INFLUENCE OF BASE COMPOSITION, BASE SEQUENCE, AND DUPLEX STRUCTURE ON DNA HYDRATION - APPARENT MOLAR VOLUMES AND APPARENT MOLAR ADIABATIC COMPRESSIBILITIES OF SYNTHETIC AND NATURAL DNA DUPLEXES AT 25-DEGREES-C

Using high-precision densimetric and ultrasonic measurements, we have determined, at 25 degrees C, the apparent molar volumes, phi V, and the apparent molar compressibilities, phi K-S, of five natural and three synthetic B-form DNA duplexes with varying base compositions and base sequences. We find that phi V ranges from 152.0 to 186.6 cm(3) mol(-1), while phi Ks ranges from -73.0 x 10(-4) to -32.6 X 10(-4) cm(3) mol(-l) bar(-1). We interpret these data in terms of DNA hydration which, by the definition employed in this work, refers to those water molecules whose density and compressibility differ from those of bulk water due to interactions with the DNA solute. This definition implies that hydration depends not just on the quantity but also on the quality of the solvent molecules perturbed by the solute. In fact, we find that the number of water molecules perturbed by the DNA duplexes (the quantity of water in their hydration shells) is approximately the same for all of the B-form double helixes studied, while the quality of this water differs as measured by its density and compressibility, thereby yielding differences in the overall hydration properties. Specifically, we find a linear relationship between the density and the coefficient of adiabatic compressibility, beta(Sh), Of water in the hydration shell of the DNA duplexes, with the range of values for beta(Sh) being only 65-80% of the value of bulk water. In the aggregate, we interpret these data as reflecting the following general features: (i) water that is perturbed by interactions with B-form DNA duplexes (i.e., the hydration shell) exhibits increased density and a diminished coefficient of adiabatic compressibility relative to bulk water; (ii) the hydration of B-form duplexes primarily depends on the base composition, while it is only weakly influenced by base sequence; (iii) duplexes with lower phi V and phi Ks values exhibit greater hydration; (iv) duplexes with 55-60% AT base composition exhibit the weakest hydration, while increases or decreases in AT content from this range of values lead to enhanced hydration; and (v) water in the hydration shell of GC base pairs is more dense and less compressible than water in the hydration shell of AT base pairs, suggesting that GC base pairs are solvated more strongly (are more hydrated) than AT base pairs, in contrast with conventional wisdom. Using the definition of hydration employed in this work (solute-induced perturbation of solvent density and compressibility), we propose molecular interpretations for the differential hydration properties we observe.