NMR determination of the torsion angle psi in alpha-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 -20 degree to -70 degree, 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 (13)C'--(15)N dipolar evolution period is followed by the C' to C(alpha) polarization transfer and by Lee--Goldburg cross polarization recoupling of the (13)C(alpha)(1)H dipolar interaction. The difference between the effective (13)C(1)H 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 approximately omega(CH)/omega(CN)) larger than the (13)C--(1)H 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-(13)C,(15)N] 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-(13)C,(15)N]Met-Leu-Phe-OMe.
Ladizhansky
Vladimir
V
Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Veshtort
Mikhail
M
Griffin
Robert G
RG
D003198:Computer Simulation; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008956:Models, Chemical; D010455:Peptides / Q000737:chemistry*; D011506:Proteins / Q000737:chemistry*
D010455:Peptides; D011506:Proteins