Theoretical considerations of the electronic spectra of methyl flavins

Methyl groups in flavins are best treated by the group‐orbital approximation. The pseudo‐heteroatom approximation overestimates methyl hyperconjugation with the Pariser–Parr–Pople SCF–MO method. Singlet π → π* transition energies are calculated by various MO methods with differing degrees of sophistication, and the results from the PPP method agree reasonably with the experimental values. 2‐ and 4‐thioflavin analogs are also treated satisfactorily. The effects of position and number of the methyl groups on the spectra of flavins are described in detail. Rough estimates of the n → π* energies of flavins suggest that the lowest singlet excited state is (π, π*), consistent with the fluorescence and phosphorescence polarization data.