[Comparative study of slow 1H to 3H exchange in synthetic polynucleotides of A- and B-type conformations].

The rate of 1H leads to 3H exchange between water and C(8)H-groups of purinic residues in synthetic polynucleotides in wide temperature range measured. At temperatures below their Tm the rate of the exchange is shown to be lower as compared with that in corresponding mononucleotides. In the case of polynucleotides of A-conformation (poly(A).poly(U), poly(A).2poly(U) and poly(dA).2poly(dT), and poly(G).poly(C) the exchange is retarded by a factor of 5.7--7.5, whereas in the case of those of B-conformation (poly(dA).poly(dT), poly(dA--dT).poly(dA--dT) and poly(dG).poly(dC)) the exchange is retarded only by a factor of 2.3--2.5. Assuming the ylide mechanism of exchange the retardation is interpreted as a consequence of sterical hidrance in polynucleotides helical structure, which hampers contacts between purinic C(8)H-groups and OH-ions of solvent. Analysis of atomic arrangement around C(8)H-group and interatomic distances calculated on the basis of published atomic coordinates support our general conclusion that the sterical hindrance is more significant in the A-form as compared with that in the B-form. Elucidated correlation between the degree of the retardation in purine-containing polynucleotides and their conformation in solution allows to estimatf the type of conformation of polynucleotide with unknown structure on account of the slow 1H leads to 3H exchange data.