Water-solubilized, cap-stabilized, helical polyalanines: calibration standards for NMR and CD analyses

J Am Chem Soc. 2005 Feb 16;127(6):1690-704. doi: 10.1021/ja0457462.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Calibration
  • Carbon Isotopes
  • Circular Dichroism / methods*
  • Circular Dichroism / standards
  • Hydrogen-Ion Concentration
  • Models, Molecular
  • Nuclear Magnetic Resonance, Biomolecular / methods*
  • Peptides / chemistry*
  • Protein Structure, Secondary
  • Protons
  • Solubility
  • Water / chemistry

Substances

  • Carbon Isotopes
  • Peptides
  • Protons
  • Water
  • polyalanine