Reduced four-dimensional exchange

Fig. 24. The basic scheme of the reduced four-dimensional exchange experiment as performed on l3C.53 All pulses are 90° pulses. Part A of the experiment is simply a two-dimensional exchange experiment with l set equal to U. The effect of this part of the pulse sequence is to select out only those spins that did not reorientate significantly during the mixing period of part A, rma. The remainder of the experiment then serves to examine whether the motional timescale of these selected components subsequently changes during the second mixing time, rmb- The experiment thus probes the timescale of motional heterogeneity.

Fig. 7. One-dimensional NMR spectra of the designed four-helix bundles SA-42 (lower trace) and GTD-43 (top two traces). The chemical shift dispersion of SA-42 in 90% H2O and 10% D2O at 323 K and pH 4.5 is poor and the resonances are severely broadened due to conformational exchange. The chemical shift dispersion of GTD-43 in the same solvent at 288 K and pH 3.0 is comparable to that of the naturally occurring four-helix bundle IL-4 and the resonances are not significantly affected by conformational exchange. Upon raising the temperature to 298 K line broadening is observed (top trace) which shows that GTD-43 is in slow exchange on the NMR time scale, unlike SA-42 where an increased temperature reduces the line width. These spectra are therefore diagnostic of structures with disordered (SA-42) and ordered (GTD-43) hydrophobic cores...

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