Stereochemical restrictions on the occurrence of amino acid residues in the collagen structure

Abstract

The primary structure of collagen is characterized by the repeating tripeptide sequence (Gly-R₂-R₃)n. The results of theoretical studies, carried out using contact criteria to compute the stereochemically allowed orientations for various side chains at locations 2 and 3, are reported here. It is found that side chains with only -atoms, as in valine, serine and threonine, or with only one δ-methyl group, as in isoleucine, can occur equally well at locations 2 and 3, as is actually the case in collagen. Side chains with two Cδ-atoms, as in leucine and phenyl-alanine, can also be accommodated at both positions. However, if they occur as R₃ their freedom of orientation is severely restricted in the presence of a proline residue as R₂ in a neighbouring chain. If water molecules bound to the chains of the triple helix are assumed to be present, then location 3 is virtually impossible for leucine and phenylalanine residues. Location 2 is, however, unaffected, and their presence as R₂ can help to shield the water molecules from disturbance by the solvent medium. This may be the reason for the preferential occurrence of Leu and Phe residues in location 2 in the collagen triplets, although the polypeptides (Gly-Pro-Leu)_n and (Gly-Pro-Phe)_n form collagen-like structures.