Synthesis and photochemistry of 8-substituted 5-deazaflavins

5-Deazaflavin has been applied as a photocatalyst in the reduction of a wide variety of biological redox systems. The use of 5-deazaflavin as a photocatalyst has serious drawbacks in that continuous UV irradiation is required to drive the process and that the needed wavelength (300-400 nm) corresponds to an energy level high enough to destroy the protein enzyme in time.To avoid photodestruction of the protein a number of 5-deazaflavins, which have a chromophoric group at position 8 of the 5-deazaisoalloxazine skeleton, causing the absorption maximum to undergo a red shift, were synthesized (chapter 2). This was accomplished i) by the oxidative cyclization of 5,5'-arylmethylenebis(6-methylaminouracil) derivatives and ii) by the cyclization of N -methylanilinouracil derivatives with a one-carbon reagent. The latter method led to the formation of impure products. Condensation and oxidation reactions with the π-electron deficient C(8) methyl group in 5-deazalumiflavin did not occur. By the former method a number of 8-substituted 5-deazaflavins with a carboxymethyl group at the N(3) position were synthesized (chapter 3). This polar group was introduced prior to the oxidative cyclization and increased the solubility of 5-deazaflavins in aqueous media, in which photoreduction of redox enzymes are to be carried out. Introduction of substituents at position 8 caused a bathochromic shift that varied between 1 and 147 nm (chapter 6).Since the 5-deazaflavin radical is the actual reducing species within the photosystem, the effect of the substituents, introduced at position 8, on the reducing power of the radical was determined by measuring the half-wave potentials of the 5-deazaflavins (chapter 4). The dissociation constants of 5-deazaflavosemiquinones were successfully determined on the basis of the polarographic properties of 5-deazaflavins. Both half- wave potentials and dissociation constants appeared to be very sensitive to the substituent at position 8 and to meet the Hammett relation.The effect of the substituent on the spectroscopic (chapter 6) and photochemical (chapters 5, 6 and 7) properties was investigated. In contrast with the findings of others that irradiation of 5-deazaflavins in the presence of oxalate or ethylenediaminetetraacetate (EDTA) as electron donor exclusively leads to the formation of dimeric compounds as end-products of the photoreduction, illumination of 8-R-5-deazaflavins (R=Cl, N(CH 3 ) 2 , NH 2 , p -NH 2 -C 6 H 4 ) in the presence of various electron donors (EDTA, oxalate and triethanolamine) yielded the dimeric species 5,5'-bis(1,5-dihydro-8-R-5-deazaflavin) and/or 6,7-dihydro-8-R-5-deazaflavin. It was found that reduction of the C(6)-C(7) bond is highly promoted by strong electron donating substituents at position 8 and bulky electron donors. 5-Deazaflavins with a reducible substituent at position 8 (R=NO 2 , p -NO 2 -C 6 H 4 , p -N(CH 3 ) 2 -C 6 H 4 -N=N) exhibited the reduction of the substituent prior to the reduction of the 5-deazaisoalloxazine skeleton.The course (chapter 6) and rate (chapter 7) of the photoreduction of 8-R-5-deazaflavins were established and explained in terms of electronic and steric effects, exerted by the substituent at position 8 and the electron donor. In some cases the rate of photoreduction appeared to contain an autocatalytic element.The catalytic effect of 8-R-5-deazaflavins in the photoreduction of methyl viologen by EDTA was investigated (chapter 7). The substituent effect on the rate of the 8-R-5-deazaflavin mediated photoreduction of methyl viologen by EDTA was found to be comparable with that on the photoreduction rate of 8-R-5-deazaflavin in the presence of EDTA with the exception of 8-R-5-deazaflavin (R=N(CH 3 ) 2 ), which showed a remarkable relative enhancement of the reactivity towards methyl viologen photoreduction.