Periodate oxidation and the shapes of glycosaminoglycuronans in solution

Abstract

Periodate oxidation of glycol groups in the repeating units of polysaccharide molecules may be used to probe differences in polymer shapes in solution. Measurement of second-order rate constants (k2) of periodate-glycol reactions may be compared between polymers and relevant monomers, to assess perturbations due to polymer configuration. Factors affecting the measurement and interpretation of k2 are discussed. Over-oxidation, free-radical side reactions, end-group effects, Donnan equilibria and polymer (or MW) effects are relevant, but their importance is either small or can be minimized in practice. A small group of glycosaminoglycuronans (chondroitin 4- and 6-sulfates and hyaluronate) are oxidized 50-100 times more slowly than 3 other glycosaminoglycuronans of similar composition, relevant monomers or 3 homopolyuronides. A stable configuration in solution is postulated for the periodate-resistant polymers, involving carboxylate, acetamido and hydroxy groups in H-bonded sequences on alternate sides of the molecule. The more easily oxidizable polyuronides are unable to form this configuration. The effect of temperature on the postulated configuration is investigated through the Arrhenius plot of k2, measured on hyaluronate, chondroitin 6-sulfate and methyl 4-0-methyl-.alpha.-D-glucopyranoside. Probable transitions at high (around 90.degree. C) temperatures were observed for both polymers, with an additional transition at about 37.degree. C in the case of hyaluronate. L-Iduronic acid can take up different conformations depending on the polymer environment.