The structure of gellan in dilute aqueous solution

A full assignment of high-field nmr spectra of gellan was obtained in dilut e aqueous solution by performing a series of selective one-dimensional nmr experiments. The observed nuclear Overhauser effects (NOEs) cannot be inter preted assuming that each sugar residue is intrinsically rigid and in a cha ir conformation. In fact, the rhamnose residue gives strong NOE contacts co herent only with an equilibrium involving both a chair as well as a boat (o r a hemiboat) conformation. Molecular dynamic calculations performed on a h eptamer with a central rhamnose support the above finding, and show a struc ture based on a very stiff single chain in which it is present a flipping o f the rhamnose residue. At low temperatures (5-20 degrees C) in very dilute solutions (0.018 mg/mL) nmr spectra show a splitting of the resonance due to the methyl group of rhamnose residue, thus confirming the presence of a slow equilibrium among different conformers. (C) John Wiley & Sons, Inc.