Mechanism of dilute-spin-exchange in solid-state NMR.
In the stationary, aligned samples used in oriented sample (OS) solid-state NMR, (1)H-(1)H 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 (15)N-(15)N spin-exchange mediated by (1)H-(1)H homonuclear dipolar couplings. The mixed-order proton-relay mechanism can be differentiated from the third spin assisted recoupling mechanism by setting the (1)H 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 (15)N-(15)N dilute-spin-exchange in OS solid-state NMR in crystalline samples. Remarkably, the "forbidden" spin-exchange condition under "magic angle" irradiation results in (15)N-(15)N 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.
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0307, USA.
George J
GJ
Lu
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0307, USA.
Stanley J
SJ
Opella
D017104:Bacteriophage M13 / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D009587:Nitrogen Isotopes; D011522:Protons; D012015:Reference Standards; D014764:Viral Proteins / Q000737:chemistry*
D009587:Nitrogen Isotopes; D011522:Protons; D014764:Viral Proteins