Fig. 1. Exchange between E and B conformers of T4L L99A can be quantified using CPMG RD NMR. (A) E and B structures of T4L L99A, as established from X-ray37,40 and CPMG RD NMR33 studies. Helices f (residues 107–113) and g (115–123) are colored in red and green respectively, with Phe 114 shown in blue. (B) ¹⁵N-¹H HSQC spectrum98 of T4L L99A, G113A, R119P recorded at 35 °C, 11.7 T, with assignments of selected peaks. Peaks aliased in the ¹⁵N dimension are shown in blue. (C) Amide ¹⁵N CPMG RD profiles of Lys135 recorded at 1, 5.3, 10.4, 15.4 and 20.4 °C, 11.7 T. Red dots are the measured R_2,eff rates and the blue line is from a global best-fit to the experimental data. (D, E) Arrhenius plots of k_BE and k_EB obtained from analysis of the CPMG data. Experimental rates are shown as red circles and the best-fit Arrhenius curves are in blue (see ESI,† Arrhenius analysis of the temperature-dependent rate constants k_BE and k_EB).