How many hydrogen-bonded α-turns are possible?

Abstract

The formation of -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 -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 -turn structures with glycine and alanine backbones fulfill the geometry criteria for the i(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 -and -turns with better hydrogen bond geometries. Sometimes, this competition weakens or even destroys the i(i+4) hydrogen bond leading to very stable double -turn structures. This is also the reason why an "ideal" -turn with three central amino acids having the perfect backbone angle values of an -helix could not be localized. There are numerous hints for stable -turns with a distance between the C  -atoms of the first and fifth amino acid smaller than 6-7 Å, but without an i(i+4) hydrogen bond.

Hydrogen Bonded Squaramide-Based Foldable Module Induces Both β- and α-Turns in Hairpin Structures of α-Peptides in Water

“…In this case as in nature, the αSQ-turn is internally stabilized by a βSQ-turn, Figure . The formation of the αSQ-turn is favored by the presence of glycine in the turn segment that allows fulfilling the geometry due its lack of bulky side chain …”