THE AROMATIC-(I+2) AMINE INTERACTION IN PEPTIDES

The interaction between an aromatic ring and an amine group has been proposed to be important for the behavior of proteins and peptides. A special case, notable in the folding pathway of BPTI, is the interaction of the aromatic side chain of the ith residue with the amine group in the peptide link to the (i+2) th residue. In this paper, molecular mechanics calculations, employing the CHARMm22 force field together with a simple model of a hydrogen bonding environment, are used to systematically search torsion space to find the favorable conformations for tripeptides containing this interaction as defined by the NMR ring shift on the amine proton. When all hydrogen-bonding atoms in the peptide are assumed to be able to make hydrogen bonds to solvent molecules, all favored conformers found have the amine group lying approximately parallel to the aromatic ring. In this geometry, the aromatic-amine interaction energy is dominated by the interaction energy of the (i+2) amine group with solvent and the rest of the (i+2)th residue with the aromatic ring. When a solvent hydrogen-bond acceptor is not available for the (i+2) amine proton, most of the favorable conformers have the amine group perpendicular to the ring. This geometry is known to be the minimum energy geometry for the isolated aromatic-amine interaction. For steric reasons, the majority of minimum energy conformers on the two potential energy surfaces are favorable for glycine-rich tripeptides. The geometries and sequence selectivities of the minimum energy conformers are found to fit the distribution observed in a set of tripeptides containing this interaction extracted from a database of 297 representative protein crystal structures.