title: beta-turn conformations in crystal structures of model peptides containing alpha,alpha-di-n-propylglycine and alpha,alpha-di-n-butylglycine.
abstract: The crystal state conformations of three peptides containing the alpha,alpha-dialkylated residues, alpha,alpha-di-n-propylglycine (Dpg) and alpha,alpha-di-n-butylglycine (Dbg), have been established by x-ray diffraction. Boc-Ala-Dpg-Ala-OMe (I) and Boc-Ala-Dbg-Ala-OMe (II) adopt distorted type II beta-turn conformations with Ala (1) and Dpg/Dbg (2) as the corner residues. In both peptides the conformational angles at the Dxg residue (I: phi = 66.2 degrees, psi = 19.3 degrees; III: phi = 66.5 degrees, psi = 21.1 degrees) deviate appreciably from ideal values for the i + 2 residue in a type II beta-turn. In both peptides the observed (N...O) distances between the Boc CO and Ala (3) NH groups are far too long (I: 3.44 A; III: 3.63 A) for an intramolecular 4-->1 hydrogen bond. Boc-Ala-Dpg-Ala-NHMe (II) crystallizes with two independent molecules in the asymmetric unit. Both molecules IIA and IIB adopt consecutive beta-turn (type III-III in IIA and type III-I in IIB) or incipient 3(10)-helical structures, stabilized by two intramolecular 4-->1 hydrogen bonds. In all four molecules the bond angle N-C alpha-C' (tau) at the Dxg residues are > or = 110 degrees. The observation of conformational angles in the helical region of phi,psi space at these residues is consistent with theoretical predictions.
authors.affiliation: Department of Organic Chemistry, University of Padova, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Crisma
authors.forename: G
authors.initials: G
authors.lastname: Valle
authors.forename: C
authors.initials: C
authors.lastname: Toniolo
authors.forename: S
authors.initials: S
authors.lastname: Prasad
authors.forename: R B
authors.initials: RB
authors.lastname: Rao
authors.forename: P
authors.initials: P
authors.lastname: Balaram
mesh_terms: D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009646:Norleucine / Q000031:analogs & derivatives*; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013329:Structure-Activity Relationship; D014633:Valine / Q000031:analogs & derivatives*
chemical_list: D009842:Oligopeptides; C038263:dipropylglycine; C092836:alpha,alpha-di-n-butylglycine; D009646:Norleucine; D014633:Valine