Figure 2.

Experimental ¹⁵N RD data obtained on perdeuterated ubiquitin at 300 K, obtained from proton-detected experiments. Plotted is the on-resonance R_1ρ, that is, corrected for chemical-shift offset (see the Supporting Information). a) RD profiles obtained at 39.5 kHz MAS and low spin-lock rf field strengths of 2–15 kHz. Solid black lines show the result of a Bloch–McConnell fit of a two-state exchange model using only R_1ρ-derived data, while orange lines are derived from a fit that includes CPMG data for residues 23, 27, and 55 at 600 MHz, as reported earlier (see Figure S8). Straight dashed lines (constant R_1ρ rate) in panels (a) and (b) show the relaxation rate constant obtained at 39.5 kHz MAS and 15 kHz spin-lock field strength, which is considered as free from exchange effects. b) RD profiles obtained at 20 kHz MAS on a fully deuterated, 20 % reprotonated sample of ubiquitin. Solid lines show simulated R_1ρ RD profiles assuming an exchange rate k_ex=2900 s⁻¹ and population p_B=9.3 %. All available RD profiles, as well as experimental details are shown in the Supporting Information.