Rocking motion in solid proteins studied by the 15 N proton-decoupled R 1ρ relaxometry.

Recently it has been revealed that proteins in solid samples undergo slow overall rocking. The parameters of this motion depend on intermolecular interactions. Therefore, the characterization of the rocking motion enables one to investigate protein-protein interactions. NMR R _1ρ relaxometry is the most suitable tool to study slow molecular motions. However, the time scale of the rocking motion is on the edge of the dynamics window of the standard R _1ρ experiment, precluding the R _1ρ data analysis from being precise and reliable. In this work, we apply a modified R _1ρ relaxation method to characterize the slow motion in solids with much higher precision and reliability. The modification is the simultaneous use of a strong ¹ H-CW pulse and a weak/moderate ¹⁵ N spin-lock pulse. We demonstrate theoretically and experimentally that under this condition, R _1ρ decays have a significantly better signal-to-noise ratio and a much shorter "dead time" caused by the initial oscillations compared to the conventional R _1ρ experiment. Moreover, the proton-decoupled R _1ρ 's can be measured at a much smaller difference between the spin-lock and MAS frequencies; thus, much slower molecular motions can be sampled. The proton decoupling during the ¹⁵ N spin-lock pulse also suppresses the interfering coherent spin-spin relaxation pathway at low spin-lock fields, which overlaps the Bloch-McConnell (chemical exchange) range of R _1ρ dispersions. The proton-decoupled and standard R _1ρ experiments were used to study the rocking motion of ¹⁵ N, ² H-enriched protein GB1 in two solid forms, microcrystals and lyophilized amorphous powder. The most striking finding is that the correlation function of this motion consists of two components with very different correlation times, 2-20 μs and a few hundred μs. The rocking motion parameters in microcrystals and powder are quite different, revealing the distinct nature of inter-protein interactions in these two samples.