Water-Solubilized,Cap-Stabilized,Helical Polyalanines:Calibration Standards for NMR and CD Analyses

摘要

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 ~betaAsp-Hel and C-capped by beta-aminoalanine beta and that are studied in water at 2 deg C,pH 1 -8.NMR analysis yields a structural characterization of the peptide Ac~betaAspHelAla_8betaNH_2 and selected members of one ~betaAspHelAla_nbeta series.At pH > 4.5 the ~betaAspHel 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 ~13C- and ~15N-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,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 ~13C NMR chemical shifts,~3J_(HNHalpha)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.