Conformational energy calculation on the neurotensin c-terminal pentapeptide Arg–Pro–Tyr–Ile–Leu OH

The conformational behavior of the C-terminal neurotensin pentapeptide, Arg–Pro–Tyr–Ile–Leu OH NT(9–13)], was investigated using empirical energy calculations. A special aim was to display the specific contribution of each residue to induce conformations able to interact with biological receptors. Restrictions were then introduced in intramolecular interactions involving the Arg side chain and the terminal COOH group. The stablest conformations include in the order of decreasing stability: a distorted helical form for the C-terminal tetrapeptide, a (Pro₂–Tyr₃) β turn I, an α helix, an extended form, and a (Tyr₂–Ile₃) β turn III, which are energetically rather close (ΔE < 3 kcal/mol). The NT(9–13) peptide appears then as a rather flexible molcule with a noteworthy ability of adaptation to a substrate. Extended forms would be in agreement with a zipper model of interactions with receptors, whereas folded forms involving helices and β, γ turns would support a lock and key model. The specific contribution of side chains, specially those of Tyr and Arg residues as well as the key position of the Pro residue emerge clearly from this study.