title: Water-solubilized, cap-stabilized, helical polyalanines: calibration standards for NMR and CD analyses.
abstract: NMR and CD studies are reported for two length series of solubilized, spaced, highly helical polyalanines that are N-capped by the optimal helix stabilizer (beta)Asp-Hel and C-capped by beta-aminoalanine beta and that are studied in water at 2 degrees C, pH 1-8. NMR analysis yields a structural characterization of the peptide Ac(beta)AspHelAla(8)betaNH(2) and selected members of one (beta)AspHelAla(n)beta series. At pH > 4.5 the (beta)AspHel cap provides a preorganized triad of carboxylate anion and two amide residues that is complementary to the helical polyalanine N-terminus. The C-terminal beta-aminoalanine assumes a helix-stabilizing conformation consistent with literature precedents. H(N)CO NMR experiments applied to capped, uniformly (13)C- and (15)N-labeled Ala(8) and Ala(12) peptides define Ala(n) hydrogen bonding signatures as alpha-helical without detectable 3(10) character. Relative NH-->ND exchange rates yield site protection factors PF(i) that define uniquely high fractional helicities FH for the peptide Ala(n) regions. These Ala(n) calibration series, studied in water and lacking helix-stabilizing tertiary structure, yield the first (13)C NMR chemical shifts, (3)J(HNH)(alpha) coupling constants, and CD ellipticities [theta(Molar)](lambda,n) characteristic of a fully helical alanine within an Ala(n) context. CD data are used to assign parameters X and [theta](lambda,infinity), required for rigorous calculation of FH values from CD ellipticities.
authors.affiliation: Department of Chemistry, Room 18-296, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Björn
authors.initials: B
authors.lastname: Heitmann
authors.forename: Gabriel E
authors.initials: GE
authors.lastname: Job
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Kennedy
authors.forename: Sharon M
authors.initials: SM
authors.lastname: Walker
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Kemp
mesh_terms: D002138:Calibration; D002247:Carbon Isotopes; D002942:Circular Dichroism / Q000379:methods* / Q000592:standards; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D011522:Protons; D012995:Solubility; D014867:Water / Q000737:chemistry
chemical_list: D002247:Carbon Isotopes; D010455:Peptides; D011522:Protons; D014867:Water; C019529:polyalanine