NMR Determination of the Torsion Angle ψ in α-Helical Peptides and Proteins: The HCCN Dipolar Correlation Experiment

摘要

Severla existing methods permit measurement of the torsion angles φ, ψ and χ in peptides and proteins with solid-state MAS NMR experiments. Currently, however, there is not an approach that is applicable to measurement of ψ in the angular range -20° to -70°, commonly found in α-helical structures. Accordingly, we have developed a HCCN dipolar correlation MAS experiment that is sensitive and accurate in this regime. An initial REDOR driven ~(13)C'-~(15)N dipolar evolution period is followed by the C' to C_α polarization transfer and by Lee-Goldburg cross polarization recoupling of the ~(13)C_α~1H dipolar interaction. The difference between the effective ~(13)C~1H and ~(13)C~(15)N dipolar interaction strengths is balanced out by incrementing the ~(13)C-~(15)N dipolar evolution period in steps that are a factor of R(R ～ ω_(CH)/ω_(CN)) larger than the ~(13)C-~1H steps. The resulting dephasing curves are sensitive to variations in ω in the angular region associated with α-helical secondary structure. To demonstrate the validity of the technique, we apply it to N-formyl-[U-~(13)C,~(15)N] Met-Leu-Phe-OH (MLF). The value of ψ extracted is consistent with the previous NMR measurements and close to that reported in diffraction studies for the methyl ester of MLF, N-formyl-[U-~(13)C,~(15)N]Met-Leu-Phe-OMe.