Collisional cross-section of water molecules in vapour studied by means of H-1 relaxation in NMR

In gas phase, collisions that affect the rotational angular momentum lead to the return of the magnetization to its equilibrium (relaxation) in Nuclear Magnetic Resonance (NMR). To the best of our knowledge, the longitudinal relaxation rates R-1 = 1/T-1 of protons in H2O and HDO have never been measured in gas phase. We report R1 in gas phase in a field of 18.8 T, i.e., at a proton Larmor frequency v(0) = 800 MHz, at temperatures between 353 and 373 K and pressures between 9 and 101 kPa. By assuming that spin rotation is the dominant relaxation mechanism, we estimated the effective cross-section sigma(J) for the transfer of angular momentum due to H2O-H2O and HDO-D2O collisions. Our results allow one to test theoretical predictions of the intermolecular potential of water in gas phase.

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

Daniele Mammoli

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 âforbidden fruitsâ 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.

EPFL2016

Structural determination of biomolecular interfaces by nuclear magnetic resonance of proteins with reduced proton density

Fabien Ferrage

Protein interactions are important for understanding many molecular mechanisms underlying cellular processes. So far, interfaces between interacting proteins have been characterized by NMR spectroscopy mostly by using chemical shift perturbations and cross-saturation via intermolecular cross-relaxation. Although powerful, these techniques cannot provide unambiguous estimates of intermolecular distances between interacting proteins. Here, we present an alternative approach, called REDSPRINT (REDduced/Standard PRoton density INTerface identification), to map protein interfaces with greater accuracy by using multiple NMR probes. Our approach is based on monitoring the cross-relaxation from a source protein (or from an arbitrary ligand that need not be a protein) with high proton density to a target protein (or other biomolecule) with low proton density by using isotope-filtered nuclear Overhauser spectroscopy (NOESY). This methodology uses different isotropic labeling for the source and target proteins to identify the source-target interface and also determine the proton density of the source protein at the interface for protein-protein or protein-ligand docking. Simulation indicates significant gains in sensitivity because of the resultant relaxation properties, and the utility of this technique, including a method for direct determination of the protein interface, is demonstrated for two different protein–protein complexes.

Springer Verlag2010

Nanodiamond Relaxometry-Based Detection of Free-Radical Species When Produced in Chemical Reactions in Biologically Relevant Conditions

Mayeul Sylvain Chipaux

Diamond magnetometry is a quantum sensing method involving detection of magnetic resonances with nanoscale resolution. For instance, T1 relaxation measurements, inspired by equivalent concepts in magnetic resonance imaging (MRI), provide a signal that is equivalent to T1 in conventional MRI but in a nanoscale environment. We use nanodiamonds (between 40 and 120 nm) containing ensembles of specific defects called nitrogen vacancy (NV) centers. To perform a T1 relaxation measurement, we pump the NV center in the ground state (using a laser at 532 nm) and observe how long the NV center can remain in this state. Here, we use this method to provide real-time measurements of free radicals when they are generated in a chemical reaction. Specifically, we focus on the photolysis of H2O2 as well as the so-called Haber-Weiss reaction. Both of these processes are important reactions in biological environments. Unlike other fluorescent probes, diamonds are able to determine spin noise from different species in real time. We also investigate different diamond probes and their ability to sense gadolinium spin labels. Although this study was performed in a clean environment, we take into account the effects of salts and proteins that are present in a biological environment. We conduct our experiments with nanodiamonds, which are compatible with intracellular measurements. We perform measurements between 0 and 10(8) nM, and we are able to reach detection limits down to the nanomolar range and typically find T1 times of a few 100 mu s. This is an important step toward label-free nano-MRI signal quantification in biological environments.

AMER CHEMICAL SOC2020

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

Daniele Mammoli

EPFL2016