4774-9. Epub 2012 Mar 14.

Department of Chemistry, University of Oxford, Centre for Advanced Electron Spin Resonance, Inorganic Chemistry Laboratory, Oxford OX1 3QR, United Kingdom.

Among the biological phenomena that fall within the emerging field of "quantum biology" is the suggestion that magnetically sensitive chemical reactions are responsible for the magnetic compass of migratory birds. It has been proposed that transient radical pairs are formed by photo-induced electron transfer reactions in cryptochrome proteins and that their coherent spin dynamics are influenced by the geomagnetic field leading to changes in the quantum yield of the signaling state of the protein. Despite a variety of supporting evidence, it is still not clear whether cryptochromes have the properties required to respond to magnetic interactions orders of magnitude weaker than the thermal energy, k(B)T. Here we demonstrate that the kinetics and quantum yields of photo-induced flavin-tryptophan radical pairs in cryptochrome are indeed magnetically sensitive. The mechanistic origin of the magnetic field effect is clarified, its dependence on the strength of the magnetic field measured, and the rates of relevant spin-dependent, spin-independent, and spin-decoherence processes determined. We argue that cryptochrome is fit for purpose as a chemical magnetoreceptor.
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http://dx.doi.org/10.1073/pnas.1118959109DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3323948PMC
March 2012

Following radical pair reactions in solution: a step change in sensitivity using cavity ring-down detection.

J Am Chem Soc 2011 Nov 17;133(44):17807-15. Epub 2011 Oct 17.

Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, Oxford, OX1 3QR, UK.

The study of radical pair intermediates in biological systems has been hampered by the low sensitivity of the optical techniques usually employed to investigate these highly reactive species. Understanding the physical principles governing the spin-selective and magneto-sensitive yields and kinetics of their reactions is essential in identifying the mechanism governing bird migration, and might have significance in the discussion of potential health hazards of electromagnetic radiation. Here, we demonstrate the powerful capabilities of optical cavity-enhanced techniques, such as cavity ring-down spectroscopy (CRDS) in monitoring radical recombination reactions and associated magnetic field effects (MFEs). These include submicrosecond time-resolution, high sensitivity (baseline noise on the order of 10(-6) absorbance units) and small (μL) sample volumes. Combined, we show that these represent significant advantages over the single-pass flash-photolysis techniques conventionally applied. The studies described here focus on photoinduced radical pair reactions involving the protein lysozyme and one of two possible photosensitizers: anthraquinone-2,6-disulphonate and flavin mononucleotide. CRDS-measured MFEs are observed in pump-probe experiments and discussed in terms of the sensitivity gains and sample-volume minimization afforded by CRDS when compared with flash photolysis methods. Finally, CRDS is applied to an in vitro MFE study of intramolecular electron transfer in the DNA-repair enzyme, Escherichia coli photolyase, a protein closely related to cryptochrome which has been proposed to mediate animal magnetoreception.
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http://dx.doi.org/10.1021/ja206783tDOI Listing
November 2011

Spin-selective recombination kinetics of a model chemical magnetoreceptor.

Chem Commun (Camb) 2011 Jun 12;47(23):6563-5. Epub 2011 May 12.

Department of Chemistry, University of Oxford, Centre for Advanced Electron Spin Resonance, Inorganic Chemistry Laboratory, Oxford, UK.

We determine the spin-selective kinetics of a carotenoid-porphyrin-fullerene triad that has previously been used to establish the principle that a photochemical reaction could form the basis of the magnetic compass sensor of migratory birds and show that its magnetic sensitivity can be understood without invoking quantum Zeno effects.
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http://dx.doi.org/10.1039/c1cc11625hDOI Listing
June 2011

Consistent treatment of spin-selective recombination of a radical pair confirms the Haberkorn approach.

J Phys Chem A 2010 Sep;114(35):9447-55

International Tomography Center SB RAS, Novosibirsk, 630090, Russia.

In the present work, we have shown that consistent derivation of the kinetic equations describing the electron spin-selective recombination of radical pairs confirms the conventional Haberkorn approach. The derivation has been based on considering the interaction of the reactive system (radical pair and product state) with the thermal bath. The consistency of this approach has also been substantiated by numerical simulations performed for the purely quantum mechanical model of the recombining radical pair. Finally, we have shown that the quantum Zeno effect on radical pair recombination is not an exclusive feature of the approach recently proposed by Kominis, as it should be present at any rate of the singlet-triplet dephasing in the radical pair, which always accompanies the recombination process.
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http://dx.doi.org/10.1021/jp1048265DOI Listing
September 2010

Protein surface interactions probed by magnetic field effects on chemical reactions.

J Am Chem Soc 2010 Feb;132(5):1466-7

Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford, Oxford OX1 3TA, UK.

Here we have employed the effects of weak static magnetic fields (not exceeding 46 mT) on radical recombination reactions to investigate protein-substrate interactions. Pulsed laser excitation of an aqueous solution of anthraquinone-2,6-disulfonate (AQDS(2-)) and either hen egg white lysozyme (HEWL) or bovine serum albumin (BSA) produces the triplet state of the radical pair (T)[AQDS(3-*) Trp(*)] by a photoinduced electron transfer reaction from tryptophan residues. Time-resolved absorption techniques were employed to study the recombination characteristics of these radical pairs at different static magnetic fields and ionic strengths. The experimental data in connection with the simulated curves unequivocally show that the radical pair has a lifetime of the order of microseconds in both systems (HEWL and BSA). However, the radical pair is embedded within a binding pocket of the BSA protein, while the (otherwise identical) radical pair, being subject to attractive Coulomb forces, resides on the protein surface in the HEWL system.
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http://dx.doi.org/10.1021/ja908988uDOI Listing
February 2010

Radiofrequency polarization effects in low-field electron paramagnetic resonance.

Phys Chem Chem Phys 2009 Aug 3;11(31):6573-9. Epub 2009 Jul 3.

Department of Chemistry, University of Oxford, Oxford, UK.

Low-field optically detected EPR spectra of photochemically formed transient radical ion pairs are reported for weak circularly and linearly polarized radiofrequency (RF) fields. The spectra are found to be strongly dependent on the polarization and frequency of the RF field and on the angle between the static magnetic field and the plane containing the RF field. The spectra are discussed in terms of resonances arising from Zeeman and hyperfine interactions; the conditions for validity of the rotating frame approximation are determined. Knowledge of the latter is important when using low-field EPR as a diagnostic test for the operation of the radical pair mechanism.
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http://dx.doi.org/10.1039/b907915gDOI Listing
August 2009

Radiofrequency polarization effects in zero-field electron paramagnetic resonance.

Phys Chem Chem Phys 2009 Aug 3;11(31):6569-72. Epub 2009 Jul 3.

Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, UK.

Optically detected zero-field electron paramagnetic resonance spectroscopy is used to show that weak linearly and circularly polarized radiofrequency magnetic fields affect the recombination reactions of spin-correlated radical pairs to different extents; the spectra are shown to be consistent with the radical pair mechanism.
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http://dx.doi.org/10.1039/b906102aDOI Listing
August 2009

Refolding of ribonuclease A monitored by real-time photo-CIDNP NMR spectroscopy.

J Biomol NMR 2009 Jun 13;44(2):77-86. Epub 2009 May 13.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX13QZ, UK.

Photo-CIDNP NMR spectroscopy is a powerful method for investigating the solvent accessibility of histidine, tyrosine and tryptophan residues in a protein. When coupled to real-time NMR, this technique allows changes in the environments of these residues to be used as a probe of protein folding. In this paper we describe experiments performed to monitor the refolding of ribonuclease A following dilution from a high concentration of chemical denaturant. These experiments provide a good example of the utility of this technique which provides information that is difficult to obtain by other biophysical methods. Real-time photo-CIDNP measurements yield residue-specific kinetic data pertaining to the folding reaction, interpreted in terms of current knowledge of the folding of bovine pancreatic ribonuclease A.
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http://dx.doi.org/10.1007/s10858-009-9322-2DOI Listing
June 2009

Quenching mechanisms and diffusional pathways in micellar systems unravelled by time-resolved magnetic-field effects.

Chemistry 2009 Jun;15(24):6058-64

Institut für Chemie, Martin-Luther-Universität Halle-Wittenberg, Kurt-Mothes-Str. 2, 06120 Halle/Saale, Germany.

Magnetic-field effects (MFEs) are used to investigate the photoreaction of xanthone (A) and DABCO (D) in anionic (SDS) or cationic (DTAC) micelles at high pH (DABCO = 1,4-diazabicyclo[2.2.2]octane, SDS = sodium dodecyl sulfate, DTAC = dodecyl trimethyl ammonium chloride). From MFE experiments with nanosecond time resolution, the radical anion A(.)(-) can be observed without any interference from the much more strongly absorbing triplet (3)A*, the different quenching processes can be separated and their rates can be measured. Triplet (3)A* is quenched dynamically both by the SDS micelle (k(1) = 5.0x10(5) s(-1)) and by DABCO approaching from the aqueous phase (k(2) = 2.0x10(9) M(-1) s(-1)). Static quenching by solubilised DABCO (association constant with the SDS micelles, 1.5 M(-1)) also participates at high DABCO concentrations, but is chemically nonproductive and does not lead to MFE generation. The MFEs stemming from the radical ion pairs A(.)(-) D(.)(+) are about 40 times larger in the anionic micelles than in the cationic ones despite a higher yield of free radicals in the latter case. This can be rationalised by different diffusional dynamics: Because of the location of their precursors, A(.)(-) and D(.)(+) are formed at opposite sides of the micelle boundary. Subsequently, the negatively charged Stern layer of the SDS micelle traps the radical cation, which then undergoes surface diffusion, so both the recombination probability and the spin mixing are high; in contrast, the positive surface charge of the DTAC micelle forces the radical cation into the bulk of the solution, thus efficiently blocking a recombination.
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http://dx.doi.org/10.1002/chem.200802502DOI Listing
June 2009

Effect of magnetic fields on cryptochrome-dependent responses in Arabidopsis thaliana.

J R Soc Interface 2009 Dec 25;6(41):1193-205. Epub 2009 Feb 25.

Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.

The scientific literature describing the effects of weak magnetic fields on living systems contains a plethora of contradictory reports, few successful independent replication studies and a dearth of plausible biophysical interaction mechanisms. Most such investigations have been unsystematic, devoid of testable theoretical predictions and, ultimately, unconvincing. A recent study, of magnetic responses in the model plant Arabidopsis thaliana, however, stands out; it has a clear hypothesis-that seedling growth is magnetically sensitive as a result of photoinduced radical-pair reactions in cryptochrome photoreceptors-tested by measuring several cryptochrome-dependent responses, all of which proved to be enhanced in a magnetic field of intensity 500 muT. The potential importance of this study in the debate on putative effects of extremely low-frequency electromagnetic fields on human health prompted us to subject it to the 'gold standard' of independent replication. With experimental conditions chosen to match those of the original study, we have measured hypocotyl lengths and anthocyanin accumulation for Arabidopsis seedlings grown in a 500 microT magnetic field, with simultaneous control experiments at 50 microT. Additionally, we have determined hypocotyl lengths of plants grown in 50 microT, 1 mT and approximately 100 mT magnetic fields (with zero-field controls), measured gene (CHS, HY5 and GST) expression levels, investigated blue-light intensity effects and explored the influence of sucrose in the growth medium. In no case were consistent, statistically significant magnetic field responses detected.
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http://dx.doi.org/10.1098/rsif.2008.0519DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817153PMC
December 2009

SQUID magnetometry as a tool for following a clock reaction in solution.

Dalton Trans 2009 Apr 18(14):2467-9. Epub 2009 Feb 18.

Inorganic Chemistry, University of Oxford, South Parks Road, Oxford, UK OX1 3QR.

SQUID magnetometry, normally used to characterise the properties of solids, was used to follow a clock reaction in solution, namely the auto-catalytic oxidation of [Co(II)EDTA]2- by H2O2, in real time and it was shown that, in combination with other methods (e.g., magnetic resonance proton relaxation studies and UV-vis absorption analysis), SQUID magnetometry can be a powerful method in elucidating and interpreting the time-profile of chemical reactions so as long as reactants, intermediates and products have suitably large differences in their respective magnetic susceptibilities.
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http://dx.doi.org/10.1039/b819977aDOI Listing
April 2009

Magnetic-field effect on the photoactivation reaction of Escherichia coli DNA photolyase.

Proc Natl Acad Sci U S A 2008 Sep 17;105(38):14395-9. Epub 2008 Sep 17.

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, U.K.

One of the two principal hypotheses put forward to explain the primary magnetoreception event underlying the magnetic compass sense of migratory birds is based on a magnetically sensitive chemical reaction. It has been proposed that a spin-correlated radical pair is produced photochemically in a cryptochrome and that the rates and yields of the subsequent chemical reactions depend on the orientation of the protein in the Earth's magnetic field. The suitability of cryptochrome for this purpose has been argued, in part, by analogy with DNA photolyase, although no effects of applied magnetic fields have yet been reported for any member of the cryptochrome/photolyase family. Here, we demonstrate a magnetic-field effect on the photochemical yield of a flavin-tryptophan radical pair in Escherichia coli photolyase. This result provides a proof of principle that photolyases, and most likely by extension also cryptochromes, have the fundamental properties needed to form the basis of a magnetic compass.
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http://dx.doi.org/10.1073/pnas.0803620105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2567148PMC
September 2008

Chemical compass model of avian magnetoreception.

Nature 2008 May 30;453(7193):387-90. Epub 2008 Apr 30.

Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK.

Approximately 50 species, including birds, mammals, reptiles, amphibians, fish, crustaceans and insects, are known to use the Earth's magnetic field for orientation and navigation. Birds in particular have been intensively studied, but the biophysical mechanisms that underlie the avian magnetic compass are still poorly understood. One proposal, based on magnetically sensitive free radical reactions, is gaining support despite the fact that no chemical reaction in vitro has been shown to respond to magnetic fields as weak as the Earth's ( approximately 50 muT) or to be sensitive to the direction of such a field. Here we use spectroscopic observation of a carotenoid-porphyrin-fullerene model system to demonstrate that the lifetime of a photochemically formed radical pair is changed by application of < or =50 microT magnetic fields, and to measure the anisotropic chemical response that is essential for its operation as a chemical compass sensor. These experiments establish the feasibility of chemical magnetoreception and give insight into the structural and dynamic design features required for optimal detection of the direction of the Earth's magnetic field.
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http://dx.doi.org/10.1038/nature06834DOI Listing
May 2008

Chemical magnetoreception: bird cryptochrome 1a is excited by blue light and forms long-lived radical-pairs.

PLoS One 2007 Oct 31;2(10):e1106. Epub 2007 Oct 31.

VW Nachswuchgruppe Animal Navigation, IBU, University of Oldenburg, Oldenburg, Germany.

Cryptochromes (Cry) have been suggested to form the basis of light-dependent magnetic compass orientation in birds. However, to function as magnetic compass sensors, the cryptochromes of migratory birds must possess a number of key biophysical characteristics. Most importantly, absorption of blue light must produce radical pairs with lifetimes longer than about a microsecond. Cryptochrome 1a (gwCry1a) and the photolyase-homology-region of Cry1 (gwCry1-PHR) from the migratory garden warbler were recombinantly expressed and purified from a baculovirus/Sf9 cell expression system. Transient absorption measurements show that these flavoproteins are indeed excited by light in the blue spectral range leading to the formation of radicals with millisecond lifetimes. These biophysical characteristics suggest that gwCry1a is ideally suited as a primary light-mediated, radical-pair-based magnetic compass receptor.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0001106PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2040520PMC
October 2007

Radical cation stabilization in a cucurbituril oligoaniline rotaxane.

J Am Chem Soc 2007 Oct 21;129(41):12384-5. Epub 2007 Sep 21.

Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford, OX1 3TA, UK.

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http://dx.doi.org/10.1021/ja074997iDOI Listing
October 2007

Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field.

J Phys Chem A 2005 Nov;109(44):9911-8

Department of Chemistry, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan.

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.
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http://dx.doi.org/10.1021/jp053989qDOI Listing
November 2005

Dynamics of intramolecular electron transfer reaction of FAD studied by magnetic field effects on transient absorption spectra.

J Phys Chem A 2005 Jul;109(26):5793-800

Department of Chemistry, University of Tsukuba, 1-1-1 Tenno-dai, Tsukuba City, Ibaraki Pref, 305-8571, Japan.

The kinetics of intermediates generated from intramolecular electron-transfer reaction by photo irradiation of the flavin adenine dinucleotide (FAD) molecule was studied by a magnetic field effect (MFE) on transient absorption (TA) spectra. Existence time of MFE and MFE action spectra have a strong dependence on the pH of solutions. The MFE action spectra have indicated the existence of interconversion between the radical pair and the cation form of the triplet excited state of flavin part. All rate constants of the triplet and the radical pair were determined by analysis of the MFE action spectra and decay kinetics of TA. The obtained values for the interconversion indicate that the formation of cation radical promotes the back electron-transfer reaction to the triplet excited state. Further, rate constants of spin relaxation and recombination have been studied by the time profiles of MFE at various pH. The drastic change of those two factors has been obtained and can be explained by SOC (spin-orbit coupling) induced back electron-transfer promoted by the formation of a stacking conformation at pH > 2.5.
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http://dx.doi.org/10.1021/jp0519722DOI Listing
July 2005

The spin mixing process of a radical pair in low magnetic field observed by transient absorption detected nanosecond pulsed magnetic field effect.

J Phys Chem A 2006 Mar;110(12):4151-6

Department of Chemistry, Faculty of Science, Shizuoka University, Ohya 836, Shizuoka City 422-8529, Japan.

The spin mixing process of the radical pair in the sodium dodecyl sulfate (SDS) micelle is studied by using a novel technique nanosecond pulsed magnetic field effect on transient absorption. We have developed the equipment for a nanosecond pulsed magnetic field and observed its effect on the radical pair reaction. A decrease of the free radical yield by a reversely directed pulsed magnetic field that cancels static field is observed, and the dependence on its magnitude, which is called pulsed MARY (magnetic field effect on reaction yield) spectra, is studied. The observed spectra reflect the spin mixing in 50-200 ns and show clear time evolution. Theoretical simulation of pulsed MARY spectra based on a single site modified Liouville equation indicates that the fast spin dephasing processes induced by the modulation of electron-electron spin interaction by molecular reencounter affect to the coherent spin mixing by a hyperfine interaction in a low magnetic field.
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http://dx.doi.org/10.1021/jp056488dDOI Listing
March 2006
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