A 2D MAS solid-state NMR method to recover the amplified heteronuclear dipolar and chemical shift anisotropic interactions

A two-dimensional solid-state NMR method for the measurement of chemical shift anisotropy tensors of X nuclei (¹⁵N or ¹³C) from multiple sites of a polypeptide powder sample is presented. This method employs rotor-synchronized π pulses to amplify the magnitude of the inhomogeneous X-CSA and ¹H-X dipolar coupling interactions. A combination of on-resonance and magic angle rf irradiation of protons is used to vary the ratio of the magnitudes of the ¹H-X dipolar and X-CSA interactions which are recovered under MAS, in addition to suppressing the ¹H-¹H dipolar interactions. The increased number of spinning sidebands in the recovered anisotropic interactions is useful to determine the CSA tensors accurately. The performance of this method is examined for powder samples of N-acetyl-¹⁵N-L-valine (NAV), N-acetyl-¹⁵N-L-valyl-¹⁵N-L-leucine (NAVL), and α-¹³C-L-leucine. The sources of experimental errors in the measurement of CSA tensors and the application of the pulse sequences under high-field fast MAS operations are discussed.