Probing Conformational Exchange in Weakly Interacting, Slowly Exchanging Protein Systems via Off-Resonance R1ρ Experiments: Application to Studies of Protein Phase Separation

R1ρ relaxation dispersion experiments are increasingly used in studies of protein dynamics on the micro- to millisecond time scale. Traditional R1ρ relaxation dispersion approaches are typically predicated on changes in chemical shifts between corresponding probe spins, ΔωGE, in the interconverting states. Here, we present a new application of off-resonance 15N R1ρ relaxation dispersion that enables the quantification of slow exchange processes even in the limit where ΔωGE = 0 so long as the spins in the exchanging states have different intrinsic transverse relaxation rates (ΔR2 = R2,E - R2,G ≠ 0). In this limit, the dispersion profiles become inverted relative to those measured in the case where ΔωGE ≠ 0, ΔR2 = 0. The theoretical background to understand this effect is presented, along with a simplified exchange matrix that is valid in the limits that are germane here. An application to the study of the dynamics of the germ granule protein Ddx4 in a highly concentrated phase-separated state is described. Notably, exchange-based dispersion profiles can be obtained despite the fact that ΔωGE ≈ 0 and ΔR2 is small, ∼20-30 s-1. Our results are consistent with the formation of a significantly populated excited conformational state that displays increased contacts between adjacent protein molecules relative to the major conformer in solution, leading to a decrease in overall motion of the protein backbone. A complete set of exchange parameters is obtained from analysis of a single set of 15N off-resonance R1ρ measurements recorded at a single static magnetic field and with a single spin-lock radio frequency field strength. This new approach holds promise for studies of weakly interacting systems, especially those involving intrinsically disordered proteins that form phase-separated organelles, where little change to chemical shifts between interconverting states would be expected, but where finite ΔR2 values are observed.