| Abstract
| - The proton NMR spectra of unlabeled alanine dipeptide (Ac-L-Ala-NHMe) at 300 MHz and of alanine dipeptidewith a single 13C label at 500 MHz are obtained in the lyotropic liquid-crystalline solvent cesiumpentadecafluorooctanoate in water (CsPFO/H2O). Simulations of the spectra yield 9 and 13 dipolar couplingsDij, respectively, many with absolute sign determined. We fit the set of dipolar couplings by systematicallyvarying the flexible dihedral angles φ and ψ while freezing local geometric details from the electronic structurecalculations of Suhai and co-workers (Han, W. G.; Jalkanen, K. J.; Elstner, M.; Suhai, S. J. Phys. Chem. B1998, 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. Theorientation tensor can be understood in terms of a simple rocking motion that dips the central methyl groupinto the fluorocarbon core of the CsPFO bicelle while alternately exposing both hydrogen-bonding pocketsof 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 thepeptide within the PII well rather than the broad sampling of φ in the range from −60° to −180° (a “β/PIIminimum”) found by certain models. We suggest that the PII geometry is stable primarily because it maximizesthe opportunity for peptide−water cooperative hydrogen bonding, whereas the αR geometry is stable primarilybecause of its large dipole moment. Our result corroborates recent work on short polypeptides suggestingthat they preferentially sample configurations that fluctuate about PII-like structures, in contrast to the usualrandom-coil models.
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