C(alpha) chemical shift tensors in helical peptides by dipolar-modulated chemical shift recoupling NMR.

The C(alpha) chemical shift tensors of proteins contain information on the backbone conformation. We have determined the magnitude and orientation of the C(alpha) chemical shift tensors of two peptides with a-helical torsion angles: the Ala residue in G*AL (phi = -65.7 degrees, psi = -40 degrees), and the Val residue in GG*V (phi = -81.5 degrees, psi = -50.7 degrees). The magnitude of the tensors was determined from quasi-static powder patterns recoupled under magic-angle spinning, while the orientation of the tensors was extracted from C(alpha)-H(alpha) and C(alpha)-N dipolar modulated powder patterns. The helical Ala C(alpha) chemical shift tensor has a span of 36 ppm and an asymmetry parameter of 0.89. Its sigma11 axis is 116 degrees +/- 5 degrees from the C(alpha)-H(alpha) bond while the sigma22 axis is 40 degrees +/- 5 degrees from the C(alpha)-N bond. The Val tensor has an anisotropic span of 25 ppm and an asymmetry parameter of 0.33, both much smaller than the values for beta-sheet Val found recently (Yao and Hong, 2002). The Val sigma33 axis is tilted by 115 degrees +/- 50 from the Ca-Halpha bond and 98 degrees +/- 5 degrees from the C(alpha)-N bond. These represent the first completely experimentally determined C(alpha) chemical shift tensors of helical peptides. Using an icosahedral representation, we compared the experimental chemical shift tensors with quantum chemical calculations and found overall good agreement. These solid-state chemical shift tensors confirm the observation from cross-correlated relaxation experiments that the projection of the C(alpha) chemical shift tensor onto the C(alpha)-H(alpha) bond is much smaller in alpha-helices than in beta-sheets.