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

Abstract

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-γ 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.