Nuclear magnetic resonance and conformational energy calculations of repeat peptides of tropoelastin: conformational characterization of the cyclododecapeptide

Abstract

N.m.r. data obtained in chloroform–dimethyl sulphoxide solutions and conformational-energy calculations are reported which deduce the preferred conformation of the cyclic dodecapeptide, cyclo-(L-Ala¹-L-Pro²-Gly³-L-Val⁴-Gly⁵-L-Val⁶)₂, having the hexapeptide repeat sequence of tropoelastin. The non-equivalence of the Gly α-CH₂ protons and the similarity of α-carbon relaxation times indicate that the molecule is quite rigid. The ¹H n.m.r. spectrum of the molecule resembles that of a hexamer, indicating that the molecule possesses two-fold symmetry on the n.m.r. time scale. The possible ranges of backbone torsion angles except ψ(Ala¹) are derived from α-CH–NH coupling constants, geminal coupling constants, and nuclear Overhauser effects. From temperature-dependence studies of the peptide NH chemical shift and from the coupling constants, secondary structural features of the molecule are obtained. The valyl α-CH–β-CH coupling constants show that the Val⁴ side chain is in a gauche conformation, while the Val⁶ side chain is in the trans-conformation. A static wire model is developed using these data and containing the following solution-derived hydrogen bonds: CO(1)⋯ HN(4)(β-turn), NH(1)⋯ OC(4) and NH(6)⋯ OC(4)(bifurcated 14- and 17-membered rings), and NH(3)⋯ OC(5)(γ-turn). In vacuo conformational-energy calculations are performed to obtain several minimum-energy conformations. The theoretical calculations utilize the Go–Scheraga method for cyclization with exact two-fold symmetry. Only one of the low-energy structures so obtained is consistent with all the experimental data, and this structure is quite similar to the static wire model based on the n.m.r. data.