Francesca Reineri

ParaHydrogen Induced Polarization of 13C carboxylate resonance in acetate and pyruvate

The advent of nuclear spins hyperpolarization techniques represents a breakthrough in the field of medical diagnoses by magnetic resonance imaging. Dynamic nuclear polarization (DNP) is the most widely used method, and hyperpolarized metabolites such as [1-(13)C]-pyruvate are shown to report on status of tumours. Parahydrogen-induced polarization (PHIP) is a chemistry-based technique, easier to handle and much less expensive in respect to DNP, with significantly shorter polarization times. Its main limitation is the availability of unsaturated precursors for the target substrates; for instance, acetate and pyruvate cannot be obtained by direct incorporation of the parahydrogen molecule. Herein we report a method that allows us to achieve hyperpolarization in this kind of molecule by means of a tailored precursor containing a hydrogenable functionality that, after polarization transfer to the target (13)C moiety, is cleaved to obtain the metabolite of interest. The reported procedure can be extended to a number of other biologically relevant substrates.

New hyperpolarized contrast agents for 13C-MRI from para-hydrogenation of oligooxyethylenic alkynes.

Two alkyne derivatives, which contain one and two oligooxyethylenic chains respectively, showed to be good substrates for para-hydrogenation reactions, yielding the corresponding hyperpolarized alkenes in good yields. A suitable theory has been developed to account for the observed results, fully explaining the different para-H 2 induced effects observed upon the para-hydrogenation of symmetrically and asymmetrically substituted alkynes in ALTADENA and PASADENA modes. The oligooxyethylenic substituent provides good water solubility to the para-hydrogenated symmetrical derivative. (13)C-MR in vitro images of the latter derivative were obtained both in acetone and in water solutions (130 mM), using the ALTADENA procedure and after application of the field cycling procedure which allows acquisition of an in-phase (13)C carbonyl resonance. The finding that the hydrogenated product is water-soluble in contrast to the parent alkyne which is not allows for the pursuit of a fast phase-transfer separation from the organic solvent, the unreacted substrate, and the catalyst to obtain a ready-to-use water solution suitable for further in vivo MRI applications.

Hyperpolarization transfer from parahydrogen to deuterium via carbon-13

Hyperpolarization arising from para-hydrogen (p-H2) can be transferred via carbon-13 to deuterium after hydrogenation of a perdeuterated substrate. The model compound is acetylene-d2, hydrogenated to yield ethylene-d2. Transfer to deuterium occurs in ALTADENA experiments (the hydrogenation reaction being performed outside the magnet of the NMR spectrometer prior to the insertion of the sample tube into the NMR probe). The proposed theory, limited to the case where the two p-H2 protons remain isochronous (same chemical shift), is based on the concept of a steady-state density operator which prevails subsequently to the hydrogenation reaction. The outcome quantity is the carbon–deuterium longitudinal spin order, denoted as IzCIzD. Calculations simply involve commutators of all relevant spin quantities with the J-coupling Hamiltonian (denoted as HJ). In particular, it is shown that the necessary condition for polarization transfer toward deuterium via carbon-13 is that IzCIzD does not commute with HJ. The st...

Para-hydrogenated Glucose Derivatives as Potential 13C-Hyperpolarized Probes for Magnetic Resonance Imaging

A set of molecules in which a glucose moiety is bound to a hydrogenable synthon has been synthesized and evaluated for hydrogenation reactions and for the corresponding para-hydrogen-induced polarization (PHIP) effects, in order to select suitable candidates for an in vivo magnetic resonance imaging (MRI) method for the assessment of glucose cellular uptake. It has been found that amidic derivatives do not yield any polarization enhancement, probably due to singlet-triplet state mixing along the reaction pathway. In contrast, ester derivatives are hydrogenated in high yield and afford enhanced (1)H and (13)C NMR spectra after para-hydrogenation. The obtained PHIP patterns are discussed and explained on the basis of the calculated spin level populations in the para-hydrogenated products. These molecules may find interesting applications in (13)C MRI as hyperpolarized probes for assessing the activity of glucose transporters in cells.

15N Magnetic Resonance Hyperpolarization via the Reaction of Parahydrogen with 15N-Propargylcholine

(15)N-Propargylcholine has been synthesized and hydrogenated with para-H(2). Through the application of a field cycling procedure, parahydrogen spin order is transferred to the (15)N resonance. Among the different isomers formed upon hydrogenation of (15)N-propargylcholine, only the nontransposed derivative contributes to the observed N-15 enhanced emission signal. The parahydrogen-induced polarization factor is about 3000. The precise identification of the isomer responsible for the observed (15)N enhancement has been attained through a retro-INEPT ((15)N-(1)H) experiment. T(1) of the hyperpolarized (15)N resonance has been estimated to be ca. 150 s, i.e., similar to that reported for the parent propargylcholine (144 s). Experimental results are accompanied by theoretical calculations that stress the role of scalar coupling constants (J(HN) and J(HH)) and of the field dependence in the formation of the observed (15)N polarized signal. Insights into the good cellular uptake of the compound have been gained.

Effects of Magnetic Field Cycle on the Polarization Transfer from Parahydrogen to Heteronuclei through Long-Range J-Couplings

Hyperpolarization of (13)C carboxylate signals of metabolically relevant molecules, such as acetate and pyruvate, was recently obtained by means of ParaHydrogen Induced Polarization by Side Arm Hydrogenation (PHIP-SAH). This method relies on functionalization of the carboxylic acid with an unsaturated alcohol (side arm), hydrogenation of the unsaturated alcohol using parahydrogen, and polarization transfer to the target (13)C signal. In this case, parahydrogen protons are added three to four bonds away from the target (13)C nucleus, while biologically relevant molecules had been hyperpolarized, using parahydrogen, through hydrogenation of an unsaturated bond adjacent to the target (13)C signal. The herein reported results show that the same polarization level can be obtained on the (13)C carboxylate signal of an ester by means of addition of parahydrogen to the acidic or to the alcoholic moiety and successive application of magnetic field cycle (MFC). Experimental results are supported by calculations that allow one to predict that, upon accurate control of magnetic field strength and speed of the passages, more than 20% polarization can be achieved on the (13)C-carboxylate resonance of the esters by means of side arm hydrogenation and MFC.

13C-MR Hyperpolarization of Lactate using ParaHydrogen and metabolic transformation in vitro.

Hyperpolarization of the 13 C magnetic resonance signal of l-[1-13 C]lactate has been obtained using the chemically based, cost-effective method called parahydrogen-induced polarization by means of side-arm hydrogenation (PHIP-SAH). Two ester derivatives of lactate were tested and the factors that determine the polarization level on the product have been investigated in detail. The metabolic conversion of hyperpolarized l-[1-13 C]lactate into pyruvate has been observed in vitro using lactate dehydrogenase (LDH) and in a cells lysate. From the acquisition of a series of 13 Cu2005NMR spectra, the metabolic build-up of the [1-13 C]pyruvate signal has been observed. These studies demonstrate that, even if the experimental set-up used for these PHIP-SAH hyperpolarization studies is still far from optimal, the attained polarization level is already sufficient to carry out in vitro metabolic studies.

How to design 13C para-hydrogen-induced polarization experiments for MRI applications

The application of hyperpolarization techniques for MRI purposes is gathering increasing attention, especially for nuclei such as (13)C or (129)Xe. Among the different proposed methods, ParaHydrogen Induced Polarization requires relatively cheap equipment. The setup of an MRI experiment by means of parahydrogen requires the application of skills and methodologies that derive from different fields of knowledge. The basic theory and a practical insight of this method are presented here. Parahydrogenation of alkynes, having a labelled (13)CO group adjacent to the triple bond, catalyzed by Rh(I) complexes containing a chelating phosphine, represents the best choice for producing and maintaining high heteronuclear polarization effect. In order to transform anti-phase into in-phase (net) (13)C polarization for MRI application it is necessary to set up the described magnetic field cycle procedure. In vitro and in vivo images have been acquired using fast imaging sequences (RARE and trueFISP).

Effect of the static magnetic field strength on parahydrogen induced polarization NMR spectra

Spin polarization transfer from parahydrogen (p-H(2)) to another molecular entity is generally thought to be mediated by longitudinal spin order (represented by the operator product I(z)(A)I(z)(B), A and B being the two hydrogen nuclei which originate from p-H(2) after a hydrogenation reaction). The longitudinal spin order leads to antiphase patterns in the proton NMR spectrum. In addition to these antiphase patterns, in-phase patterns, arising from polarization differences (represented by (I(z)(A)-I(z)(B))), have been experimentally observed. A complete theory, based on a density operator treatment, has been worked out and applied to the two types of parahydrogen induced polarization experiments: PASADENA (PArahydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment; hydrogenation reaction inside the NMR magnet) and (ALTADENA) (Adiabatic Longitudinal Transport After Dissociation Engenders Nuclear Alignment; hydrogenation reaction outside the NMR magnet). It is shown that polarization differences are always created in the case of a PASADENA experiment but that their amplitude depends critically on the ratio of the J coupling over the frequency difference between A and B. In the case of an ALTADENA experiment, if the sample is slowly transferred toward the NMR magnet, polarization differences are definitely created and their amplitude can be larger than the amplitude of the longitudinal spin order. Some test experiments demonstrate the validity of the proposed theory.

Creation and evolution of net proton hyperpolarization arising from para-hydrogenation.

When a hydrogenation reaction is carried out with gaseous hydrogen enriched in its para- isomer in the earth magnetic field (prior to adiabatic insertion of the sample in the NMR magnet), enhanced proton longitudinal order (represented by 2I(z)(A)I(z)(B)) is created but also difference of enhanced polarizations (I(z)(A)-I(z)(B)). In a first part, it is shown theoretically and experimentally that the longitudinal relaxation time of this polarization difference is roughly twice the ones of individual polarizations. The second part is devoted to a pulse sequence designed for transforming this difference into net hyperpolarization. The evolution of this global hyperpolarization is studied experimentally in a third part and it is observed that a fraction of hyperpolarization possesses an effective longitudinal relaxation time similar to the one of the initial polarization difference. Those experimental results are interpreted by numerical calculations based on Solomon-type equations including the longitudinal order and possibly dipolar-csa cross correlation rates.