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Oligosaccharides and recognition--a 'shape' problem probed by n.m.r. and molecular modelling.

There is now adequate experimental evidence that demonstrates that oligosaccharides are specific ligands in a variety of biological-recognition phenomena. While the precise nature of the oligosaccharide ligand involved in a particular process is known for certain systems [ 11, the molecular basis of the interaction remains obscure. Recognition is clearly a ‘shape’ problem and ideally we require a highresolution structure of the oligosaccharide-protein complex to aid our understanding of the mechanisms involved in the interaction. Classically this would require the crystallization of the complex [Z-41, but with the advent of new multi-nuclear, multi-dimensional n.m.r. methods, it becomes feasible in principle to determine the high-resolution three-dimensional structure of the complex in solution. In particular the delineation of the interaction of the carbohydrate recognition domains of a variety of lectins with their ligands becomes a real possibility using n.m.r. methods. However, while the necessary theoretical and experimental techniques for the conformational analysis of the protein receptor by n.m.r. are well developed [5], there is much work to be done in developing similar techniques for the conformational analysis of the oligosaccharide ligand. This state of affairs is undoubtedly due in part to the relatively small number of groups working on these problems, which in turn probably derives from a historic regard for oligosaccharides as ‘without function’ and ‘uninteresting’, a myth which appears to have been dispelled only in the light of recent experimental data. Here we describe some recent experimental and theoretical approaches that we have developed to simplify the task of oligosaccharide conformational analysis. We focus on two particular ‘bottlenecks’ in the field, namely the problem of resonance assignment in the n.m.r. spectra of oligosaccharides and the treatment of distances and angles derived from n.m.r. measurements, given the possibility of significant internal motions about glycosidic linkages.