Exploiting level anti-crossings for efficient and selective transfer of hyperpolarization in coupled nuclear spin systems

Abstract

Spin hyperpolarization can be coherently transferred to other nuclei in field-cycling NMR experiments.At low magnetic fields spin polarization is redistributed in a strongly coupled network of spins.Polarization transfer is most efficient at fields where level anti-crossings (LACs) occur for the nuclear spin-states. A further condition is that field switching to the LAC positions is non-adiabatic in order to convert the starting population differences into spin coherences that cause time-dependent mixing of states. The power of this method has been demonstrated by studying transfer of photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) in N-acetyl-tryptophan. We have investigated the magnetic field dependence and time dependence of coherent CIDNP transfer and directly assessed nuclear spin LACs by studying polarization transfer at specific field positions. The proposed approach based on LACs is not limited to CIDNP but is advantageous for enhancing NMR signals by spin order transfer from any type of hyper-polarized nuclei.

High resolution NMR study of T1 magnetic relaxation dispersion. IV. Proton relaxation in amino acids and Met-enkephalin pentapeptide

“…Although this process is dynamic (in contrast to stochastic spin relaxation) it also relies on strong coupling (because polarization is evenly distributed among strongly coupled spins) and is particularly efficient at LACs. 30,[34][35][36][37][38][39] Thus, both concepts, strong coupling and LACs, are also of importance for other low-field NMR experiments.…”

Iridium(III) Hydrido N-Heterocyclic Carbene–Phosphine Complexes as Catalysts in Magnetization Transfer Reactions

“…2−4 In PHIP, detection of a product that contains two nuclei that were originally located in a single p -H 2 molecule is achieved and therefore a simultaneous change in the chemical identity is required. Nonetheless, this approach is very powerful, and detection of the true reaction intermediates has been achieved, 5−7 the development of NMR theoretical approaches facilitated, 8,9 and the monitoring of hydrogenation products in organic transformation enabled. 10,11 The latter situation has led to significant interest in PHIP as a tool for the in vivo monitoring of living systems.…”

A fast field-cycling device for high-resolution NMR: Design and application to spin relaxation and hyperpolarization experiments

“…In these pairs, the states differ by projections of the H4-H7 and H5-H6 spins. For this reason, polarization exchange occurs predominantly within these pairs of spins; the coherences are excited by non-adiabatic passages through the corresponding LACs as has been shown before [77].…”

“…Such a spin mixing is of coherent nature and it is mediated by zero-quantum coherences (ZQCs). Here we do not want to discuss all details of the transfer mechanism, as they can be found in previous publications [47,77]. We give only one example demonstrating advantages of our setup.…”

“…The necessary prerequisites are: (i) spins are strongly coupled at the field where the spin mixing occurs and (ii) field variations are non-adiabatic. In two-spin ½ systems this method enables [47,77] complete transfer of net polarization between the spins; in higher-spin systems mixing of such a type usually occurs for pairs of levels, which have LACs at low fields, resulting in selective spin order transfer [47,78]. Although the formation of ZQCs requires relatively high field variation speed, it is feasible with our setup.…”