Determination of torsion angles in proteins and peptides using solid state NMR

A combination of Solid State NMR (SSNMR) dipolar recoupling and double quantum CSA-CSA correlation experiments are used to determine the Ramachandran angles phi and iota in a crystalline tripeptide (AGG) and a 14 amino acid peptide designed to be helical. The advantage of the SSNMR approach described herein is the ability to measure both phi and psi to high resolution using a single noncrystalline, doubly carbonyl labeled peptide. It is also shown that DRAWS and DQDRAWS data are insensitive to N-14-C-13 and N-15-C-13 dipolar couplings making corrections for these effects unnecessary. Extremes in secondary structure (e.g., alpha-helix vs beta-sheet) can be discerned by simple inspection of DQDRAWS spectra. Subtleties in secondary structure (alpha-helix vs 3(10)-helix) can be distinguished by simulation of the DQDRAWS spectrum.