Redox Splitting Mechanism of C(5)-C(5)-linked Dihydrothymine Dimers

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Redox mechanism by which meso- and racemic forms of C(5)-C(5)-linked dihydrothymine dimers (5a-c), as derived from various thymine derivatives [1-methylthymine (1a) ; 1,3-dimethylthymine (1b) ; thymidine (1c)], undergo splitting to regenerate 1a-c was studied and the following results were obtained.(1) Product studies on radiolytic and photosensitized redox splittings of 5a-c in aqueous solution were carried out to correlate with their structural characteristics.(2) A structure-dependent reactivity was elucidated that 5a undergoes splitting by one-electron reduction, while 5b, c favor two-electron reductive splitting.(3) Meso-form of 5a showed fluorescence spectrum with a maximum intensity at 370 nm, although the other dimers were of non-fluorescent.(4) Conformation of 5a in aqueous solution was determined by NMR, indicating that the meso-form favors a conformation similar to that in the crystal phase.(5) One-electron reduction potentials of 5a-c were evaluated from the corresponding rate constants of electron-transfer quenching for the fluorescence of ruthenium complex (Ru(bpy)_3Cl_2 6H_20).(6) One-electron oxidative splitting of 5a by SO_4- as generated by laser photolysis was studied to observe transient spectra due to 1-methyl-5,6-dihydrothymin-5-yl radicals that were derived from both meso- and racemic- forms. The rate constants of second-order decay were evaluated as 2.2 - 2.7 x 10^9 M^<-1> s^<-1>.(7) No transient spectra due to 5-yl radicals were observed in the laser photolysis of 5a-c in aqueous solution containing N,N-dimethylaniline as a reducing photosensitizer.