Dynamics of Intramolecular Electron Transfer Reaction of FAD Studied by Magnetic Field Effects on Transient Absorption Spectra

Abstract

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.

Fluorescence-detected magnetic field effects on radical pair reactions from femtolitre volumes

“…In the absence of Trp, the signals at the two wavelengths (red traces in Figs 2a & b) are very similar and differ only in amplitude, reflecting the wavelengthdependence of the extinction coefficients. This indicates that the decay of 3 FAD in the absence of Trp also dominates the 580 nm signal in agreement with Murakami et al 21 In the presence of Trp, the absorbance at 650 nm was rapidly quenched (Fig. 2b) and the amplitude of the slow phase observed at 580 nm increased (Fig.…”

“…20,21 The fluorescence of FAD-Trp solutions at low pH, generated by a 473 nm continuous-wave laser, was monitored by TIRF microscopy ( Fig. 1a) in the presence of a square-wave modulated static magnetic field.…”

“…The rapid reversibility of the electron transfer step is indicated by the MFE on 3 FAD as observed previously. 21 Reverse electron transfer in the singlet state of the flavin-adenine radical pair provides a return route to ground state FAD. Additionally, 3 (Fig.…”

Magnetic field effects in flavoproteins and related systems

“…In a similar reaction, photoexcited FAD forms an intramolecular triplet SCRP in which the adenine moiety of triplet FAD donates an electron to the isoalloxazine ring [48,49]. Through photochemically induced dynamic nuclear polarization (photo-CIDNP) NMR, Kaptein and colleagues [50,51] demonstrated that the radical pair is produced by photoexcitation in the pH range 1.5-4.0.…”

“…p*) (figure 4; [52,53]). For convenience, the third harmonic of the commonly available Nd:YAG laser (l ¼ 355 nm) is typically used to generate the excited singlet flavin which may subsequently form a radical pair with a suitable electron donor [44][45][46]48,49]. When working with cryptochrome and photolyase protein samples, it is important to minimize UV-induced photodegradation, hence care is taken to match the first absorption band more closely using a Nd:YAG pumped dye laser (l % 460 nm) for photoexcitation [54,55].…”