Conformational transition of hyaluronic acid in aqueous-organic solvent monitored by vacuum ultraviolet circular dichroism

The chiroptical transition of hyaluronic acid (HA) in aqueous-organic solvent has been investigated by circular dichroism (CD) spectroscopy into the vacuum ultraviolet region. The CD of HA changes dramatically, monitoring a cooperative transition as the dielectric constant of an aqueous solution is reduced by adding organic solvents. This transition results in a high-intensity CD band at 188 nm, indicating an ordered structure in the mixed solvent. Heating HA in the mixed solvent also causes a cooperative transition, reducing the CD to that found for the polymer in aqueous solution. In contrast, heating HA in aqueous solution results in small, noncooperative changes in the CD spectrum. This indicates an unordered structure in aqueous solution. The CD as the dielectric constant is reduced exhibits isodichroic points, showing that there are only two environments for chromophores contributing to the CD. This is confirmed by singular value decomposition of CD spectra recorded as a function of solvent composition, which shows the spectra to contain only two principal components. The data describing the thermally induced transition of HA in mixed solvent are not consistent with infinite cooperativity. The van't Hoff relation yields thermodynamic parameters for the conformational transition in terms of the cooperative unit of -60 kcal mol-1 for delta H degrees and -180 eu mol-1 for delta S degrees.