Archive | 2021
Exchange interaction in short-lived flavine adenine dinucleotide biradical in aqueous solution revisited by CIDNP (chemically induced dynamic nuclear polarization) and nuclear magnetic relaxation dispersion
Abstract
Abstract. Flavin adenine dinucleotide (FAD) is an important\ncofactor in many light-sensitive enzymes. The role of the adenine moiety of\nFAD in light-induced electron transfer was obscured, because it involves an\nadenine radical, which is short-lived with a weak chromophore. However, an\nintramolecular electron transfer from adenine to flavin was revealed several\nyears ago by Robert\xa0Kaptein by using chemically induced dynamic nuclear\npolarization (CIDNP). The question of whether one or two types of biradicals of\nFAD in aqueous solution are formed stays unresolved so far. In the present\nwork, we revisited the CIDNP study of FAD using a robust mechanical sample\nshuttling setup covering a wide magnetic field range with sample\nillumination by a light-emitting diode. Also, a cost efficient fast field\ncycling apparatus with high spectral resolution detection up to 16.4\u2009T for\nnuclear magnetic relaxation dispersion studies was built based on a 700\u2009MHz\nNMR spectrometer. Site-specific proton relaxation dispersion data for FAD\nshow a strong restriction of the relative motion of its isoalloxazine and\nadenine rings with coincident correlation times for adenine, flavin, and\ntheir ribityl phosphate linker. This finding is consistent with the\nassumption that the molecular structure of FAD is rigid and compact. The\nstructure with close proximity of the isoalloxazine and purine moieties is\nfavorable for reversible light-induced intramolecular electron transfer from\nadenine to triplet excited flavin with formation of a transient\nspin-correlated triplet biradical F ⚫ − -A ⚫ + . Spin-selective recombination of the biradical leads to the formation of CIDNP\nwith a common emissive maximum at 4.0\u2009mT detected for adenine and flavin\nprotons. Careful correction of the CIDNP data for relaxation losses during\nsample shuttling shows that only a single maximum of CIDNP is formed in the\nmagnetic field range from 0.1\u2009mT to 9\u2009T; thus, only one type of FAD\nbiradical is detectable. Modeling of the CIDNP field dependence provides\ngood agreement with the experimental data for a normal distance distribution\nbetween the two radical centers around 0.89\u2009nm and an effective electron\nexchange interaction of − 2.0\u2009mT.