Nuclear magnetic resonance studies of cis-syn, trans-syn, and 6-4 photodimers of thymidylyl(3'-5')thymidine monophosphate and cis-syn photodimers of thymidylyl(3'-5')thymidine cyanoethyl phosphotriester

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Abstract
Three out of four possible photodimers of thymidylyl(3''-5'')thymidine monophosphates (i.e., cis-syn, 6-4, and one of the trans-syn) and two structural isomers (i.e., R and S forms) of cis-syn-thymidylyl(3''-5'')thymidine cyanoethyl phosphotriester have been isolated and purified from the reaction mixtures after UV irradiation and studied by multinuclear magnetic resonance spectroscopy. All five inter thymine base linked photodimers have grossly similar structures which are quite different from those of the parent thymidylyl(3''-5'')thymidine. The base of Tp- is in the syn conformation, and that of -pT is in the anti conformation. The sugar puckering of Tp- is dominated by the 2E conformer, but in -pT it is in 4E; except for the conformer around the C5 .sbd.O5'' bond, the 6-4 isomer is very similar to those of cis-syn and trans-syn conformation. As expected, there are sugar-phosphate backbone distortions in the phosphotriesters, due to the neutralization of the negative charge of the phosphate. In general the structures of all five photodimers are very close to those of the cis-syn photodimer of thymidylyl(3''-5'')thymidine monophosphate cyanoethyl ester as studied by X-ray diffraction [Cadet, J., Voituriez, L., Hruska, F.E., and Grand, A. (1985)Biopolymers 24, 897-903; Hruska, F.E., Voituriez, L., Grand, A., and Cadet, J. (1986) Biopolymers 25, 1401-1417]. While the trans-syn photodimer has two structural isomers, only one [C6(of Tp-)-R] was produced by the UV irradiation and studied.

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