C(alpha) chemical shift tensors in helical peptides by dipolar-modulated chemical shift recoupling NMR |
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Authors: | Yao Xiaolan Yamaguchi Satoru Hong Mei |
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Institution: | (1) Department of Chemistry, Iowa State University, Ames, IA 50011, USA;(2) Present address: Department of Life Science, Himeji Institute of Technology, Harima Science Garden City, Kamigori, Hyogo, 678-1297, Japan |
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Abstract: | The C chemical shift tensors of proteins contain information on the backbone conformation. We have determined the magnitude and orientation of the C chemical shift tensors of two peptides with -helical torsion angles: the Ala residue in G*AL (=–65.7°, =–40°), and the Val residue in GG*V (=–81.5°, =–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 C–H and C–N dipolar modulated powder patterns. The helical Ala C chemical shift tensor has a span of 36 ppm and an asymmetry parameter of 0.89. Its 11 axis is 116° ± 5° from the C–H bond while the 22 axis is 40° ± 5° from the C–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 -sheet Val found recently (Yao and Hong, 2002). The Val 33 axis is tilted by 115° ± 5° from the C–H bond and 98° ± 5° from the C–N bond. These represent the first completely experimentally determined C 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 chemical shift tensor onto the C–H bond is much smaller in -helices than in -sheets. |
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Keywords: | chemical shift tensor dipolar modulation helical peptides icosahedral representation solid-state NMR |
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