4a‐Alkyldihydroflavin Coenzyme Synthesis and Modification of Flavodoxin

Abstract

A stable (i.e., non-oxidizable), reduced flavocoenzyme, 4a(n-propyl)-4a,5-dihydro-riboflavin 5''-monophosphate, was 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 removed the allyl substituent. The mode of this substituent mobility was checked with deuterated photosubstrate. Upon hydrogenation, the allyl substituent becomes firmly and irreversibly fixed to the flavin surface. The diastereomers thus formed were separated and characterized by their UV, circular dichroic and NMR spectra. After subsequent side chain phosphorylation, only 1 of the diastereomeric dihydro-flavocoenzymes is firmly bound by Peptostreptococcus elsdenii apoflavodoxin. The non-planar 4a,5-dihydro-flavin isomer and, thus, C(4a) as active flavin center, may have biological relevance while the flavin site in flavodoxin is probably accessible to some extent for a stacking interaction with a redox-partner group.