title: Conformational investigation of alpha,beta-dehydropeptides. XVI. Beta-turn tendency in Ac-Pro-DeltaXaa-NHMe: crystallographic and theoretical studies.
abstract: The crystal structures of two diastereomeric alpha,beta-dehydrobutyrine peptides Ac-Pro-(Z)-DeltaAbu-NHMe (I) and Ac-Pro-(E)-DeltaAbu-NHMe (II) have been determined. Both dehydropeptides adopt betaI-turn conformation characterized by the pairs of (phi(i+1), psi(i+1)) and (phi(i+2), psi(i+2)) angles as -66, -19, -97, 11 degrees for I and -59, -27, -119, 29 degrees for II. In each peptide, the betaI turn is stabilized by (i + 3) --> i intramolecular hydrogen bonds with N...O distance of 3.12 A for I and 2.93 A for II. These structures have been compared to the crystal structures of homologous peptides Ac-Pro-DeltaVal-NHMe and Ac-Pro-DeltaAla-NHMe. Theoretical analyses by DFT/B3LYP/6-31 + G** method of conformers formed by these four peptides and by the saturated peptide Ac-Pro-Ala-NHMe revealed that peptides with a (Z) substituent at the C(beta) (i+2) atom of dehydroamino acid, i.e. Ac-Pro-DeltaVal-NHMe and Ac-Pro-(Z)-DeltaAbu-NHMe, predominantly form beta turns, both in vacuo and in polar environment. The tendency to adopt beta-turn conformation is much weaker for the peptides lacking the (Z) substituent, Ac-Pro-(E)-DeltaAbu-NHMe and Ac-Pro-DeltaAla-NHMe. The latter adopts a semi-extended or an extended conformation in every polar environment, including a weakly polar solvent. The saturated peptide Ac-Pro-Ala-NHMe in vacuo prefers a beta-turn conformation, but in polar environment the differences between various conformers are small. The role of pi-electron correlation and intramolecular hydrogen bonds interaction in stabilizing the hairpin structures are discussed.
authors.affiliation: Institute of Chemistry, University of Opole, Olesja Street 48, 45-052 Opole, Poland. broda@uni-opole.pl
authors.forename: Malgorzata A
authors.initials: MA
authors.lastname: Broda
authors.forename: Ewa M
authors.initials: EM
authors.lastname: Ciszak
authors.forename: Anna E
authors.initials: AE
authors.lastname: Koziol
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Pietrzynski
authors.forename: Barbara
authors.initials: B
authors.lastname: Rzeszotarska
mesh_terms: D003198:Computer Simulation; D018360:Crystallography, X-Ray / Q000379:methods*; D006860:Hydrogen Bonding; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012997:Solvents / Q000737:chemistry
chemical_list: D010455:Peptides; D012997:Solvents