Structure and dynamics of sialic acid at the surface of a magnetically oriented membrane system.

A structural analysis of a sialic acid containing glycolipid alpha-dodecyl-N-acetylneuraminic acid anchored to the surface of a phospholipid-based membrane-like fragment is presented. The analysis is based on measurement of dipolar interactions between 13C-13C and 13C-1H spin pairs in a magnetically oriented membrane phase and the interpretation of data using an order matrix formalism. Structural analysis in this medium allows an assessment of properties in an environment approximating the environment where similar molecules function as cell surface receptors. The results indicate an extended headgroup conformation with the carboxyl group of the sialic acid near the membrane interface. Anisotropic motion of the sialic acid headgroup occurs with the most restriction along an axis which is significantly out of the plane of the sialic acid ring suggesting an important role for the carboxylate group in membrane surface interaction. The effect of calcium on the headgroup orientation was also investigated, but the results showed no significant change in the preferred conformation, possibly due to a weak binding under the conditions of the experiment.