Communications proximity , we assumed that these residues are involved in the same exchange process . Consequently , the exchange rate constant , k ex = k AB + k BA and minor - state population , p B , were assumed to be identical for all these residues , and only the chemical - shift differences between the major and minor states , Dd , which are sensitive to changes in the local environment , were assumed to be specific to individual residues . Solid lines in Figure 2 a show best - fit curves of this fit . The fit yields an exchange rate constant k ex of 8600 AE 1700 s À1 and a minor - state population p B of 3.1 AE 1.2 % . Residue - wise chemical - shift differences Dd are in the range of 2 - 5 ppm ( all values are reported in Table S1 ) . In order to investigate the reliability of these results , which were obtained from a single R 11 RD measurement , we explored the inclusion of additional , independent data sets : a first possibility , often used in solution - state NMR , would be the measurement of R 11 RD at additional static magnetic field strengths ( requiring , however , access to another spectrometer equipped with a fast - MAS probe ) . As an alternative , we use here a combined fit with CPMG RD data , obtained previously under similar conditions of fast MAS and deuteration . [ 11 ] CPMG RD is sensitive to exchange processes on msms timescales , making a combined fit with R 11 possible . Such an analysis of the present R 11 RD data with CPMG data , also obtained at a magnetic field strength of 14.1 T , yields values of Dd that are very similar to those obtained from the above fit of only R 11 RD data ( see Table S1 ) . The obtained exchange rate k ex is 2900 AE 140 s À1 , and the population p B is 9.3 AE 0.6 % . Although these values slightly differ from the values obtained from fitting only a single R 11 RD data set ( where k ex = 8600 AE 1700 s À1 , p B = 3.1 AE 1.2 % ) it is noteworthy that the fit curves of the combined R 11 /CPMG fit , shown as orange lines in Figure 2 a , are almost indistinguishable from the fits of R 11 data only , which shows that the present data are in excellent agreement with independent CPMG data . The differences of the fitted parameters point to the well - known fact that it is   ( PDB 3ons ) . Amides 24 and 25 ( black spheres ) are invisible in NH correlation spectra , presumably due to exchange broadening . [ 11 ] c ) Residue - wise differences of the N - H orientations in the crystal structure used here ( type - II b - turn ) and in a structure featuring a type - I b - turn ( PDB 1ubi ) . These angles were obtained by aligning the two structures to all secondary structure elements and extracting the direction of the respective NÀH bonds . -state exchange model using only R 11derived 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 11 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 11 RD profiles assuming an exchange rate k ex = 2900 s À1 and population p B = 9.3 % . All available RD profiles , as well as experimental details are shown in the Supporting Information .