Rayleigh scattering of Mössbauer radiation in oriented fibres of hydrated biopolymers

Abstract

The Rayleigh scattering of Mössbauer radiation (RSMR) has been measured on films of highly oriented hydrated polynucleotides (A-NaDNA) and polysaccharides (Na-hyaluronate). Both DNA and hyaluronate (HA) have helical secondary structures with a similar pitch (28.2 Å for A-DNA, and 32.8 Å for Na-HA), but they differ in the basic elements which make up the helices and in the extent of water-biopolymer interactions. These differences are responsible for the diverse stiffness of the polymer backbone, and also affect the dynamics of the first hydration layers. For both samples the elastic scattering intensity shows a sharp peak at about 2 Å⁻¹ only for samples oriented withQ parallel to the fibre direction. Its position is close to that of the first maximum in the structure factor of bulk water; it is, however, much narrower than in pure H₂O and it is similar to a crystalline Bragg peak. It can be attributed to an ordered structure of water along the double helices. From the temperature dependence of the elastic intensity under the peak maximum, the mean square displacement of water oxygens in the direction parallel to the helices has been deduced. The thermal diffuse scattering intensity is also peaked at the sameQ values of the elastic intensity, indicating the presence of coherent vibrational excitations propagating along the ordered water filaments.