Development of High-Temperature Superconducting Resonators in NMR Probes for 13C NMR Spectroscopy

Most biomolecular NMR experiments rely on 1H detection because it offers high sensitivity, but 1H NMR experiments are limited by the chemical shift dispersion of 1H which presents significant challenges for accurate peak identification and quantification. Direct detection 13C NMR experiments offer improved peak resolution and provide more information about molecular dynamics and structure, but 13C has inherently low sensitivity arising from its small magnetogyric ratio and the limited natural abundance of 13C. Therefore, improvements in 13C sensitivity which make direct detection 13C experiments feasible are especially advantageous in the metabolomics and natural products studies. This work is primarily focused on the application of high-temperature superconducting resonators in cryogenic microprobes with the aim of developing high sensitivity NMR probes for direct detection 13C experiments. It is challenging to design HTS resonators that produce a sufficient B1 field for 13C NMR experiments due to the limited current capacity of thin-film HTS resonators. In this project, the current distribution of HTS resonators with varying numbers of turns and numbers of arms has been simulated to better understand the limiting factors and opportunities for improvement. A trial multi-arm spiral was fabricated, and the current capacity compared experimentally to a similar single-arm design. The multi-arm spiral resonator design has an improved current capacity which results in a significant reduction in the B1 pulse length. Typically, high Q factor NMR probes offer significant sensitivity gains but also are challenged by distorted transmit pulse shapes and insufficient bandwidth due to long ring up and ring down times. NMR tests of a 1.5-mm 13C-optimized NMR probe are reported here. The probe demonstrated a 33% improvement in 13C mass sensitivity over the most sensitive 13C-optimized NMR probe. While 1H performance was unexpectedly poor in comparison to a previous version of this probe, it was adequate for 1H decoupling. Finally, we report progress towards designing a pulse sequence to simultaneously detect de novo lipogenesis and chain lengthening in fatty acids. By using selective pulses, the long T1 time of the carboxyl group can be leveraged against the relatively short T1 of the methyl group to create a nested pulse sequence. However, in uniformly labeled samples, phase distortions likely arising from long-range carbon-carbon coupling presented a significant challenge to quantification.