Toward Understanding of Nuclear Spin Relaxation at Zero-to-Ultralow Fields

Chengtong Zhang (New York University, United States)

Abstract: NMR experiments can interrogate a broad spectrum of molecular tumbling regimes and can accurately measure interatomic distances in solution with sub-nanometer resolution. Relaxation rates of nuclear spin polarization can unravel many dynamical and structural aspects of biomolecules in their native form. The high field relaxometry/dispersion techniques are widely used to monitor protein folding, molecular size, intermolecular interactions etc. However, little effort was devoted to extracting molecular information from relaxation rates at zero-to-ultralow-field (ZULF) mainly due to poor SNR and intricate spin dynamics. We develop the theoretical framework to understand relaxation rates measured in a ZULF-NMR setup with detection using optical-atomic magnetometers. In regimes where the scalar coupling constants and differences in Larmor frequencies of heteronuclear systems AX(N-1) have similar magnitudes, the spectrum reaches its maximum complexity with 2^N peaks. Populations and coherences’ lifetime measurements will greatly depend on the choice of monitored peaks’ decay. This multifaceted analysis of the same relaxation interaction can lead to a more accurate and robust determination of its strength and correlation time leading to new strategies for simultaneous extraction of structural and dynamical. We highlight how several factors impact the observed rates, such as (i) the shuttling profile from the (pre)polarizing magnet to the detection region, (ii) the measurement field, (iii) the detection method (single- or dual-channel) and even (iv) the nutation angle induced by the detection pulse. Our findings are compared to experimental relaxation rates measured for two [13C]-labelled molecules, showing how structural constraints and rotational tumbling can be inferred from ZULF relaxometry studies.