Magic Angle Spinning Effects on Longitudinal NMR Relaxation: 15N in L-Histidine

TL;DR

This study investigates how magic angle spinning rate influences 15N longitudinal relaxation in solid-state NMR, revealing significant effects that impact measurements of molecular dynamics in biological and material systems.

Contribution

It demonstrates the effect of magic angle spinning on 15N relaxation rates in biological model compounds, highlighting the importance of rotational resonance conditions.

Findings

15N relaxation in histidine is reduced by nearly three orders of magnitude at rotational resonance.

Amine relaxation rate increases at rotational resonance conditions.

The phenomenon has implications for measuring dynamics and enhancing DNP techniques.

Abstract

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.