Determination of the chemical shift tensor anisotropy and asymmetry of strongly dipolar coupled protons under fast MAS

Abstract

<p>Orientationally-dependent interactions such as dipolar coupling, quadrupolar coupling, and chemical shift anisotropy (CSA) contain a wealth of spatial information that can be used to elucidate molecular conformations and dynamics. To determine the sign of the chemical shift tensor anisotropy parameter (delta aniso), both the |m| = 1 and |m| = 2 components of the CSA need to be symmetry allowed, while the recoupling of the |m| = 1 term is accompanied with the reintroduction of homonuclear dipolar coupling components. Therefore, previously suggested sequences which solely recouple the |m| = 2 term cannot determine the sign a 1H's delta aniso in a denselycoupled network. In this study, we demonstrate the CSA recoupling of strongly dipolar coupled 1H spins using the Cn1n(9003601805400360180900) sequence. This pulse scheme recouples both the |m| = 1 and |m| = 2 CSA terms but the scaling factors for the homonuclear dipolar coupling terms are zeroed. Consequently, the sequence is sensitive to the sign of delta aniso but is not influenced by homonuclear dipolar interactions.</p>