Theoretical studies on shortest ��-helix

We present the fully optimized structures of 4-residue and 5-residue ??-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 ??-helices, the fraying of the C-terminal is observed. For short polyalanine peptide, 310-helix is significantly more stable than ??-helix in enthalpy. However, the 310-helix is destabilized by entropy effect. In addition, it has been found that the capping effect by an aspartic acid at N2 (second residue from the N-terminal) significantly stabilizes the ??-helix over the 310-helix. Thus, the statistics that there are more shortest ??-helices than 310-helices of the same length can be understood. It has been also found that the C-terminal of the shortest ??-helix tends to end with a ??-turn structure.EI0122371-23762