Structure of Flavin Adducts with Acetylenic Substrates

Publisher Website
Google Scholar
Abstract
The photoreaction of flavoquinones (lumiflavin, riboflavin, FMN etc. and their 3‐alkylated derivatives) with propargylamine‐type acetylenic substrates, R4–Cα≡ Cβ–CγHR3–NR2R1, yields a mixture of two adducts, which result from covalent Cα fixation of the Cγ‐deprotonated substrate to either position C(4a) or N(5) in the flavin nucleus.The N(5) adduct is a dihydroflavin‐5‐trimethine‐cyanine with very intense (ɛ > 20000 M−1 cm−1) absorption maxima in the region 380–450 nm depending on the R1, R2· This absorption allows recognition of minute amounts of this species of flavocyanine even in complex mixtures. Flavocyanines can be reconverted to starting flavin by base. Its spectral properties are identical with those obtained for the pargyline‐flavin inhibitor complex from bovine kidney or pig liver monoamine oxidase.The C(4a) adduct is a 4a: 5‐propenylidene‐dihydroflavin, the cyclisation probably arising through secondary attack of the Cγ carbon at N(5). It cannot be reconverted to starting flavin in relevant yields. Its chromophor (σmax∼ 360 nm) is identical with that of lactate‐oxidase inhibitor complex. If N(3) is alkylated, these cyclic adducts undergo cleavage of the 4–4a bond upon treatment with base, followed by aromatization of the substrate substituent to yield novel pyrroloquinoxal‐4‐ones. Upon treatment with LiAlH4, however, the adducts are hydrogenated at the N(1)=C(10a) double bond, which by concomitant base hydrolysis yields 1,2‐dihydroquinoxaline derivatives. Their autoxidation yields a pyrrolo‐quinoitaiinium cation which exhibits spectral identity with the product isolated from the α‐hydroxybutynoate‐flavin inhibitor complex of Mycobacterium smegmatis lactate oxidase, after borohydride treatment.The scope and limitations of covalent adduct formation in flavin‐dependent biocatalysis will be discussed.