Conformational and electronic properties of the two cis (5S,6R) and (5R,6S) diastereoisomers of 5,6-dihydroxy-5,6-dihydrothymidine: X-ray and theoretical studies.

The structure of (+)-cis-(5S,6R)-5,6-dihydroxy-5,6-dihydrothymidine was obtained using X-ray crystallography [space group P2(1) with a = 10.130(3) angstroms, b = 6.434(9) angstroms, c = 11.02(5) angstroms, and beta = 112.646(2) angstroms]. The comparison of the two cis diastereoisomers of thymidine glycol (I, II) showed several structural and conformational differences. The solid state structures appear to be in agreement with the results of 1H NMR studies which were carried out in aqueous solution. Conformational and electronic properties of the ground state of the molecules I and II were obtained using ab initio LSD-DFT theory. Only slight differences between the crystal structure and the optimized geometry are observed for each of the two oxidized nucleosides. On the other hand, molecules I and II exhibit significant differences in their electronic properties. In particular, the dipole moment of (5S,6R)-thymidine glycol (I) is twice smaller than that of the (5R,6S) diastereoisomer (II). It is noteworthy that these differences in the electronic properties between the two compounds may be related to changes in the rotameric population around the C4'-C5' bond. The repartition of the electrostatic potential is different in the two compounds. These observations lead to a better understanding of the structural changes when the above lesions are included within a DNA molecule.