Laser Raman inelastic light scattering investigations of hyaluronic acid primary and secondary structure

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Abstract
The solvent‐subtracted Raman spectrum of hyaluronic acid is reported. The spectrum is unchanged by (i) variations in the pH of the solution 6.0–8.5, when buffered at constant ionic strength 0.1, and (ii) changes in temperature 10–50°C.As rheological, flow birefringence, and linear dichroism studies of hyaluronate solutions indicate a large and abrupt structural transition across the physiological pH range 7.0–7.5, the negative finding reported here is of considerable interest in contrast with those previous studies. When considering the polysaccharides in general, as precise rheological, flow birefringence and linear dichroism data are only available for hyaluronic acid, the present finding is of interest for hyaluronic acid considered as a model polysaccharide system exhibiting elasticity.The result reported here is discussed within the context of, on the one hand, the possible limitations of the Raman technique which is a probe of g‐state vibrational modes and, on the other hand, the possible extension of the Raman technique to probing oriented (stressed) samples. As a previous study demonstrated a small limiting birefringence but a large limiting extinction angle with minimum change in monomer anisotropy for these solutions, the suggestion is favored that the abrupt structural transition across the physiological pH range previously reported is due to long‐range Van der Waals interactions.