L-alanine (LA, as an amino acid residue) pentamer model was used to investigate changes in the dihedral angle, intramolecular hydrogen bonding and formation energies during structural optimization. LA pentamers having four conformation types [β: φ/ψ=t−/t+, α: φ/ψ=g−/g−, PPII: φ/ψ=g−/t+ and P-like: φ/ψ= g−/g+] were carried out by quantum chemical calculations (QCC) [B3LYP/6-31G(d,p)]. In LA, β, α, and P-like types did not change by optimization, having an intra-molecular hydrogen bond: NH···OC (H-bond), and PPII types in the absence of H-bond were transformed into P-like at the designated ψ of 140°, and to β at that of 160° or 175°. P-like and α were about 0.5 kcal/mol/mu more stable than β. In order to understand the processes of the transformations, the changes of φ/ψ, distances of NH-OC (dNH/CO) and formation energies (ΔE, kcal/mol/mu) were examined.