Previously, we derived a PII propensity scale using N- and C-terminally blocked host–guest peptide model AcGGXGGNH2 (X ≠ Gly) and concluded that PII 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 PII 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, AcGXGNH2, and AcGGXGGNH2) as a function of pH in this study. Our data show that PII 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 AcGXGNH2/AcGGXGGNH2 series. Our results indicate that PII scales are sequence and context dependent and the presence of proximal charged end groups exerts a strong effect on PII population in short model peptides.