a tilt is introduced, models C and D become nonequivalent with respect to powder averaging, and an additional tilt angle is needed to define the plane of the benzene ring. In view of the experimental spectral distortions (cf. Experimental Section), we did not attempt further simulations to pursue this point. Thus, qualitative analysis seems to favor either model C or D, in agreement with ref 49.We note in passing that the g" state for the y-position has slightly lower conformational energy than the t state (cf. Table I), thereby favoring the parallel orientation of the para axis with respect to the helix axis.
Abstract— Several high energy ketone triplet sensitizers, e.g. carvone, camphor, 3‐methylcyclohexanone, benzoin and 3‐methylindanone, were studied as photosensitizers for the splitting of dimethylthymine dimers. The absence of splitting in all cases and the lack of quenching of benzoin and 3‐methylindanone triplets by the trans‐anti dimer of dimethylthymine strongly suggests that cleavage of dimethylthymine dimers cannot be achieved by a triplet mechanism on irradiation at Λ> 300 nm. The absence of optical rotation in the recovered chiral cis‐anti dimethylthymine dimer after sensitized photolysis (12% splitting) in the presence of (—)‐tryptophan suggests that. in highly polar solvents, such as methanol, where reaction probably takes place according to an electron transfer mechanism involving ion‐pair intermediates, close approach of the sensitizer and substrate does not occur. To the extent that these results can be extrapolated to sensitized cleavage of cis‐syn pyrimidine dimers in DNA brought about by action of photoreactivating enzyme or conventional photosensitizers, a mechanism involving dimer triplet states appears highly unlikely.
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