Hydrogen bond connectivity patterns and hydrophobic interactions in crystal structures of small, acyclic peptides

Crystal structures of all available unblocked linear peptides with two to five residues were retrieved from the Cambridge Structural Database and their intermolecular contacts and packing modes studied using molecular graphics. This survey reveals that interactions between hydrophobic portions of the molecules are critically important in determining the overall features of their crystal packing patterns. Distinct hydrophobic columns or layers are observed in almost all crystal structures. Analyses of the relationships between these interactions and crystal growth properties of small peptides are given. It is suggested that needle growth is promoted by hydrophobic packing, usually along a short crystallographic axis (4.6-6.0 angstroms). Also contributing to these morphologic characteristics are entropic factors associated with hydrophobic aggregation as well as tightly bound water molecules on hydrophobic faces. The paper also provides a comprehensive overview of hydrogen bond patterns in acyclic peptide crystals. It is demonstrated that one of their primary roles is to provide a scaffolding within which hydrophobic groups can aggregate. Even though there is a high density of hydrogen bonds in the crystals, often with complex patterns and networks, certain motifs are found to recur in a number of structures indicating specific hydrogen bond preferences. Water, for example, is an integral part of the hydrogen bond networks in these crystals, usually acting as the primary donor for main-chain carboxylate groups in peptide hydrates.