Figure 8. Residues which undergo microsecond exchange are identified by ΔR_1ρ experiments, and are in qualitative agreement with the ModelFree results.

A schematic representation of ΔR_1ρ is shown in (a) with dispersion curves for a residue with microsecond exchange (blue curve) and a residue with no microsecond exchange (grey curve). Importantly, this figure shows the propensity of the method to underestimate the exchange contribution to R₂ because the R_1ρ rates at the spin lock field strength of 0 Hz cannot be extrapolated for non-two-state systems. ΔR_1ρ results are shown in (b), plotted against residue number, and mapped onto the 3-dimensional structure in (c) (PDB ID: 2EVZ). These results indicate that residues throughout PTB1:34 are in microsecond exchange, and substantiate the R_ex terms obtained from ModelFree.