pH-Dependence, Isotope Effect and Products of Flavin-sensitized Photodecarboxylation and Photodehydrogenation

Abstract

The pH-dependence of flavin-sensitized photodehydrogenation and oxidative photodecarboxylation shows a change of reaction rate with an apparent pK of about 5.5. This “photo-pK” cannot be assigned to any known ground state or excited flavin or substrate species, nor is it related to the kind of bond being broken (C-H or C-COO-). Therefore, it is attributed to the pK of a covalent flavin-substrate intermediate. Carboxylates R-COO~ react by decarboxylation, the nature of R determining only the rate of reaction. The acids R -COOH themselves are unreactive, unless functionally substituted at the α-carbon. Hence X-CH(R)-COOH behaves as does X-CH₂-R, for X=OH, OR, NH₂, NR₂. R₃N⁺- CH(R) -COO- is unreactive, while H₃N⁺-CH(R)-COO-is slowly dehydrogenated through its neutral tautomer which is favored in the flavin-substrate exciplex.