| Abstract: | Various two-electron reduced flavin derivatives have been investigated by natural abundance 13C-NMR. spectroscopy. Some selectively 13C-enriched compounds were synthesized to ensure the assignment of some of the quaternary C-atoms of the flavin molecule. Addition of two electrons to oxidized flavin leads to upfield shifts of all resonances except for those due to C(5a), C(9) and C(10′α). The largest upfield shift is observed for C(4a). Also some direct and two-bond coupling constants are reported. Theoretical calculations by INDO show that a rather good correlation exists between the calculated π-electron densities and the observed chemical shifts of the two-electron reduced molecule. For the oxidized molecule, the correlation is less satisfactory. Most substitution effects are additive, but some deviations in some compounds are observed indicating structural differences between the compounds in question. The chemical shifts are also discussed in terms of the chemical reactivity of the oxidized and reduced flavin molecule. |
