Light Scattering Spectroscopy Studies of the Water Molecules in DNA

Abstract

The water molecules surrounding DNA are usually described in terms of a primary hydration shell and a secondary hydration shell (Saenger, 1984; Texter, 1978). Various experiments indicate that the primary hydration contains 20–25 water molecules per nucleotide pair. The size of the secondary hydration shell is less clear (Saenger, 1984; Texter, 1978). To understand the roles water plays in DNA properties at a microscopic level, it is essential to know the structure of the DNA hydration shells. There have been many studies of the structure of the DNA hydration shells, although a complete understanding of the structure is still lacking. The most direct method is probably X-ray crystallography, and it has been used to locate the water molecules in DNA single crystals. For example, Kopka et al. (1983) and Kennard et al. (1986) studied water structure in various oligonucleotide single crystals using high-resolution X-ray diffraction. Various water bridges that depend on DNA conformation have been observed in DNA. However, this method can only locate a small fraction of the water molecules in the hydration shells, since most water molecules diffuse too rapidly, and it is also limited to DNA single crystals. Infrared spectroscopy has been used by many workers (e.g. Blinska and Wilczk, 1976; Falk et al., 1970; Pilet et al., 1975). Falk et al. (1970) studied the infrared spectrum of DNA and water in the region of 400–4000 cm−1 (1 cm−1 = 30 GHz) as a function of relative humidity (r.h.).