Multidimensional magic angle spinning NMR spectroscopy for site-resolved measurement of proton chemical shift anisotropy in biological solids

The proton chemical shift (CS) tensor is a sensitive probe of structure and hydrogen bonding. Highly accurate quantum-chemical protocols exist for computation of (1)H magnetic shieldings in the various contexts, making proton chemical shifts potentially a powerful predictor of structural and electronic properties. However, (1)H CS tensors are not yet widely used in protein structure calculation due to scarcity of experimental data. While isotropic proton shifts can be readily measured in proteins even in the solid state, determination of the (1)H chemical shift anisotropy (CSA) tensors remains challenging, particularly in molecules containing multiple proton sites. We present a method for site-resolved measurement of amide proton CSAs in fully protonated solids under magic angle spinning. The approach consists of three concomitant 3D experiments yielding spectra determined by either mainly (1)H CSA, mainly (1)H–(15)N dipolar, or combined (1)H CSA and (1)H–(15)N dipolar interactions. The anisotropic interactions are recoupled using RN-sequences of appropriate symmetry, such as R12(1)(4), and (15)N/(13)C isotropic CS dimensions are introduced via a short selective (1)H–(15)N cross-polarization step. Accurate (1)H chemical shift tensor parameters are extracted by simultaneous fit of the lineshapes recorded in the three spectra. An application of this method is presented for an 89-residue protein, U-(13)C,(15)N-CAP-Gly domain of dynactin. The CSA parameters determined from the triple fits correlate with the hydrogen-bonding distances, and the trends are in excellent agreement with the prior solution NMR results. This approach is generally suited for recording proton CSA parameters in various biological and organic systems, including protein assemblies and nucleic acids.