NMR determination of the torsion angle Ψ in α-helical peptides and proteins:: The HCCN dipolar correlation experiment

Several existing methods permit measurement of the torsion angles phi, psi and chi in peptides and proteins with solid-state MAS NMR experiments. Currently, however, there is not an approach that is applicable to measurement of psi in the angular range -20degrees to -70degrees, commonly found in alpha-helical structures. Accordingly, we have developed a HCCN dipolar correlation MAS experiment that is sensitive and accurate in this regime. An initial REDOR driven C-13'-N-15 dipolar evolution period is followed by the C' to C-alpha polarization transfer and by Lee-Goldburg cross polarization recoupling of the C-13(alpha) H-1 dipolar interaction. The difference between the effective (CH)-C-13-H-1 and (CN)-C-13-N-15 dipolar interaction strengths is balanced out by incrementing the C-13-N-15 dipolar evolution period in steps that are a factor of R(R similar to omega(CH)/omega(CN)) larger than the C-13-H-1 steps. The resulting dephasing curves are sensitive to variations in psi in the angular region associated with alpha-helical secondary structure. To demonstrate the validity of the technique, we apply it to N-formyl-[U-C-13,N-15] Met-Leu-Phe-OH (MLF). The value of psi 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-C-13,N-15]Met-Leu-Phe-OMe. (C) 2002 Elsevier Science (USA).