Preprints
https://doi.org/10.5194/mr-2021-17
https://doi.org/10.5194/mr-2021-17

  16 Feb 2021

16 Feb 2021

Review status: this preprint is currently under review for the journal MR.

Exchange interaction in short lived flavin adenine dinucleotide biradical in aqueous solution revisited by CIDNP and nuclear magnetic relaxation dispersion

Ivan V. Zhukov1,2, Alexey S. Kiryutin1,2, Mikhail S. Panov1,2, Natalya N. Fishman1,2, Olga B. Morozova1,2, Nikita N. Lukzen1,2, Konstantin L. Ivanov1,2, Hans-Martin Vieth1,3, Renad Z. Sagdeev1,2, and Alexandra V. Yurkovskaya1,2 Ivan V. Zhukov et al.
  • 1International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090, Russia
  • 2Novosibirsk State University, Novosibirsk, 630090, Russia
  • 3Freie Universität Berlin, 14195 Berlin, Germany

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 T for nuclear magnetic relaxation dispersion studies was built based on a 700 MHz 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 mT 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 mT to 9 T; 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 nm and an effective electron exchange interaction of −2.0 mT.

Ivan V. Zhukov et al.

Status: open (until 21 Mar 2021)

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Ivan V. Zhukov et al.

Ivan V. Zhukov et al.

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Short summary
We report data on nuclear polarization formed in photoreaction of intramolecular electron transfer from adenine to flavin that was revealed several years ago by R. Kaptein by CIDNP. The question whether one or two types of biradicals are formed stays unresolved so far. This work clarified the problem and confirmed that single common emissive maximum in magnetic field dependence of CIDNP may originate from single level crossing of short-lived biradical and that FAD molecule has a compact shape.