title: How many hydrogen-bonded α-turns are possible?
abstract: The formation of α-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 α-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 α-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 β- and γ-turns with better hydrogen bond geometries. Sometimes, this competition weakens or even destroys the i←(i + 4) hydrogen bond leading to very stable double β-turn structures. This is also the reason why an "ideal" α-turn with three central amino acids having the perfect backbone angle values of an α-helix could not be localized. There are numerous hints for stable α-turns with a distance between the C(α)-atoms of the first and fifth amino acid smaller than 6-7 Å, but without an i←(i + 4) hydrogen bond.
authors.affiliation: Institute of Biochemistry, Faculty of Biosciences, Pharmacy, and Psychology, University of Leipzig, Leipzig, Germany.
authors.forename: Anette
authors.initials: A
authors.lastname: Schreiber
authors.forename: Peter
authors.initials: P
authors.lastname: Schramm
authors.forename: Hans-Jörg
authors.initials: HJ
authors.lastname: Hofmann
mesh_terms: D000409:Alanine / Q000737:chemistry; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D003198:Computer Simulation; D005998:Glycine / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D013816:Thermodynamics
chemical_list: D000409:Alanine; D005998:Glycine