Hydration of hyaluronan polysaccharide observed by IR spectrometry. III. Structure and mechanism of hydration

The results of the analysis of hydration spectra of Na+ hyaluronan (HA) performed in a companion study are translated in terms of chemical mechanisms. We find that dried HA is characterized by chains having ordered parts of at least 6 disaccharide repeat units that extend over 60 A. The order is mainly due to C3O3H…O5 and C4O4H…O5 hydrogen bonds that hinder rotations around β1–4 and β1–3 glycoside bonds. Along one chain there are two-thirds of the NH amide groups and carboxyl groups that are directly hydrogen bonded, with no water intermediate, to form NH…−OCO hydrogen bonds, which are collateral to C3O3H…O5 hydrogen bonds. The existence of these NH…−OCO bonds is somewhat in opposition to literature descriptions. In this dry state a “water wire” of 4–5 H2O molecules, which are anchored on CO carboxyl groups and hydrating the Na+ CO− ionic group, establishes hydrogen bonds on other hydrophilic groups of the same chain or other chains and remains embedded in HA, even at 104°C. Hydration occurs at low hygrometry around the remaining one-third of the NH and −OCO pairs that are not hydrogen bonded. Each of these NH and −OCO groups is hydrated by a nanodroplet of some 25 H2O molecules that finds other sites for binding and hydrates 2 disaccharide repeat units. At higher hygrometry bigger nanodroplets hydrate all hydrophilic sites. © 2003 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 72: 162–173, 2003