The cause of the twist in β sheets appears to lie in noncovalent interactions between hydrogen atoms on the β-carbon atoms of side chains and the peptide backbone atoms. For side chains of most L- amino acids these interactions provide a small tendency towards the observed right-handed twist. Nonplanarity in the amide groups may also contribute. Interstrand interactions seem to be important.
A “ribbon” drawing of the 307- residue proteinhydrolyzing enzyme carboxypeptidase A. In this type of drawing wide ribbons are used to show β strands and helical turns while narrower ribbons are used for bends and loops of the peptide chains. The direction from the N terminus to C terminus is indicated by the arrowheads on the β strands. No individual atoms are shown and side chains are omitted. Courtesy of Jane Richardson.