Graduate Thesis Or Dissertation
 

A double-stranded secondary structure for hyaluronic acid in aqueous-organic solvent as revealed by vacuum ultraviolet circular dichroism

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  • The association of hyaluronic acid (HA) into a double stranded structure 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 structural transition results in a high intensity CD band at 190 nm, indicating an ordered structure in the mixed solvent. Heating hyaluronic acid in the mixed solvent also causes a cooperative transition reducing the CD to that found for the polymer in aqueous solution. The CD for the polymer in aqueous solution is not very different from some of the CD's for the constituent monomers, and the CD of the monomers is similar in both aqueous solution and mixed solvent. Furthermore, heating HA in aqueous solution results in small, noncooperative changes in the CD spectrum. The CD as dielectric constant is reduced exhibits isodichroic points, showing that there are two states. Singular value decomposition of the CD as a function of organic solvent has only two principal components, confirming that there are only two states. Oligomers of HA show a chain length effect with approximately nine disaccharides required for the structural transition as a function of organic solvent. This proves that the transition is indeed cooperative. The concentration dependence for the transition was investigated in detail for the 12 mer and 16 mer, and it was possible to prevent completely the transition in mixed solvent with sufficient dilution of the oligomer. The CD data in mixed solvent as a function of oligomer concentration were fit with various models for association of two and more strands. Simplex directed search methods were used to investigate the vector space of unknown model parameters, and two strand models were shown to consistently give a better fit. A cooperative two-strand zipper model which allows relative sliding of the chains gave the smallest error and produced the following thermodynamic parameters for the ordered structure: enthalpy of growth, -1000 ± 300 cal/mole; entropy of growth, -2.25 ± 1.30 eu/mole: enthalpy of initiation, -20,000 ± 3000 cal/mole; entropy of initiation, -71 ± 15 eu/mole. These results are consistent with a double-stranded and helical structure for hyaluronic acid in solutions of reduced dielectric constant.
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