Controlling spin diffusion by tailored rf-irradiation schemes

Spin-diffusion type polarization transfer among heteronuclei, e.g., C-13 or N-15, plays an important role in many solid-state NMR experiments for structure determination of proteins. In such experiments, chemical-shift compensation is provided by the proton bath ('proton-driven') which can be improved by radio-frequency irradiation of the protons. Here, we address the problem that the polarization-transfer rates depend not only on the intermolecular distance but also on the chemical-shift difference of the two spins. We introduce rf-modulation schemes that allow eliminating the chemical-shift dependence to a large extent over a predefined range. At the same time, the rate constants are maximized. (c) 2014 Elsevier B.V. All rights reserved.