We evaluate the effect of the amino acid mutations of glycine, leucine, valine, phenylalanine, serine, and proline for the 10th alanine in the capped peptide, acetly(Ala)(17)NH2, upon the energies of the a-helices and beta-strands using ONIOM DFT/AM1 molecular orbital calculations. The relative stabilities of the a-helix (to the beta-strand) derive from the differences between the effects upon not only the helix but the strand as well. Thus, Ala -> Pro significantly destabilizes both but destabilizes the a-helix more, while Ala -> Gly stabilizes both but stabilizes the beta-strand more. The theoretical results are discussed in the context of the known experimental reports. We suggest that the solvation of the unfolded state drives the helix/coil equilibrium in solution.