Electronic structure studies of flavin radicals
Organic Molecular Chemistry - Synthesis and Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
The following achievements have been made: (i) A global fitting approach was developed to extract consistent and precise sets of hyperfine principal values for all 13C nuclei of the 7,8-dimethyl isoalloxazine moiety of flavin from orientation-selective 13C-ENDOR spectra recorded at W-band frequencies; (ii) photo-CIDNP NMR experiments were conducted to obtain isotropic 13C hyperfine couplings that turned out to be useful to double-check signal assignments based on DFT; (iii) QM/MM computations were performed to obtain improved structure data from the cofactor binding site of two flavoproteins, Escherichia coli DNA photolyase and the LOV2 domain of Avena sativa phototropin, based on which DFT calculations were conducted with the aim to obtain a better agreement between experiment and theory. DFT at the B3LYP/EPR-II level of theory turned out to be of limited value for predictions of the subtle modulations a specific protein environment imposes onto the electronic structure of a flavin cofactor in its neutral radical form. For an experimentalist, this is good news, as the efforts invested in characterizations of the paramagnetic redox state are rewarding as long as theory does not make any significant progress. For theoreticians, our data provides the means to refine the methodology to compute hyperfine data from more complex organic cofactors, such as the flavin. In addition to isotropic hyperfine values, we provide information on hyperfine anisotropies as well. A systematic examination of hyperfine tensor asymmetries and orientations with respect to the flavin’s g-tensor is ongoing and will most certainly keep us busy for quite some time. We are confident, that in addition to the publications mentioned above, several further papers will emerge from the present studies. We achieved, for the first time, a nearly complete hyperfine mapping of the redox-active 7,8-dimethyl isoalloxazine moiety of the neutral flavin radical in bulk aqueous solution and in representative protein surroundings. Our novel strategy establishes a basis for a more detailed understanding of the electronic characteristics of flavins and flavoproteins in general. A thorough hyperfine mapping of various flavoproteins is now feasible and will allow for quantitative analyses of reactivity patterns of paramagnetic flavin cofactors. With quantitative information directly related to spin densities at hand, we have now the potential to tackle the fundamental question on the specific role of the protein surrounding on a cofactor and to assess the contributions of electronic versus sterical control of a catalyzed reaction.
Publications
- (2020) Coupled Methyl Group Rotation in FMN Radicals Revealed by Selective Deuterium Labeling. The journal of physical chemistry. B 124 (9) 1678–1690
Brosi, Richard; Illarionov, Boris; Heidinger, Lorenz; Kim, Ryu-Ryun; Fischer, Markus; Weber, Stefan; Bacher, Adelbert; Bittl, Robert; Schleicher, Erik
(See online at https://doi.org/10.1021/acs.jpcb.9b11331) - “Long-lived hydrated FMN radicals: EPR characterization and implications for catalytic variability in flavoproteins” Journal of the American Chemical Society 140 (2018) 16521–16527
A. Rostas, C. Einholz, B. Illarionov, L. Heidinger, T. Al Said, A. Bauss, M. Fischer, A. Bacher, S. Weber, E. Schleicher
(See online at https://doi.org/10.1021/jacs.8b07544) - “Methyl groups matter: photo-CIDNP characterizations of the semiquinone radicals of FMN and demethylated FMN analogs” Journal of Chemical Physics 151 (2019) Art.-No. 235103
N. Pompe, J. Chen, B. Illarionov, S. Panter, M. Fischer, A. Bacher, S. Weber
(See online at https://doi.org/10.1063/1.5130557)
