Progress in C13 and H1 solid-state nuclear magnetic resonance for paramagnetic systems under very fast magic angle spinning
Authors: Wickramasinghe Nalinda P. , Shaibat Medhat A. , Jones Christopher R. , Casabianca Leah B. , de Dios Angel C. , Harwood John S. , Ishii Yoshitaka 
Published: February 2008

High-resolution solid-state NMR (SSNMR) of paramagnetic systems has been largely unexplored because of various technical difficulties due to large hyperfine shifts, which have limited the success of previous studies through depressed sensitivity/resolution and lack of suitable assignment methods. Our group recently introduced an approach using “very fast” magic angle spinning (VFMAS) for SSNMR of paramagnetic systems, which opened an avenue toward routine analyses of small paramagnetic systems by C13 and H1 SSNMR [Y. Ishii et al., J. Am. Chem. Soc. 125, 3438 (2003); N. P. Wickramasinghe et al., ibid. 127, 5796 (2005)]. In this review, we discuss our recent progress in establishing this approach, which offers solutions to a series of problems associated with large hyperfine shifts. First, we demonstrate that MAS at a spinning speed of 20kHz or higher greatly improves sensitivity and resolution in both H1 and C13 SSNMR for paramagnetic systems such as Cu(II)(DL-alanine)2⋅H2O (Cu(DL-Ala)2) and Mn(acac)3, for which the spectral dispersions due to H1 hyperfine shifts reach 200 and 700ppm, respectively. Then, we introduce polarization transfer methods from H1 spins to C13 spins with high-power cross polarization and dipolar insensitive nuclei enhanced by polarization transfer (INEPT) in order to attain further sensitivity enhancement and to correlate H1 and C13 spins in two-dimensional (2D) SSNMR for the paramagnetic systems. Comparison of C13 VFMAS SSNMR spectra with C13 solution NMR spectra revealed superior sensitivity in SSNMR for Cu(DL-Ala)2, Cu(Gly)2, and V(acac)3. We discuss signal assignment methods using one-dimensional (1D) C13 SSNMR C13–H1 rotational echo double resonance (REDOR) and dipolar INEPT methods and 2D C13∕H1 correlation SSNMR under VFMAS, which yield reliable assignments of H1 and C13 resonances for Cu(Ala-Thr). Based on the excellent sensitivity/resolution and signal assignments attained in the VFMAS approach, we discuss methods of elucidating multiple distance constraints in unlabeled paramagnetic systems by combing simple measurements of C13 T1 values and anisotropic hyperfine shifts. Comparison of experimental C13 hyperfine shifts and ab initio calculated shifts for α- and β-forms of Cu(8-quinolinol)2 demonstrates that C13 hyperfine shifts are parameters exceptionally sensitive to small structural difference between the two polymorphs. Finally, we discuss sensitivity enhancement with paramagnetic ion doping in C13 SSNMR of nonparamagnetic proteins in microcrystals. Fast recycling with exceptionally short recycle delays matched to short H1 T1 of ∼60ms in the presence of Cu(II) doping accelerated 1D C13 SSNMR for ubiquitin and lysozyme by a factor of 7.3–8.4 under fast MAS at a spinning speed of 40kHz. It is likely that the VFMAS approach and use of paramagnetic interactions are applicable to a variety of paramagnetic systems and nonparamagnetic biomolecules.