NMR.-Study of nitrogen inversion and conformation of 1,5-dihydro-isoalloxazines ("reduced flavin"). Studies in the flavin series, XIX

The pyramidal inversion of the N(5)-centre of several reduced flavins was measured by NMR. The inversion barrier was found to be ～10 kcal/mol in acetone solutions and to be independent of the size of the N(5) substituent. An increase of the inversion barrier of ～5 kcal/mol was observed in the case where the N(5) substituent could only be in axial position, and an increase of ～3.5 kcal/mol was observed for an acyl-like N(5) substituent. In aqueous solution the inversion barrier increases by ～3 kcal/mol. The stereochemistry of reduced flavin and its potential relevance in flavin-dependent biological dehydrogenations is discussed.