title: Consistent helicities from CD and template t/c data for N-templated polyalanines: progress toward resolution of the alanine helicity problem.
abstract: The helicity reporting parameters t/c and [theta](222) have been measured at 2, 25, and 60 degrees C in water for the solubilized polyalanine series Ac-Hel-A(n)-(t)LInp(2)K(4)W-NH(2) of length 4 < or = n < or = 14 that bears the helix-initiating and monitoring N-cap Ac-Hel and the spaced solubilizer (t)LInp(2)K(4)W-NH(2) as a C-cap. Correlation of t/c with length shows that the helical propensity for n < or = 6 is ca. 1.0, consistent with our early reports, but that a dramatic increase in temperature dependence and helical propensity occurs for n > or = 8. A model based on hydrogen-bonding cooperativity is proposed to explain this finding, and both t/c and [theta](222) are modeled successfully by length-dependent alanine propensities at 2 degrees C of 1.03 for n = 6, 1.15, for 7 < or = n < or = 9 and 1.26 for n > or = 10. The implications of these results for the energetics of helix formation by alanine-rich peptide sequences are discussed.
authors.affiliation: Department of Chemistry, Room 18-582, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Kennedy
authors.forename: Kwok-Yin
authors.initials: KY
authors.lastname: Tsang
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Kemp
mesh_terms: D000409:Alanine / Q000737:chemistry*; D000465:Algorithms; D002942:Circular Dichroism; D008956:Models, Chemical; D009842:Oligopeptides / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D012996:Solutions; D013698:Templates, Genetic; D014867:Water / Q000737:chemistry
chemical_list: D009842:Oligopeptides; D010455:Peptides; D012996:Solutions; D014867:Water; C019529:polyalanine; D000409:Alanine