Theory of pressure-dependent melting of the DNA double helix: Role of strained hydrogen bonds

In this paper we study the effect of hydrostatic pressure on the thermal stability of DNA base pairs. The thermal stability of a base pair is predominantly determined by the fluctuational motion of the interbase hydrogen bonds in the base pair. We postulate that the pressure exerted on the surface of DNA creates a compressive stress in the interbase hydrogen bonds. This stress in turn induces a strain in these hydrogen bonds. The effect of this compressive stress on the thermal fluctuational motion of the strained interbase hydrogen bonds in DNA can be analyzed by the modified self-consistent phonon approximation theory. We calculate individual hydrogen bond disruption probabilities and base pair opening probabilities of two B-conformation DNA polymers, an adenine-thymine copolymer and a guanine-cytosine homopolymer, at various pressures and at temperatures both in the premelting region and in the helix-coil transition region. Our calculated pressure dependence of the melting temperature for both polymers is in fair agreement with experimental observations. Our values for the melting temperatures are also in agreement with observation and our theory is successful as a theory of cooperative melting.