Improved Resolution in Dipolar NMR Spectra Using Constant Time Evolution PISEMA Experiment.

The atomic structure of small molecules and polypeptides can be attained from anisotropic NMR parameters such as dipolar couplings (DC) and chemical shifts (CS). Separated local field experiments resolve DC and CS correlations into two dimensions. However, crowded NMR spectra represent a significant obstacle for the complete resolution of these anisotropic parameters. Using the PISEMA (Polarization Inversion Spin Exchange at the Magic Angle) experiment as a foundation, we designed new pulse schemes that use a constant time evolution in the dipolar (indirect) dimension to measure DC and CS correlations at high resolution. We demonstrated this approach on a 4-pentyl-4'-cyanobiphenyl (5CB) liquid crystal sample, achieving a resolution enhancement ranging from 30 to 60 % for the resonances in the dipolar dimension. These new experiments open the possibility of obtaining significant resolution enhancement for multidimensional NMR experiments carried out on oriented liquid crystalline samples as well as oriented membrane proteins.