β Scribed by Morris, Edwin R. ;Rees, David A. ;Thom, David ;Welsh, E. Jane
No coin nor oath required. For personal study only.
For consideration of their conformations and interactions, carbohydrate chains can conveniently be divided into 3 classes on the basis of their covalent structure; namely periodic (a), interrupted periodic (b), and aperiodic (c) types. In aqueous solution carbohydrate chains often exist as highly disordered random coils. Under appropriate conditions, however, polysaccharides of types (a) and (b) can adopt a variety of ordered conformations. Physical methods, and in particular optical rotation, circular dichroism, and nuclear magnetic resonance, provide sensitive probes for the study of the mechanism and specificity of these disorderβorder transitions in aqueous solution.
Intermolecular interactions between such polysaccharide chains arise from coβoperative associations of long structurally regular regions which adopt the ordered conformations. For acidic polysaccharides these cooperative associations may involve alignment of extended ribbons with cations sandwhiched between them. In other systems the interactions involve double belices which may then aggregate further, and geometric βmatchingβ of different polysaccharide chains can also occur. These ordered, associated regions are generally terminated by deviations from structural regularity or by βkinksβ which prevent complete aggregation of the molecules.
The complex carbohydrate chains which occur at the periphery of animal cells have very different, aperiodic structures and although their conformations are as yet poorly understood, preliminary indications are considered.
π SIMILAR VOLUMES
The known protein structures have been analyzed to find out if there is any pattern in the type of residues used and their conformation at the two terminal positions of the polypeptide chains. While the N-terminal position is overwhelmingly occupied by Met (followed by Ala and Ser), the preference f