Conformational Preferences of Gas-Phase Helices: Experiment and Theory Struggle to Agree: The Seven-Residue Peptide Ac-Phe-(Ala)5-Lys-H+

The physical chemistry of bio molecules in the gas phase is progressing at a rapid pace. One of the most promising areas is the determination of the conformations of biologically important molecules. Quantitative determination of their relative energies is a difficult experimental problem. Although triple-resonance methods exist to measure to within 1 kJmol-1 the barriers to conformational isomerization, only in a select few cases has the information derived from those measurements been completed enough to allow deduction of the conformers relative energies. Because the electronic excitation in the helices seems to be localized on the phenylalanine ring in all four conformers, the latter assumption is reasonable. A final possible concern may be that for a short peptide in the gas phase, an ensemble of structures is needed to represent every single subpopulation.