INVESTIGATION OF FLAVIN-CONTAINING DNA-REPAIR MODEL COMPOUNDS

Irradiation of DNA with UV-B light causes the formation of mutagenic D NA lesions such as cis-syn and trans-syn cyclobutane pyrimidine dimers . DNA photolyases are flavin-dependent repair enzymes which directly r evert the mutagenic cis-syn pyrimidine dimers into the corresponding m onomers by a light-facilitated repair reaction. To gain deeper insight into the repair process, we recently prepared flavin-containing model compounds which are able to mimic the repair reaction (Carell, T.; Ep ple, R.; Gramlich, V. Angew. Chem., Int. Ed. Engl. 1996, 35, 620-623). This publication now contains a detailed description of the synthesis of a series of related model compounds and a comprehensive investigat ion of their cleavage properties. The results obtained help to unravel the requirements necessary for an efficient, flavin-mediated cleavage of pyrimidine dimers and provide insight into the factors on which th e enzymatic repair process depends. The investigation of the cleavage reaction with cis-syn, trans-syn, and hans-anti cyclobutane pyrimidine dimer model compounds reveal an enhanced vulnerability for the cis-sy n isomer. The trans-syn dimer is 10 times more stable, These results a re comparable to those observed in a recent study on the E. coli enzym e. The excellent solubility of some of the model compounds has allowed a medium-dependent investigation of the flavin-initiated cleavage rea ction. increased cleavage efficiencies are observed in polar solvents such as water (phi = 0.06) and acetonitrile (phi = 0.05). The quantum yields decrease by a factor of 4 in solvents with very low polarity su ch as dioxane (phi = 0.01). These results are not in agreement with ea rlier solvent-dependent evaluations performed with non-flavin-containi ng model compounds (Hartzfeld, D. G.; Rose, S. D. J. Am. Chem. Soc. 19 93, 115,850-854). The results, however, suggest that the unusually pol ar flavin-binding pocket, observed in the X-ray crystal structure of t he E, coli. photolyase, might be required to increase the catalytic re pair efficiency. Investigations of the cleavage reaction in the presen ce of acid and base in organic solvents emphasize the strict requireme nt for a deprotonated reduced riboflavin chromophore. The determined p H values for half-maximal (pH = 6.5) and maximal (7 less than or equal to pH less than or equal to 9) cleavage efficiencies are in agreement with the pK(a) value (pK(a) = 6.3) of the reduced riboflavin and reve al that physiological conditions are required to reach maximum catalyt ic cleavage efficiency.