AB-INITIO MOLECULAR-ORBITAL STUDY OF THE STRUCTURES OF PURINE HYDRATES

The structures of the isomers of purine hydrates [4(5)-hydroxy-5(4)-hy dropurines] have been geometry optimized with ab initio quantum chemic al methods at the 6-31G basis set and with the semiempirical method P M3. These hydrates which can result from reduction of radical species formed by attack of hydroxyl radical at the 4,5 double bond in the pur ines, show significant geometrical distortion when compared to the nat ural bases. More specifically, the cis isomers adopt a ''butterfly'' c onformation, while in the trans isomers, the pyrimidine and imidazole rings tilt opposite to each other. Our results predict the cis purine hydrate isomers are far more stable than the trans isomers by 10-18 kc al/mol at the 6-31G level, whereas the 4-hydroxy-5-hydropurines are f ound to be slightly more energetically stable than the 5-hydroxy-4-hyd ropurines. The ''butterfly'' conformation of the cis isomers constitut es a bulky lesion which will result in a significant distortion of the DNA helix.