title: Comparison of helix-stabilizing effects of alpha,alpha-dialkyl glycines with linear and cycloalkyl side chains.
abstract: The ability of alpha, alpha-di-n-alkyl glycines with linear and cyclic alkyl side chains to stabilize helical conformations has been compared using a model heptapeptide sequence. The conformations of five synthetic heptapeptides (Boc-Val-Ala-Leu-Xxx-Val-Ala-Leu-OMe, Xxx = Ac8c, Ac7c, Aib, Dpg, and Deg, where Ac8c = 1-aminocyclooctane-1-carboxylic acid, Ac7c = 1-aminocycloheptane-1-carboxylic acid, Aib = alpha-aminoisobutyric acid, Dpg = alpha,alpha-di-n-propyl glycine, Deg = alpha,alpha-di-n-ethyl glycine) have been investigated. In crystals, helical conformations have been demonstrated by x-ray crystallography for the peptides, R-Val-Ala-Leu-Dpg-Val-Ala-Leu-OMe, (R = Boc and acetyl). Solution conformations of the five peptides have been studied by 1H-nmr. In the apolar solvent CDCl3, all five peptides favor helical conformations in which the NH groups of residues 3-7 are shielded from the solvent. Successive NiH<-->Ni + 1H nuclear Overhauser effects over the length of the sequence support a major population of continuous helical conformations. Solvent titration experiments in mixtures of CDCl3/DMSO provide evidence for solvent-dependent conformational transitions that are more pronounced for the Deg and Dpg peptides. Solvent-dependent chemical shift variations and temperature coefficients in DMSO suggest that the conformational distributions in the Deg/Dpg peptides are distinctly different from the Aib/Acnc peptides in a strongly solvating medium. Nuclear Overhauser effects provide additional evidence for the population of extended backbone conformations in the Dpg peptide, while a significant residual population of helical conformations is still detectable in the isomeric Ac7c peptide in DMSO.
authors.affiliation: Department of Chemistry, Banaras Hindu University, Varanasi, India.
authors.forename: S
authors.initials: S
authors.lastname: Vijayalakshmi
authors.forename: R B
authors.initials: RB
authors.lastname: Rao
authors.forename: I L
authors.initials: IL
authors.lastname: Karle
authors.forename: P
authors.initials: P
authors.lastname: Balaram
mesh_terms: D018360:Crystallography, X-Ray; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D013329:Structure-Activity Relationship
chemical_list: D009842:Oligopeptides; D005998:Glycine