title: Temperature- and length-dependent energetics of formation for polyalanine helices in water: assignment of w(Ala)(n,T) and temperature-dependent CD ellipticity standards.
abstract: Length-dependent helical propensities w(Ala)(n,T) at T = 10, 25, and 60 degrees C are assigned from t/c values and NMR 13C chemical shifts for series 1 peptides TrpLys(m)Inp2(t)Leu-Ala(n)(t)LeuInp2Lys(m)NH2, n = 15, 19, and 25, m = 5, in water. Van't Hoff analysis of w(Ala)(n,T) show that alpha-helix formation is primarily enthalpy-driven. For series 2 peptides Ac-Trp Lys5Inp2(t)Leu-(beta)AspHel-Ala(n)-beta-(t)LeuInp2Lys5NH2, n = 12 and 22, which contain exceptionally helical Ala(n) cores, protection factor-derived fractional helicities FH are assigned in the range 10-30 degrees C in water and used to calibrate temperature-dependent CD ellipticities [theta](lambda,H,n,T). These are applied to CD data for series 1 peptides, 12 < or = n < or = 45, to confirm the w(Ala)(n,T) assignments at T = 25 and 60 degrees C. The [theta](lambda,H,n,T) are temperature dependent within the wavelength region, 222 +/- 12 nm, and yield a temperature correction for calculation of FH from experimental values of [theta](222,n,T,Exp).
authors.affiliation: Department of Chemistry, Room 18-296, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Gabriel E
authors.initials: GE
authors.lastname: Job
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Kennedy
authors.forename: Björn
authors.initials: B
authors.lastname: Heitmann
authors.forename: Justin S
authors.initials: JS
authors.lastname: Miller
authors.forename: Sharon M
authors.initials: SM
authors.lastname: Walker
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Kemp
mesh_terms: D002942:Circular Dichroism; D003198:Computer Simulation; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015394:Molecular Structure; D010455:Peptides / Q000138:chemical synthesis* / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D010455:Peptides; D014867:Water; C019529:polyalanine