End effects influence short model peptide conformation

Previously, we derived a P _II propensity scale using N- and C-terminally blocked host-guest peptide model AcGGXGGNH ₂ (X * Gly) and concluded that P _II represents a dominant conformation in the majority of this series of 19 peptides (Shi et al. Proc. Natl. Acad. Sci. U.S.A.2005, 102, 17964-17968). Recently, Schweitzer-Stenner and co-workers examined a series of eight short host-guest tripeptides with the sequence GXG (X = A, V, F, S, E, L, M, and K) in which both N- and C-ends were unblocked and reported major differences in P _II content for F, V, and S compared to our scale (Hagarman et al. J. Am. Chem. Soc.2010, 132, 540-551). We have investigated four representative amino acids (X = A, V, F, and S) in three series of peptides (GXG, AcGXGNH ₂, and AcGGXGGNH ₂) as a function of pH in this study. Our data show that P _II content in the GXG series (X = A, V, F, and S) is pH-dependent and that the conformations of each amino acid differ markedly between the GXG and AcGXGNH ₂/ AcGGXGGNH ₂ series. Our results indicate that P _II scales are sequence and context dependent and the presence of proximal charged end groups exerts a strong effect on P _II population in short model peptides.