[In a b-sheet two or more polypeptide chains run alongside each other and are linked in a regular manner by hydrogen bonds between the main chain C=O and N-H groups. The R-groups (side chains) of neighbouring residues in a b-strand point in opposite directions; beta sheets can be parallel, anti-parallel or mixed. (from expasy.org) ( NCI )]
UMLS (CSP) C0599215