Intramolecular hydrogen bonding in 2o-deoxyribonucleosides : an AIM topological study of the electronic density

The theoretical analysis of the energy of the conformers of 2@-deoxyribonucleosides presents some subtle but signiÐcant di†erences depending on the nature of the base linked to the sugar. In particular, 2@-deoxycytidine behaves uniquely. Among the structural parameters invoked to explain this phenomenon, one of the most intriguing is the CÈHÉ É ÉO intramolecular hydrogen bond in which the donor atom is H6 (H8) in the pyrimidine (purine) base and the acceptor atom is O5@ of the sugar. In the present work, the network of intramolecular weak bonds is thoroughly investigated for every 2@-deoxynucleoside in both the North (C3@endo/anti) and South (C2@-endo/anti) conformations. In this respect, we use BaderÏs atoms in molecules (AIM) theory to perform a topological study of the electronic density, emphasizing the weak bonding in the nucleosides. Criteria for hydrogen bonding are comprehensively reviewed for each hydrogen bond revealed, and the concept of ““ improper ÏÏ hydrogen bonding is addressed. The AIM analysis thus allows us to gain insight into the intrinsic reasons for the strange conformational behaviour of 2@-deoxycytidine.