title: Short alanine-based peptides may form 3(10)-helices and not alpha-helices in aqueous solution.
abstract: Short alanine peptides, containing 16 or 17 residues, appear to form alpha-helices in aqueous solution. But the main spectroscopic analyses used on helical peptides (circular dichroism and nuclear magnetic resonance) cannot distinguish between an alpha-helix (in which the ith residue is hydrogen-bonded to residue i + 4; ref. 9) and the next most common peptide helix, the 3(10)-helix10 (i-->i + 3 hydrogen-bonding). To address this problem we have designed single and doubly spin-labelled analogues of alanine-based peptides in which the nitroxide spin label forms an unbranched side chain extending from the sulphur atom of a cysteine residue. Here we report the circular dichroism, Fourier-transform infrared and electron-spin resonance spectra of these peptides under helix-forming conditions. The infrared absorbance gives an amide I' band with a frequency that is substantially different from that observed for alpha-helices. The electron-spin resonance spectra of doubly labelled helices show that the ranking of distances between side chains, around a single turn (residues 4-8), is inconsistent with an alpha-helical structure. Our experiments suggest that the more likely peptide geometry is a 3(10)-helix.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Santa Cruz 95064.
authors.forename: S M
authors.initials: SM
authors.lastname: Miick
authors.forename: G V
authors.initials: GV
authors.lastname: Martinez
authors.forename: W R
authors.initials: WR
authors.lastname: Fiori
authors.forename: A P
authors.initials: AP
authors.lastname: Todd
authors.forename: G L
authors.initials: GL
authors.lastname: Millhauser
mesh_terms: D000409:Alanine / Q000737:chemistry*; D000595:Amino Acid Sequence; D002942:Circular Dichroism; D004578:Electron Spin Resonance Spectroscopy; D005583:Fourier Analysis; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013055:Spectrophotometry, Infrared; D013113:Spin Labels
chemical_list: D010455:Peptides; D013113:Spin Labels; D000409:Alanine