Long-lived nuclear spin states of magnetically equivalent protons by means of dissolution dynamic nuclear polarization

Nuclear Magnetic Resonance (NMR) is one of the most versatile techniques since it enables the characterization of solid, liquid and gaseous systems in a plethora of in-vitro and in-vivo experiments. Despite its multidisciplinary scope, it still suffers from two drawbacks, namely the intrinsic low sensitivity and the fast return to equilibrium. Dissolution Dynamic Nuclear Polarization (D-DNP) is a powerful scheme to enhance the signals of nuclear spins by up to five orders of magnitude and it is at the forefront of current research in NMR. The main detriment is that the great efforts spent in producing the hard-earned hyperpolarized signals fade because of rapid longitudinal relaxation with a time constant T1. Long-Lived States (LLS) are states with lifetimes that may be much longer than T1. In a two-spin system, they can be defined as a T/S Imbalance (TSI) between the average population of the three triplet (T) states on the one hand and the population of the singlet (S) state on the other. So far, para-hydrogen is the only molecule routinely used as a source of long-lived hyperpolarization. In recent years, it has been shown that it is possible to create a TSI in magnetically in-equivalent spins, by exploiting DNP to lower the spin temperature well below the Zeeman splitting. The present thesis is devoted to combine D-DNP with LLS in the perspective of preparing enhanced and long-lasting NMR signals in systems comprising magnetically equivalent protons. We proved that that is possible to induce the TSI via D-DNP in partly deuterated ethanol and in fumaric acid. In ethanol the TSI has been revealed by 1H-13C cross-relaxation that leads to population differences across observable transitions. In fumarate, we resorted to a symmetry-breaking addition of D2O catalysed by fumarase which makes the TSI observable on the two magnetically in-equivalent protons of malate. In principle, our strategy should allow one to prepare either an excess or a deficiency of para-water, which we like to refer to as “forbidden fruits” of spectroscopy. Water is challenging since its TSI may suffer from relaxation due to fast proton exchange and to spin-rotation. We assessed conditions suitable to slow down proton exchange by dilution in aprotic solvents, yielding lifetimes of water as a molecular entity on a timescale compatible with D-DNP experiments. We designed an arrangement of coaxial tubes to measure T1 of water vapour at known pressures and temperatures. The experimental T1 are rather small (i.e., tens of ms) and in fair agreement with predictions if spin-rotation is the dominant relaxation mechanism. Empirical evidence of the survival of the hyperpolarized signal of water after dissolution in our lab suggests that the exchange of water molecules between the gaseous and liquid phases is either inefficient or slow in most of cases we studied. I report preliminary results of the application of our D-DNP scheme to induce a TSI in water. Considering the complexity of the project, further investigation is needed to unravel and optimize recent observations in view of identifying an unambiguous fingerprint of dilute para-water.


Related material