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

The proton NMR spectra of unlabeled alanine dipeptide (Ac-L-Ala-NHMe) at 300 MHz and of alanine dipeptide with a single 13 C label at 500 MHz are obtained in the lyotropic liquid-crystalline solvent cesium pentadecafluorooctanoate in water (CsPFO/H 2 O). Simulations of the spectra yield 9 and 13 dipolar couplings D ij, 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 P II (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 P II to interfacial or bulk water. The search for a minority conformer such as α R (right-handed alpha helix, often favored by theory) using the larger data set was inconclusive. The data support localization of the peptide within the P II well rather than the broad sampling of φ in the range from −60° to −180° (a “β/P II minimum”) found by certain models. We suggest that the P II geometry is stable primarily because it maximizes the opportunity for peptide−water cooperative hydrogen bonding, whereas the α R 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 P II -like structures, in contrast to the usual random-coil models.