Singlet-triplet dephasing in radical pairs in avian cryptochromes leads to time-dependent magnetic field effects.

Cryptochrome 4a (Cry4a) has been proposed as the sensor at the heart of the magnetic compass in migratory songbirds. Blue-light excitation of this protein produces magnetically sensitive flavin-tryptophan radical pairs whose properties suggest that Cry4a could indeed be suitable as a magnetoreceptor. Here, we use cavity ring-down spectroscopy to measure magnetic field effects on the kinetics of these radical pairs in modified Cry4a proteins from the migratory European robin and from nonmigratory pigeon and chicken.

Magnetic sensitivity of cryptochrome 4 from a migratory songbird.

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.

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

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.

Light-Induced Triplet-Triplet Electron Resonance Spectroscopy.

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.

Magnetically Sensitive Radical Photochemistry of Non-natural Flavoproteins.

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.

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

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.

Engineering an Artificial Flavoprotein Magnetosensor.

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.

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

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.

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

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.

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

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.

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

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.

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

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).

Spin-selective recombination kinetics of a model chemical magnetoreceptor.

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.

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

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.

Radiofrequency polarization effects in low-field electron paramagnetic resonance.

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.

Radiofrequency polarization effects in zero-field electron paramagnetic resonance.

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.

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

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).

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

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.

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

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.

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

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.

Chemical compass model of avian magnetoreception.

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.

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

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.

Visible luminescence of carbon nanotubes and dependence on functionalization.

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.

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

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.

A study of spin chemistry in weak magnetic fields.

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.