Heterogeneity and stability of helical conformations in peptides: crystallographic and NMR studies of a model heptapeptide

The helix is a common secondary structural element in proteins. Several subtypes of helical structures are possible, depending on the nature of the backbone hydrogen-bonding pattern. For short peptides in solution, more than one helical conformation may be in equilibrium. Most of the time it is extremely difficult to identify experimentally the interconversion between different helix forms in solution, although one of the helical structures may be trapped in single crystals. This report analyzes crystallographic and NMR studies of a model heptapeptide, Boc-Leu-Aib-Val-Ala-Leu-Aib-Val-OMe, designed to probe conformational heterogeneity and relative stability of different helical structures. In crystals the peptide adopts a right-handed 310-helical structure, terminated by a lefthanded helical conformation (alphaL) at Aib(6) resulting in a 6-1 hydrogen bond at the C-terminus. The crystals are in space group P21, a = 9.843(4) A, b = 19.944(4) A, c = 12.456(2) A, beta = 102.51(2)°, Z = 2, R = 10.1%, and Rw = 9.21% for 2301 reflections; [F] > 3sigma(F). In CDCl3 NMR studies establish that the NH groups of residues 3-6 are hydrogen bonded, consistent with a continuous 310-helix or the conformation observed in crystals. In (CD3)2SO, NMR studies reveal that the NH groups of residues 1-3 are solvent exposed, supporting an R-helical structure. However, NMR data suggest appreciable conformational heterogeneity in solution, with population of extended states and conformations resembling the structure in crystals. Helical conformations terminated by alphaL residues may be an important subset of helical structure in peptides, particularly when the penultimate residue is achiral or has a propensity to adopt alphaL conformations.