Exchange interaction in short lived flavin adenine dinucleotide\nbiradical in aqueous solution revisited by CIDNP and nuclear\nmagnetic relaxation dispersion

Abstract

Abstract. Flavin adenine dinucleotide (FAD) is an important cofactor in many light-sensitive enzymes. The role of the adenine moiety of FAD in light induced electron transfer was obscured because it involves an adenine radical, short-lived with a weak chromophore. However, an intramolecular electron transfer from adenine to flavin was revealed several years ago by R. Kaptein by using chemically induced dynamic nuclear polarization (CIDNP). The question whether one or two types of biradicals of FAD in aqueous solution are formed stays unresolved so far. In the present work, we revisited the CIDNP study of FAD using a robust mechanical sample shuttling setup covering a wide magnetic field range with sample illumination by a light emitting diode. Also, a cost efficient fast field cycling apparatus with high spectral resolution detection up to 16.4\u2009T for nuclear magnetic relaxation dispersion studies was built based on a 700\u2009MHz NMR spectrometer. Site-specific proton relaxation dispersion data for FAD show a strong restriction of the relative motion of its isoalloxazine and adenine rings with coincident correlation times for adenine, flavin and their ribityl-phosphate linker. This finding is consistent with the assumption that the molecular structure of FAD is rigid and compact. The structure with close proximity of the isoalloxazine and purine moieties is favorable for reversible light induced intramolecular electron transfer from adenine to triplet excited flavin with formation of a transient spin-correlated triplet biradical F•−-A•+. Spin selective recombination of the biradical leads to the formation of CIDNP with a common emissive maximum at 4.0\u2009mT detected for adenine and flavin protons. Careful correction of the CIDNP data for relaxation losses during sample shuttling shows that only a single maximum of CIDNP is formed in the magnetic field range from 0.1\u2009mT to 9\u2009T; thus, only one type of FAD biradical is detectable. Modeling of the CIDNP field dependence provides good agreement with the experimental data for a normal distance distribution between the two radical centers around 0.89\u2009nm and an effective electron exchange interaction of −2.0\u2009mT.