[Study of conformation characteristics of "X-form" of alternating polynucleotides by the method of slow 1H----3H transition].

The rate constants of 1H----3H exchange between water and C8H-groups of purine residues of alternating polynucleotides: poly[d(A-C)].poly[d(G-T)] and poly[d(A-T)].poly[d(A-T)], as well as Escherichia coli DNA, dAMP and dGMP, in solutions with high concentration (4.3 or 6 M) CsF, in water ethanol (60%) solution and (in comparison) in 0.15 M NaCl were determined at 25 degrees C. The 1H----3H exchange rate exchange rate constants for adenylic (kA) and guanylic (kG) residues of polynucleotides were compared with the corresponding constant for DNA and mononucleotides. It was shown that at conditions when poly[d(G-T)] and poly[d(A-T)].poly[d(A-T)] exhibit the "X-form" CD spectrum, alteration of exchange rates in polynucleotides (approximately 2-fold increase in kA in CSF and approximately 1.5-fold decrease in kA and kG in 60% ethanol with 0.15 M NaCl) is due to the effect of solvents on the chemical reactivity of purine residues, but does not reflect a conformational transition. The analysis of these results allows us to conclude, that alternating polynucleotides under the above mentioned conditions retain roughly the conformations inherent in them in 0.15 M NaCl: poly[d(A-C)].poly[d(G-T)] conformation in 4.3 m CsF or 60% ethanol differs only insignificantly from the "canonic" B-DNA, whereas the poly[d(A-T)].poly[d(A-T)] conformation in 6 M CSF corresponds to B-alternating DNA.