CONFORMATIONAL CHARACTERISTICS OF THE HIGH-SALT FORM OF POLY(D2NH2A-DT) BASED ON H-1-]H-3 EXCHANGE BETWEEN WATER AND C8H GROUPS OF ITS ADENYLIC RESIDUES

Rate constants were determined for H-1-->H-3 exchange between water an d the C8H groups of adenylic residues in poly(d2NH2A-dT) and d2NH2-ade nosine in buffer solutions containing 0.15 and 2.2 M NaCl. The exchang e retardation coefficient (K(r)) was used as a parameter characterizin g the secondary structure of poly(d2NH2A-dT). It is numerically equal to the ratio between the exchange rate constants in a monomer and a po lymer and characterizes the accessibility of C8H groups of adenylic re sidues (located in the major groove of the double helix) to the OH- io ns of a solvent. It was shown that for the low-salt form of the polyme r the K(r) is equal to 2.9, whereas for the high-salt form it increase s to 6.8. Although the K(r) value of 2.9 for the low-salt form of poly (d2NH2A-dT) is characteristic of the B family (K(r) for B-DNA is equal to 2.5-3.0), the value of 6.8 for the high-salt form is not typical o f any known conformational family (K(r) is at least 15 for the A form and close to unity for the Z form). Consequently, if the polynucleotid e conformational type is judged by the accessibility of its purine res idue C8H groups to the OH- ions of the solvent, then the poly(d2NH2A-d T) conformation in 2.2 M NaCl is either intermediate between the A and B forms of DNA, or we are dealing with a B reversible A conformationa l equilibrium. If the first supposition is correct, then our data indi cate that the major groove of the poly(d2NH2A-dT) double helix is deep er and/or more closed than in the B form of DNA, but is wider and/or m ore open to solvent molecules than in the DNA A form.