Characterization of chemical exchange using residual dipolar coupling

NMR line shape analysis and relaxation dispersion measurements for N ,N -dimethyltrichloroacetamide (DMTCA) weakly aligned using poly-gamma-benzyl-L-glutamate, which forms a lyotropic nematic phase when dissolved in chloroform, were used to characterize chemical exchange kinetics for the rotation around the C-N amide bond. At low temperatures (<312 K), slow-exchange H-1-coupled C-13 NMR spectra show two resolved C-13 methyl quartets with different residual dipolar coupling constants (RDCs). At high temperatures (>312 K), a single population-averaged C-13 methyl quartet is observed; in this regime, the differences in RDCs contribute to differential line broadening of the quartet components. Self-consistent measurements of the difference in RDCs are obtained from both line shape and relaxation dispersion techniques. The results show that NMR spectroscopy of weakly aligned molecules allows complete characterization of chemical exchange processes using RDCs, even if exchange broadening is absent in isotropic samples.