DYNAMIC STRUCTURING OF WATER IN POLYSACCHARIDES BY NEUTRON SCATTERING AND RAYLEIGH SCATTERING OF MOSSBAUER RADIATION

A. Deriu, G. Albanese, F. Cavatorta

Dipartimento di Fisica ed Istituto Nazionale per la Fisica della Materia, Universita di Parma, Viale delle Scienze, I-43100 Parma, Italy

Haluronic acid, HA, is a biological polyelectrolyte which is the major component of the extended water-retaining network of polysaccharides and proteoglycans which make up the gel-like extracellular matrix of mammalian connective tissue [1], and it is also present in the capsules of some bacteria. X-ray fibre diffraction studies have shown that HA can adopt a number of distinct double helical conformations in a variety of packing arrangements as a function of its hydration, ionic environment, and temperature [2].

Here results of neutron scattering and Rayleigh scattering of Mössbauer radiation measurements will be reported. For both experiments highly oriented thin films of HA fibres were prepared using the 'wet-spinning' process [3].

Neutron diffraction data to a resolution of 3 A were collected on the D19 diffractometer at the ILL (Grenoble) keeping the sample at 75% relative humidity. The data showed a dominant phase which could be indexed on the basis of a large orthorombic cell of a = 34.4 A, b = 11.7 A, and c = 28.5 A corresponding to 3-fold extended double helices [4].

The RSMR experiments performed on the same samples made it possible to determine the elastic and integrated-inelastic contributions to the scattering profile [5]. The elastic scattering intensity (Iel ~ S(Q,,w =0)) shows a sharp peak at about 2 A-1 only for samples oriented with Q 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 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 Iel 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 same Q values of the elastic intensity, indicating the presence of coherent vibrational excitations propagating along the ordered water filaments.

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