Publications by authors named "Kevin B Henbest"

25 Publications

  • Page 1 of 1

Magnetic sensitivity of cryptochrome 4 from a migratory songbird.

Nature 2021 Jun 23;594(7864):535-540. Epub 2021 Jun 23.

Institut für Physikalische Chemie, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany.

Night-migratory songbirds are remarkably proficient navigators. Flying alone and often over great distances, they use various directional cues including, crucially, a light-dependent magnetic compass. The mechanism of this compass has been suggested to rely on the quantum spin dynamics of photoinduced radical pairs in cryptochrome flavoproteins located in the retinas of the birds. Here we show that the photochemistry of cryptochrome 4 (CRY4) from the night-migratory European robin (Erithacus rubecula) is magnetically sensitive in vitro, and more so than CRY4 from two non-migratory bird species, chicken (Gallus gallus) and pigeon (Columba livia). Site-specific mutations of ErCRY4 reveal the roles of four successive flavin-tryptophan radical pairs in generating magnetic field effects and in stabilizing potential signalling states in a way that could enable sensing and signalling functions to be independently optimized in night-migratory birds.
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http://dx.doi.org/10.1038/s41586-021-03618-9DOI Listing
June 2021

Orientation-Selective and Frequency-Correlated Light-Induced Pulsed Dipolar Spectroscopy.

J Phys Chem Lett 2021 Apr 15;12(15):3819-3826. Epub 2021 Apr 15.

Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy.

We explore the potential of orientation-resolved pulsed dipolar spectroscopy (PDS) in light-induced versions of the experiment. The use of triplets as spin-active moieties for PDS offers an attractive tool for studying biochemical systems containing optically active cofactors. Cofactors are often rigidly bound within the protein structure, providing an accurate positional marker. The rigidity leads to orientation selection effects in PDS, which can be analyzed to give both distance and mutual orientation information. Herein we present a comprehensive analysis of the orientation selection of a full set of light-induced PDS experiments. We exploit the complementary information provided by the different light-induced techniques to yield atomic-level structural information. For the first time, we measure a 2D frequency-correlated laser-induced magnetic dipolar spectrum, and we are able to monitor the complete orientation dependence of the system in a single experiment. Alternatively, the summed spectrum enables an orientation-independent analysis to determine the distance distribution.
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http://dx.doi.org/10.1021/acs.jpclett.1c00595DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8154851PMC
April 2021

Light-Induced Triplet-Triplet Electron Resonance Spectroscopy.

J Phys Chem Lett 2021 Jan 11;12(1):80-85. Epub 2020 Dec 11.

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

We present a new technique, light-induced triplet-triplet electron resonance spectroscopy (LITTER), which measures the dipolar interaction between two photoexcited triplet states, enabling both the distance and angular distributions between the two triplet moieties to be determined on a nanometer scale. This is demonstrated for a model bis-porphyrin peptide that renders dipolar traces with strong orientation selection effects. Using simulations and density functional theory calculations, we extract distance distributions and relative orientations of the porphyrin moieties, allowing the dominant conformation of the peptide in a frozen solution to be identified. LITTER removes the requirement of current light-induced electron spin resonance pulse dipolar spectroscopy techniques to have a permanent paramagnetic moiety, becoming more suitable for in-cell applications and facilitating access to distance determination in unmodified macromolecular systems containing photoexcitable moieties. LITTER also has the potential to enable direct comparison with Förster resonance energy transfer and combination with microscopy inside cells.
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http://dx.doi.org/10.1021/acs.jpclett.0c02884DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8016185PMC
January 2021

Magnetically Sensitive Radical Photochemistry of Non-natural Flavoproteins.

J Am Chem Soc 2018 07 10;140(28):8705-8713. Epub 2018 Jul 10.

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

It is a remarkable fact that ∼50 μT magnetic fields can alter the rates and yields of certain free-radical reactions and that such effects might be the basis of the light-dependent ability of migratory birds to sense the direction of the Earth's magnetic field. The most likely sensory molecule at the heart of this chemical compass is cryptochrome, a flavin-containing protein that undergoes intramolecular, blue-light-induced electron transfer to produce magnetically sensitive radical pairs. To learn more about the factors that control the magnetic sensitivity of cryptochromes, we have used a set of de novo designed protein maquettes that self-assemble as four-α-helical proteins incorporating a single tryptophan residue as an electron donor placed approximately 0.6, 1.1, or 1.7 nm away from a covalently attached riboflavin as chromophore and electron acceptor. Using a specifically developed form of cavity ring-down spectroscopy, we have characterized the photochemistry of these designed flavoprotein maquettes to determine the identities and kinetics of the transient radicals responsible for the magnetic field effects. Given the gross structural and dynamic differences from the natural proteins, it is remarkable that the maquettes show magnetic field effects that are so similar to those observed for cryptochromes.
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http://dx.doi.org/10.1021/jacs.8b03104DOI Listing
July 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

Engineering an Artificial Flavoprotein Magnetosensor.

J Am Chem Soc 2016 12 16;138(51):16584-16587. Epub 2016 Dec 16.

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

Migratory birds use the Earth's magnetic field as a source of navigational information. This light-dependent magnetic compass is thought to be mediated by cryptochrome proteins in the retina. Upon light activation, electron transfer between the flavin adenine dinucleotide cofactor and tryptophan residues leads to the formation of a spin-correlated radical pair, whose subsequent fate is sensitive to external magnetic fields. To learn more about the functional requirements of this complex chemical compass, we have created a family of simplified, adaptable proteins-maquettes-that contain a single tryptophan residue at different distances from a covalently bound flavin. Despite the complete absence of structural resemblance to the native cryptochrome fold or sequence, the maquettes exhibit a strong magnetic field effect that rivals those observed in the natural proteins in vitro. These novel maquette designs offer unprecedented flexibility to explore the basic requirements for magnetic sensing in a protein environment.
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http://dx.doi.org/10.1021/jacs.6b09682DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5711731PMC
December 2016

Sub-millitesla magnetic field effects on the recombination reaction of flavin and ascorbic acid radicals.

J Chem Phys 2016 Aug;145(8):085101

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

Even though the interaction of a <1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (kBT), it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub-millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo-excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green's function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin-selective recombination reactions, and the effects of the inter-radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field-dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ∼500 μT magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin-containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes.
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http://dx.doi.org/10.1063/1.4961266DOI Listing
August 2016

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

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

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

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

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

Determination of radical re-encounter probability distributions from magnetic field effects on reaction yields.

J Am Chem Soc 2007 May 1;129(21):6746-55. Epub 2007 May 1.

Contribution from the Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom.

Measurements are reported of the effects of 0-23 mT applied magnetic fields on the spin-selective recombination of Py*- and DMA*+ radicals formed in the photochemical reaction of pyrene and N,N-dimethylaniline. Singlet <--> triplet interconversion in [Py*- DMA*+] radical pairs is probed by investigating combinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2 values for the four isotopomeric radical pairs agree with predictions based on the Weller equation using known hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with the ratio of effective hyperfine couplings, aDMA/aPy. An efficient method is introduced for calculating the spin evolution of [Py*- DMA*+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highly ill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairs and have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. This interval reflects the time scale of re-encounters that are crucial for generating the magnetic field effect. Computer simulations of generalized radical pairs containing six spin-1/2 nuclei show that Weller's equation holds approximately only when the radical pair recombination rate is comparable to the two effective hyperfine couplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effective hyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentially any radical pair should show a significant LFE.
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http://dx.doi.org/10.1021/ja068209lDOI Listing
May 2007

Visible luminescence of carbon nanotubes and dependence on functionalization.

J Phys Chem B 2005 Aug;109(31):14779-82

Strong luminescence emissions over a broad wavelength region were detected from well-dispersed carbon nanotubes in most functionalized samples, even with excitation wavelengths into the near-IR. Apparently, the better dispersion and functionalization of the nanotubes resulted in more intense luminescence emissions. These emissions may logically be attributed to the trapping of excitation energy by defect sites in the nanotube structure, which are passivated upon the appropriate functionalization of the nanotubes. Better functionalization improves not only the nanotube dispersion (thus diminishing the quenching due to intertube interactions) but also the surface passivation to make the energy trapping sites more emissive, leading to stronger luminescence emissions. Because of such high sensitivity, the visible luminescence emissions may prove valuable in the evaluation of dispersion in functionalized carbon nanotube samples and related nanocomposite materials.
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http://dx.doi.org/10.1021/jp053073jDOI Listing
August 2005

Low-field optically detected EPR spectroscopy of transient photoinduced radical pairs.

J Phys Chem A 2005 Jun;109(23):5035-41

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

The effects of simultaneously applied weak static and weak radio frequency magnetic fields on the recombination of transient (<100 ns) radical pairs have been investigated using a low-field optically detected electron paramagnetic resonance technique. Measurements on the photoinduced electron-transfer reaction of perdeuterated pyrene with 1,3-dicyanobenzene using a approximately 0.3 mT radio frequency field at three separate frequencies (5, 20, and 65 MHz) in the presence of 0-4 mT static fields yield spectra that are strikingly sensitive to the frequency of the time-dependent field, to the strength of the static field, and to the relative orientation of the two fields. The spectra are simulated using a modified form of the gamma-COMPUTE algorithm originally devised for calculating magic angle spinning NMR spectra of polycrystalline samples. The essential features of the spectra are consistent with the radical pair mechanism and were satisfactorily simulated using parameters whose values are either known independently or for which estimates are readily available. The calculations included hyperfine couplings to four deuterons in the pyrene cation radical and three protons in the 1,3-dicyanobenzene anion radical. Spin-selective recombination was modeled using an exponential distribution of radical encounter times. The results are discussed in the context of the proposal that radical pair chemistry forms the basis of the magnetoreceptor that allows birds to sense the Earth's magnetic field as a source of compass information during migration.
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http://dx.doi.org/10.1021/jp050765zDOI Listing
June 2005

A study of spin chemistry in weak magnetic fields.

Philos Trans A Math Phys Eng Sci 2004 Dec;362(1825):2573-89

Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, UK.

This paper reviews the latest developments in the field of spin chemistry with a particular focus on the effects of weak static and/or oscillating magnetic fields (typically smaller than the average hyperfine coupling) on radical recombination reactions. Anisotropic magnetic field effects and their significance in the debate about potential mechanisms controlling magnetoreception in birds are discussed.
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http://dx.doi.org/10.1098/rsta.2004.1459DOI Listing
December 2004

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism.

J Am Chem Soc 2004 Jul;126(26):8102-3

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

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.
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http://dx.doi.org/10.1021/ja048220qDOI Listing
July 2004
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