How many hydrogen-bonded α-turns are possible?

The formation of alpha-turns is a possibility to reverse the direction of peptide sequences via five amino acids. In this paper, a systematic conformational analysis was performed to find the possible isolated alpha-turns with a hydrogen bond between the first and fifth amino acid employing the methods of ab initio MO theory in vacuum (HF/6-31G*, B3LYP/6-311 + G*) and in solution (CPCM/HF/6-31G*). Only few alpha-turn structures with glycine and alanine backbones fulfill the geometry criteria for the ia dagger(i + 4) hydrogen bond satisfactorily. The most stable representatives agree with structures found in the Protein Data Bank. There is a general tendency to form additional hydrogen bonds for smaller pseudocycles corresponding to beta- and gamma-turns with better hydrogen bond geometries. Sometimes, this competition weakens or even destroys the ia dagger(i + 4) hydrogen bond leading to very stable double beta-turn structures. This is also the reason why an "ideal" alpha-turn with three central amino acids having the perfect backbone angle values of an alpha-helix could not be localized. There are numerous hints for stable alpha-turns with a distance between the C-alpha-atoms of the first and fifth amino acid smaller than 6-7 , but without an ia dagger(i + 4) hydrogen bond.