title: The helical s constant for alanine in water derived from template-nucleated helices.
abstract: Formation of alpha helices from disordered polypeptides depends on the degree to which amino acids favour the helical state. The folding of helical oligopeptides can be modelled by two parameters: sigma which reflects helix initiation and s which reflects propagation of a pre-existing helix and measures helical bias. Scheraga has reported s values for oligopeptides of about 1.1, implying a weak helical bias for amino-acid residues. By contrast, certain helical peptides studied by Baldwin seem to require much larger s values for alanine. Resolution of this inconsistency requires experiments that disentangle the ease of propagation from that of initiation. In this study varying lengths of polyalanine are linked to a 'template' that initiates helical structure and permits study solely of propagation. We report here that the s value for alanine in water is close to 1, supporting the earlier results of Scheraga but not the more recent results of Baldwin.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge 02139.
authors.forename: D S
authors.initials: DS
authors.lastname: Kemp
authors.forename: J G
authors.initials: JG
authors.lastname: Boyd
authors.forename: C C
authors.initials: CC
authors.lastname: Muendel
mesh_terms: D000409:Alanine / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D013698:Templates, Genetic; D014867:Water; D014961:X-Ray Diffraction
chemical_list: D010455:Peptides; D014867:Water; D000409:Alanine