Theoretical Studies on Shortest alpha-Helix

We present the fully optimized structures of 4-residue and 5-residue alpha-helices at the B3LYP/6-31 + G{*} {*} level in solvent. The calculated backbone conformations are in good agreement with the statistical results from protein crystal structures. Similar to long regular alpha-helices, the fraying of the C-teriminal is observed. For short polyalamine peptide. 3(10)-helix is significantly more stable than alpha-helix in enthalpy. However, the 3(10)-helix is destabilized by entropy effect. In addition, it has been found that the capping effect by an aspartic acid at N-2 (second residue from the N-terminal) significantly stabilizes the alpha-helix over the 3(10)-helix. Thus, the statistics that there are more shortest alpha-helices than 3(10)-helices of the same length can be understood. It has been also found that the C-terminal of the shortest alpha-helix tends to end with a beta-turn structure