  Solid-state magnetic resonance is a unique technique that can reveal the
dynamics of complex biological systems with atomic resolution. Longitudinal
relaxation is a mechanism that returns longitudinal nuclear magnetization to
its thermal equilibrium by incoherent processes. The measured longitudinal
relaxation rate constant however represents the combination of both incoherent
and coherent contributions to the change of nuclear magnetization. This work
demonstrates the effect of magic angle spinning rate on the longitudinal
relaxation rate constant in two model compounds: L-histidine hydrochloride
monohydrate and glycine serving as proxies for isotopically-enriched biological
materials. Most notably, it is demonstrated that the longitudinal 15N
relaxation of the two nitrogen nuclei in the imidazole ring in histidine is
reduced by almost three orders of magnitude at the condition of rotational
resonance with the amine, while the amine relaxation rate constant is increased
at these conditions. The observed phenomenon may have radical implications for
the solid-state magnetic resonance in biophysics and materials, especially in
the proper measurement of dynamics and as a selective serial transfer step in
dynamic nuclear polarization.
