Cα chemical shift tensors in helical peptides by dipolar-modulated chemical shift recoupling NMR

The Cα chemical shift tensors of proteins contain information on the backbone conformation. We have determined the magnitude and orientation of the Cα chemical shift tensors of two peptides with α-helical torsion angles: the Ala residue in G*AL (φ=−65.7°, ψ=−40°), and the Val residue in GG*V (φ=−81.5°, ψ=−50.7°). 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 Cα–Hα and Cα–N dipolar modulated powder patterns. The helical Ala Cα chemical shift tensor has a span of 36 ppm and an asymmetry parameter of 0.89. Its σ11 axis is 116° ± 5° from the Cα–Hα bond while the σ22 axis is 40° ± 5° from the Cα–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 β-sheet Val found recently (Yao and Hong, 2002). The Val σ33 axis is tilted by 115° ± 5° from the Cα–Hα bond and 98° ± 5° from the Cα–N bond. These represent the first completely experimentally determined Cα 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 Cα chemical shift tensor onto the Cα–Hα bond is much smaller in α-helices than in β-sheets.