We present time-resolved optical absorption and magnetic field effect data on the photochemistry following blue light excitation of flavin adenine dinucleotide (FAD) in aqueous solution in the pH range 2.3 to 8.0. Effects of closed form conformations of FAD in ground, excited singlet, and radical pair states exhibit significant influence on the observed kinetics and magnetic field dependence and remarkably, magnetic field effects are observed even at physiological pH where the FAD radical pairs are only 75% less magnetic field sensitive than at pH 2.3.

Short-lived radicals generated in the photoexcitation of flavin adenine dinucleotide (FAD) in aqueous solution at low pH are detected with high sensitivity and spatial resolution using a newly developed transient optical absorption detection (TOAD) imaging microscope. Radicals can be studied under both flash photolysis and continuous irradiation conditions, providing a means of directly probing potential biological magnetoreception within sub-cellular structures. Direct spatial imaging of magnetic field effects (MFEs) by magnetic intensity modulation (MIM) imaging is demonstrated along with transfer and inversion of the magnetic field sensitivity of the flavin semiquinone radical concentration to that of the ground state of the flavin under strongly pumped reaction cycling conditions. A low field effect (LFE) on the flavin semiquinone-adenine radical pair is resolved for the first time, with important implications for biological magnetoreception through the radical pair mechanism.

2014) Ionic liquid crystals derived from guanidinium salts: induction of columnar mesophases by bending of the cationic core, Liquid Crystals, 41:7, 976-985, Meta-alkyloxyguanidinium salt-based ionic liquid crystals 3-5, 7 and 9 were synthesised and investigated with respect to the influence of meta-substitution of the cation on the mesomorphic properties. As expected, bending of the mesogenic cation in ion pairs with simple counterions (3-5) decreased melting points irrespective of the anion, but clearing points were influenced by the anion radii. SmA mesophases were formed in all cases. The mesophase formation in guanidinium sulphonates 7 and 9, however, depended not only on meta-substitution but also on the anion, the respective difference between alkyl chain lengths in cation and anion and the number of alkyloxy substituents on the sulphonate, for which a change of mesophase type from smectic to columnar phases was observed. For two derivatives, 7e and 9b, room temperature SmA and Col h mesophases could be obtained that were stable for a temperature range of 91 K and 55 K, respectively. A packing model for both smectic and columnar phases based on powder XRD data was proposed.

The photochemical reactions of blue-light receptor proteins have received much attention due to their very important biological functions. In addition, there is also growing evidence that the one particular class of such proteins, the cryptochromes, may be associated with not only a biological photo-response but also a magneto-response, which may be responsible for the mechanism by which many animals can respond to the weak geomagnetic field. Therefore, there is an important scientific question over whether it is possible to directly observe such photochemical processes, and indeed the effects of weak magnetic fields thereon, taking place both in purified protein samples in vitro and in actual biochemical cells and tissues. For the former samples, the key lies in being able to make sensitive spectroscopic measurements on very small volumes of samples at potentially low protein concentrations, while the latter requires, in addition, spatially resolved measurements on length scales smaller than typical cellular components, i.e., sub-micron resolution. In this work, we discuss a two- and three-color confocal pump-probe microscopic approach to this question which satisfies these requirements and is thus useful for experimental measurements in both cases.

Figure 4. a) Simplified reaction scheme for FAD photoexcitation at pH 2.3. b) Reduction of the MFE on RP/PTS concentration with increasing pump light pulse length. c) Singlet excited state FAD fluorescence detected MARY curve under high power CW pump light modulation,revealing an inverse MFE on the FAD GS concentration.

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Homo-dimer formation is important for the function of many proteins. Although dimeric forms of cryptochromes (Cry) have been found by crystallography and were recently observed in vitro for European robin Cry4a, little is known about the dimerization of avian Crys and the role it could play in the mechanism of magnetic sensing in migratory birds. Here, we present a combined experimental and computational investigation of the dimerization of robin Cry4a resulting from covalent and non-covalent interactions. Experimental studies using native mass spectrometry, mass spectrometric analysis of disulfide bonds, chemical cross-linking, and photometric measurements show that disulfide-linked dimers are routinely formed, that their formation is promoted by exposure to blue light, and that the most likely cysteines are C317 and C412. Computational modeling and molecular dynamics simulations were used to generate and assess a number of possible dimer structures. The relevance of these findings to the proposed role of Cry4a in avian magnetoreception is discussed.

Homo-dimer formation is important for the function of many proteins. Although dimeric forms of cryptochromes (Cry) have been found by crystallography and were recently observed in vitro for European robin Cry4a, little is known about the dimerisation of avian cryptochromes and the role it could play in the mechanism of magnetic sensing in migratory birds. Here we present a combined experimental and computational investigation of the dimerisation of robin Cry4a resulting from covalent and non-covalent interactions. Experimental studies using native mass spectrometry, mass spectrometric analysis of disulphide bonds, chemical cross-linking and photometric measurements show that disulphide-linked dimers are routinely formed, the most likely cysteines being C317 and C412. Computational modelling and molecular dynamics simulations were used to generate and assess a number of possible dimer structures. The relevance of these findings to the proposed role of Cry4a in avian magnetoreception is discussed.