| Abstract: | With the use of experimentally measured spin-lattice relaxation rates of H(5), H(6), H(2), H(8) and H(1') the analysis of the conformational situation in aqueous solution of CpC, CpA, ApC and ApA has been carried out. In all cases, the close state is shown to be described by the equilibrium of right- and left-handed forms. The right-handed forms are favourable for all DNP studied and are related to the "canonical" Pba type. On the contrary, the possibility of realization of one or the other noncanonical forms (these are mainly left-handed conformers) is determined strictly by nucleotide sequences. The noncanonical conformers for CpC and CpA are Mbb forms, in which both monomeric units are characterized by anti-conformation, and the base of the 3'-end is turned 180 degrees around the glycosidic bond as compared to the canonical form. The forms of Mab type are possible for ApC. In this case, the purine monomer exists in syn-, but the pyrimidine monomer--in anti-conformation. Both bases are turned to 180 degrees. Finally, in the case of ApA, the forms of Mba type with high-anti-conformation of both monomers are realized. The conformational behaviour of oligonucleotides and the possible biological role of noncanonical conformations are discussed taking into consideration the results obtained. |
