Conformational Analysis of Alanine Dipeptide from Dipolar Couplings in a Water-Based Liquid Crystal

Abstract

The proton NMR spectra of unlabeled alanine dipeptide (Ac-L-Ala-NHMe) at 300 MHz and of alanine dipeptide with a single 13C label at 500 MHz are obtained in the lyotropic liquid-crystalline solvent cesium pentadecafluorooctanoate in water (CsPFO/H2O). Simulations of the spectra yield 9 and 13 dipolar couplings Dij, respectively, many with absolute sign determined. We fit the set of dipolar couplings by systematically varying the flexible dihedral angles φ and ψ while freezing local geometric details from the electronic structure calculations of Suhai and co-workers (Han, W. G.; Jalkanen, K. J.; Elstner, M.; Suhai, S. J. Phys. Chem. B 1998, 102, 2587). The orientation tensor is optimized at each combination of dihedral angles. Remarkably, a single conformer PII (φ ≈ −85°, ψ ≈ +160°) fits both sets of couplings within experimental error. The orientation tensor can be understood in terms of a simple rocking motion that dips the central methyl group into the fluorocarbon core of the CsPFO bicelle while alternately exposing both hydrogen-bonding pockets of PII to interfacial or bulk water. The search for a minority conformer such as αR (right-handed alpha helix, often favored by theory) using the larger data set was inconclusive. The data support localization of the peptide within the PII well rather than the broad sampling of φ in the range from −60° to −180° (a “β/PII minimum”) found by certain models. We suggest that the PII geometry is stable primarily because it maximizes the opportunity for peptide−water cooperative hydrogen bonding, whereas the αR geometry is stable primarily because of its large dipole moment. Our result corroborates recent work on short polypeptides suggesting that they preferentially sample configurations that fluctuate about PII-like structures, in contrast to the usual random-coil models.