Solid-state NMR determination of 13Cα chemical shift anisotropies for the identification of protein secondary structure

A solid-state nuclear magnetic resonance (NMR) method for the site-resolved identification of the secondary structure of solid peptides and proteins is presented. This technique exploits the correlation between the backbone conformation and the Ca chemical shift anisotropies (CSA) of proteins. The C-13 alpha CSAs are measured under fast magic-angle-spinning using a new sequence of sixteen 180 degrees pulses with special timing to reintroduce the CSA interaction selectively. Quantitative values of the shielding anisotropies are determined from the magnetization decay, as demonstrated for several amino acids. To achieve high-resolution spectra, this CSA filter experiment is combined with 2D N-15-C-13 correlation spectroscopy. Applied to selectively and extensively C-13-labeled and uniformly N-15-labeled ubiquitin at the largest dephasing time, the 2D experiment yields a spectral pattern that corresponds primarily to alpha-helical residues. This agrees with the previous finding that helical residues have smaller CSAs than sheet residues. However, the quantitative CSA differences between the helical and sheet conformations are less pronounced than indicated by solution-state NMR. This CSA filter technique provides an efficient and site-resolved method for characterizing the secondary structure of extensively isotopically labeled proteins.