Changes in the zero point energy of the protons as the source of the binding energy of water to A phase DNA

Abstract

The zero point kinetic energy of protons in water is large on the scale of chemical interaction energies(29 Kj/mol in bulk room temperature water). Its value depends upon the structure of the hydrogen bond network, and can change as the network is confined or as water interacts with surfaces. These changes have been observed to be large on a chemical scale for water confined in carbon nanotubes and in the pores of xerogel, and may play a fundamental, and neglected, role in biological processes involving confined water. We measure the average momentum distribution of the protons in salmon Na-DNA using Deep Inelastic Neutron Scattering, for a weakly hydrated (6w/bp) and a dehydrated fiber sample. This permits the determination of the change in total kinetic energy of the system per water molecule removed from the DNA and placed in the bulk liquid. This energy is equal, within errors, to the measured enthalpy for the same process, demonstrating that changes in the zero point motion of the protons, arising from changes in structure as water molecules are incorporated in the DNA, are a significant factor in the energetics of the transition from the A to B phase with hydration, in this case, providing the entire binding energy of the water molecules to the DNA. The shape of the momentum distribution in the dehydrated phase is consistent with coherent delocalization of some of the protons in a double well potential, with a separation of the wells of .2 Angstroms.