Multiple-spin effects in fast magic angle spinning Lee–Goldburg cross-polarization experiments in uniformly labeled compounds

Abstract

The proton–carbon polarization exchange in Lee–Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly C13-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon–proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear C13–H1 spin pairs, it is difficult to unambiguously extract structural constraints from experimental data. To better understand the complexity of the LG-CP processes, experiments on simple three- and four-spin systems are simulated and analyzed. More specifically, it is shown that in CH13–CH13 and CH213–C13 spin systems, a significant amount of the proton polarization can be transferred to both carbons, despite the fact that the individual proton–carbon heteronuclear couplings between each proton and the carbon spins are very different. The dependence of the polarization transfer on the position of the proton carrier frequency is analyzed and it is shown that by an appropriate choice of this frequency, specific polarization transfer pathways can be selected. Experimental results from [U–13C] tyrosine.HCl and [U–13C, N15] histidine.HCl.H2O samples are in satisfactory agreement with simulations.