Polarization transfer by cross-correlated relaxation in solution NMR with very large molecules

In common multidimensional NMR experiments for studies of biological macromolecules in solution, magnetization transfers via spin-spin couplings [insensitive nuclei enhanced by polarization transfer (INEPT)] are key elements of the pulse schemes, For molecular weights beyond 100,000, transverse relaxation during the transfer time may become a limiting factor. This paper presents a transfer technique for work with big molecules, cross relaxation-enhanced polarization transfer (CRINEPT), which largely reduces the size limitation of INEPT transfers with the use of cross-correlated relaxation-induced polarization transfer. The rate of polarization transfer by cross-correlated relaxation is proportional to the rotational correlation time, so that it becomes a highly efficient transfer mechanism for solution NMR with very high molecular weights. As a first implementation, [N-15,H-1]-correlation experiments were designed that make use of cross-correlation between dipole-dipole coupling and chemical shift anisotropy of the N-15-H-1-moieties for both CRINEPT and transverse relaxation-optimized spectroscopy (TROSY). When compared with INEPT-based [N-15,H-1]-TROSY, these experiments yielded up to 3-fold signal enhancement for amide groups of a 110,000-Da protein in aqueous solution at 4 degrees C. CRINEPT opens avenues for solution NMR with supramolecular structures such as membrane proteins solubilized in micelles or lipid vesicles, proteins attached to nucleic acid fragments, or oligomeric proteins.