Experimental evidence for the role of paramagnetic oxygen concentration on the decay of long-lived nuclear spin order.

Stuart J Elliott,Bryan Erriah

doi:10.1039/c9ra03748a

Stuart J Elliott, Bryan Erriah

Open Access

https://doi.org/10.1039/c9ra03748a

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Abstract

Nuclear singlet lifetimes are often dependent on the quantity of paramagnetic oxygen species present in solution, although the extent to which quenching or removing molecular oxygen has on extending singlet lifetimes is typically an unknown factor. Here we investigate the behaviour of the singlet relaxation time constant as a function of the oxygen concentration in solution. An experimental demonstration is presented for a chemically inequivalent proton pair of the tripeptide alanine–glycine–glycine in solution. We introduce a simple methodology to ensure the solution is saturated with predetermined concentrations of oxygen gas prior to measurements of the singlet lifetime. Singlet lifetimes were measured by using the spin-lock induced crossing pulse sequence. We present a linear relationship between the amount of oxygen dissolved in solution and the singlet relaxation rate constant. Singlet relaxation was found to be ∼2.7 times less sensitive to relaxation induced by paramagnetic oxygen compared with longitudinal relaxation. The relaxation behaviour is described by using a model of correlated fluctuating fields. We additionally examine the extension of singlet lifetimes by doping solutions with the chelating agent sodium ascorbate, which scavenges oxygen radicals in solution.

Highlights

The observation of non-equilibrium magnetization in solution is limited by the spin–lattice relaxation time constant T1
Nuclear singlet lifetimes are often dependent on the quantity of paramagnetic oxygen species present in solution, the extent to which quenching or removing molecular oxygen has on extending singlet lifetimes is typically an unknown factor
The singlet relaxation time constants plateau with an increasing gas bubbling time, as demonstrated in Fig. S1.† A plateau of the singlet relaxation time is reached at 180 s, and implies saturation of the gases in solution since no further extension/depletion of the singlet lifetime is observed, see the Electronic supplementary information (ESI)† for details

Summary

Introduction

The observation of non-equilibrium magnetization in solution is limited by the spin–lattice relaxation time constant T1. LLS lifetimes dwar ng T1 by a factor of 50 have been observed,[26,27] with a LLS lifetime exceeding 1 hour recorded for a 13C2-labelled naphthalene derivative to room temperature solution.[28] LLS have applications to ligand-binding,[29,30,31,32] reaction monitoring[33] and imaging contrast.[34] The combination of LLS with hyperpolarization techniques has been proposed.[35,36,37,38,39,40,41]

Results

Discussion

Conclusion