Polychromatic Selective Population Inversion for TROSY Experiments with Large Proteins

This paper presents polychromatic selective polarization inversion (PC−SPI) as an alternative to the polarization transfer methods recently developed for the application of NMR to large biological molecules. Theoretical and numerical considerations indicate that PC−SPI has the potential for more efficient polarization transfer under conditions of rapid transverse relaxation compared to J coupling- and cross-correlated relaxation-based transfers. The main advantage offered by the method presented here is the maintenance of near-optimal trajectories of inversion of the individual components of the spin magnetization while using broadband optimized pulses. A 2D experiment was implemented combining PC−SPI with TROSY-based chemical shift correlation. The experiment was applied to detect ¹⁵N−¹H chemical shift correlation spectra of a 200 kDa complex consisting of an 80% ²H- and uniformly ¹⁵N,¹³C-labeled 22 kDa portion of complement receptor type 1 and unlabeled C3b of complement (180 kDa).