Cross-correlations between low-c nuclei in solids via a common dipolar bath

Correlation of chemical shifts of low-γ nuclei (such as ¹⁵N) is an important method for assignment of resonances in uniformly-labeled biological solids. Under static experimental conditions, an efficient mixing of low-c nuclear spin magnetization can be achieved by a thermal contact to the common reservoir of dipole-dipole interactions in order to create ¹⁵N-¹⁵N, ¹³C-¹³C, or ¹⁵N-¹³C cross-peaks in a 2D correlation spectrum. A thermodynamic approach can be used to understand the mechanism of magnetization mixing in various 2D correlation pulse sequences. This mechanism is suppressed under magic-angle spinning, when mixing via direct cross-polarization with protons becomes more efficient. Experimental results are presented for single-crystalline and powder samples of ¹⁵N-labeled N-acetyl-L- ¹⁵N-valyl-L-¹⁵N-leucine (NAVL). In addition to the thermodynamic analysis of mixing pulse sequences, two different new mixing sequences utilizing adiabatic pulses are also experimentally demonstrated.