Photochemically Induced Oxidative and Reductive Regeneration of NAD(P)+/NAD(P)H Cofactors: Applications in Biotransformations

Photosensitized regeneration of NAD(P)H cofactors is accomplished by biocatalyzed and artificially catalyzed transformations in photochemical assemblies. Photogenerated N,N'-dimethyl-4,4′-bipyridinium radical cation, MV^+., acts as electron carrier for the reduction of NADPH in the presence of the enzyme ferredoxin reductase and for the reduction of NADH in the presence of lipoamide dehydrogenase. For the photogeneration of MV^+. and subsequent NADPH formation, three different photosensitizers are applied: Ru(bpz)²₃⁺, Ru(bpy)²₃⁺, and Zn—TMPyP⁴⁺. The highest quantum yield for NADPH formation is observed with Ru(bpz)₃²⁺ and is ? = 1.7 × 10^?1. For NADH regeneration only Zn-TMPyP⁴⁺ can be applied. Ru(bpy)₃²⁺ and Ru(bpz)²⁺₃ interact with NADH in their excited or oxidized forms and therefore cannot be used as light-active compounds in the system. The NADPH regeneration cycle has been coupled to the biocatalyzed synthesis of glutamic acid. Although Ru(bpz)₃²⁺ is 42.5-fold more efficient than Ru(bpy)₃²⁺ in the regeneration of NADPH, the synthesis of glutamic acid is improved only by a factor of 2 in the presence of Ru(bpz)₃²⁺, implying that the coupled process is rate limiting. Oxidative regeneration of the NAD⁺ cofactor is accomplished in a photosystem that includes Ru(bpy)₃²⁺ as photosensitizer. The photoprocess is coupled to dehydrogenation of ethanol, propanol, lactic acid, and alanine with concomitant H₂ evolution. A photosystem that includes Ru(bpy)₃²⁺ as photosensitizer, ascorbate as electron donor, and chloro-tris-(3-diphenylphosphinobenzene sulfonate)Rh(I), RhCl(dpm)₃^3-, is catalytically active in the photoinduced regeneration of NAD(P)H cofactors. Mechanistic investigations show that photogenerated Ru(bpy)₃⁺ mediates the generation of a hydrido-rhodium complex that acts as a charge relay for the production of NAD(P)H.