DELTA-VBARO OF THE NA+-INDUCED B-Z TRANSITION OF POLY[D(G-C)] IS POSITIVE

The equilibrium between the right‐ and left‐handed conformations of poly[d(G‐C)] in aqueous NaCl shifts towards the right‐handed (B) form with increasing pressure. The optical density at 290 and 260 nm was determined at 50 and 180 MPa for solutions in which approximately equal amounts of the two conformations were present at 0.1 MPa (atmospheric pressure). Interpretation of the observed changes in terms of a two‐state unimolecular reaction mechanism results in an average molar reaction volume (ΔV̄0) equal to 26 cm3 mol−1 at 22°C; that is, the partial molar volume of B form poly [d(G‐C)] is smaller than that of the left‐handed (Z) form. Based upon the thermodynamics of ion‐pair formation in polar solvents, it is proposed that the positive ΔV̄0 reflects a favorable entropy change for the reaction toward the Z conformation. The larger entropy change of the Z form may derive from the release of water molecules from the hydration spheres of the cation and the poly[d(G‐C)] due to the formation of ionic interactions with the Z conformer. The ΔV̄0 of the transition is similar in sign and and magnitude to the calculated molar volume change of the interaction of Na+ with H2PO 4− in water. Copyright © 1990 John Wiley & Sons, Inc.