Peptide models XLII. Ab initio study on conformational changes of N-formyl- l-histidinamide caused by protonation or deprotonation of its side chain

Abstract

The imidazole ring in the side chain of N-formyl- l-histidinamide may have four forms of protonation, two tautomeric neutral forms and two charged forms. The four molecules, though they only differ in the protonation state of the imidazole nitrogen atoms, have potential energy hypersurfaces of significantly different topology; i.e. the hypersurfaces have minima at different ϕ and ψ values of the peptide backbone. A complete conformational library of the above mentioned single amino acid dipeptide model [J. Mol. Struct. (Theochem) 583 (2002) 117; J. Mol. Struct. (Theochem) 583 (2002) 199] is evaluated to map the possible conformational changes caused by protonation or deprotonation. It was determined, that a protonation or deprotonation itself can induce significant conformational changes involving the side chain, the backbone or both. Such conformational changes can lower the relative energy by up to 10 kcal mol −1. On acquiring a net positive charge α-helical ( α L) and polyproline II ( ε L) structures are willing to adopt extended ( β L) and inverse γ-turn ( γ L) conformation, respectively, while acquiring a net negative charge by an extended ( β L) conformer results in the formation of α-helical ( α L) or inverse γ-turn ( γ L) structures. The formation of a charged entity, is frequently accompanied by conformational changes, in contrast to the process of neutralization, that is seldom followed by significant modification of torsional angles.