Mechanism of dilute-spin-exchange in solid-state NMR

In the stationary, aligned samples used in oriented sample (OS) solid-state NMR, H-1-H-1 homonuclear dipolar couplings are not attenuated as they are in magic angle spinning solid-state NMR; consequently, they are available for participation in dipolar coupling-based spin-exchange processes. Here we describe analytically the pathways of N-15-N-15 spin-exchange mediated by H-1-H-1 homonuclear dipolar couplings. The mixed-order proton-relay mechanism can be differentiated from the third spin assisted recoupling mechanism by setting the H-1 to an off-resonance frequency so that it is at the "magic angle" during the spin-exchange interval in the experiment, since the "magic angle" irradiation nearly quenches the former but only slightly attenuates the latter. Experimental spectra from a single crystal of N-acetyl leucine confirm that this proton-relay mechanism plays the dominant role in 15N-15N dilute-spin-exchange in OS solid-state NMR in crystalline samples. Remarkably, the "forbidden" spin-exchange condition under "magic angle" irradiation results in N-15-N-15 cross-peaks intensities that are comparable to those observed with on-resonance irradiation in applications to proteins. The mechanism of the proton relay in dilute-spin-exchange is crucial for the design of polarization transfer experiments. (C) 2014 AIP Publishing LLC.