4-Alkyldihydroflavin: coenzyme synthesis and modification of flavodoxin.

A stable (i.e. non-oxidisable), reduced flavocoenzyme, 4a-(n-propyl)-4a,5-dihydro-riboflavin 5'-monophosphate, has been synthesized. The method of choice was selective photo-allylation in position 4a. Stabilization of this substituent was achieved by hydrogenation after N(5) protection, while hydrogenation of the unprotected compound would have led to removal of the allyl substituent. The mode of this substituent mobility has been checked with deuterated photosubstrate. Upon hydrogenation, the allyl substituent becomes firmly and irreversibly fixed to the flavin surface. The diastereomers thus formed have been separated and characterized by their ultraviolet, circular dichroic and nuclear magnetic resonance spectra. After subsequent side chain phosphorylation it has been shown that only one of the diastereomeric dihydro-flavocoenzymes is firmly bound by Peptostreptococcus elsdenii apoflavodoxin. This leads to the hypothesis that (a) the non-planar 4a,5-dihydro-flavin isomer and, therefore, C(4a) as active flavin center, may have biological relevance, while (b) the flavin site in flavodoxin is accessible to some extent for a stacking interaction with a redox-partner group.