Oligosaccharides as model systems for understanding water-biopolymer interaction: Hydrated dynamics of a hyaluronan decamer

Hyaluronan is a mammalian extracellular maxtrix polysaccharide with unusual physiochemical properties. In this study, a five-nanosecond simulation of a hyaluronan decasaccharide in aqueous solution was compared with previous experimental data and hence used as a model system for studying carbohydrate-water interactions. The predicted average conformation is in agreement with X-ray fiber diffraction and hydrodynamic predictions, but the calculated persistence length is larger than that inferred from experiments, as is expected from the lack of counterions in this simulation. Intramolecular hydrogen bonds were observed in the simulation across all sugar linkages, but they are predicted to be in exchange with water and with themselves. Some 80 distinct intramolecular hydrogen-bond interactions were documented over a period of 1 nanosecond in this short length of polymer. Therefore, it is predicted that individual intramolecular hydrogen bonds will be difficult to observe directly by nuclear magnetic resonance spectroscopy or other experimental techniques. However, the simulation also predicts that this exchange is atypical at the terminal linkages of the polymer. Such end-effects have been proposed from interpretation of nuclear magnetic resonance spectroscopy and circular dichroism measurements. In summary, water interaction resulted in a molecular end-to-end distance that was consistently close to the maximum. However, there were strong but ephemeral fluctuations on the subnanosecond time scale away from the most extended state, involving large rearrangements of the surrounding water structure. This dynamic model for hyaluronan extends the current static microscopic view, while also being consistent with large-scale stiffened random-coil models. Therefore, it is proposed that these microscopic properties, emergent from water interaction, are responsible for the macroscopic physicochemical properties of this molecule. These results motivate the design of novel prediction-testing experiments, which can throw new light on water-biopolymer interactions.