Experimental techniques and major equipment
1) Magnetic resonance spectroscopy and microimaging
The NMR microimaging studies can yield information about the spatial distribution of liquids and gases within any cross-section of the sample under study non-destructively, with a spatial resolution of the order of hundreds or even tens of microns. Therefore, this technique can be employed to study in situ various dynamic processes in real time without interrupting the process under investigation. One of the advantages of MR imaging compared to other tomographic techniques is that it can spatially map not only the quantity of a substance, but also a broad range of other properties of the objects under study and processes within them. Besides, NMR is a spectroscopic technique, therefore combining NMR and MRM techniques one can get access to the spatially resolved information on chemical composition (e.g., separate spatial distributions of the reactant and the product in a functioning reactor).
The experiments are carried out with two Bruker NMR systems, an Avance III 400 MHz NMR microimaging instrument (magnetic field gradient up to 150 G/cm or 1.5 T/m)
and an AV 300 SB NMR spectrometer.
2) Signal enhancement in NMR/MRI and parahydrogen-induced polarization
Many applications of NMR/MRI suffer fr om (or even made impossible by) a relatively low sensitivity of the technique, caused by a weak interaction of nuclear spins with the external magnetic fields and thus their poor orientation with respect to the static magnetic field. A family of the so-called hyperpolarization methods make it possible to force the nuclear spins to preferentially orient in the same direction to a much greater degree compared to the thermal equilibrium even in the very high magnetic fields of modern NMR/MRI instruments. Hyperpolarization of spins of the nuclei in diamagnetic molecules and materials is one of the hot topics in modern magnetic resonance. This is because it can provide NMR signal enhancements of about 4 orders of magnitude in the high magnetic fields of modern NMR/MRI magnets. In low magnetic fields, where sensitivity issues are particularly severe, hyperpolarization enhancements of NMR signals can in fact be significantly larger.
Parahydrogen-induced polarization (PHIP) is one of the members of the family of hyperpolarization techniques. It is based on the use of the nuclear spin isomers of molecular hydrogen H2 (either parahydrogen with the total nuclear spin of the two H atoms I=0, or sometimes orthohydrogen with I=1) in a catalytic hydrogenation reaction of an appropriate substrate. Upon the reaction, the symmetry of the H2 molecule is usually broken, and the initial correlation of the nuclear spins of para-H2 molecule is converted to a strong enhancement of the NMR signals of the reaction product. Polarization of the two H atoms originally coming from the H2 molecule can be transferred to other atoms (hydrogens or other nuclei with a non-zero nuclear spin such as 13C, 19F, etc.) in the product molecule, significantly broadening the scope of many potential applications of PHIP in NMR spectroscopy and imaging. Some examples include the hypersensitive studies of the mechanisms of homogeneous and heterogeneous catalytic reactions and of the in vivo studies of metabolism in lab animals.
A simple parahydrogen converter is available in the lab for production of the H2 mixtures enriched with parahydrogen (the ratio para-H2:ortho-H2 =1:1). A parahydrogen generator from Bruker (BPHG 90, para-H2:ortho-H2>9:1) installed in 2013.
A setup was constructed to perform heterogeneous catalytic hydrogenation of unsaturated substrates either in the Earth’s magnetic field (ALTADENA experiment) or in the high magnetic field of the NMR spectrometer (PASADENA experiment).
3) In vivo MRI and MRS
The in vivo MRI studies of lab animals are conducted within the framework of the Inter-Institute Research Sector for Imaging of Lab Animals founded jointly by the International Tomography Center, SB RAS and the Institute of Cytology and Genetics, SB RAS (head of the Sector – Dr. Sci., Prof. Igor V. Koptyug). The MRI studies are performed on the high-field "BioSpec 117/16" MRI system (Bruker) at 11.7 T magnetic field (500 MHz 1Н NMR frequency). The instrument is located at the SPF vivarium of the Institute of Cytology and Genetics, SB RAS wh ere it was installed in 2009.
4) Analytical methods
Since the PHIP technique is based on the catalytic addition of parahydrogen molecule, a great emphasis is placed on the screening of catalysts which are suitable for pairwise hydrogen addition. Originally, PHIP effects were observed for transition metal complexes used as homogeneous hydrogenation catalysts, e.g. Wilkinson’s catalyst, due to a well-defined nature of an isolated catalytic center which is responsible for the pairwise hydrogen addition. However, utilization of homogeneous hydrogenation catalysts significantly limits the biomedical application of the PHIP technique. For example, MRI of living systems requires the separation of the hyperpolarized contrast agent from a toxic homogeneous catalyst. One possible approach is to use transition metal complexes immobilized on a solid support, which can combine advantages of homogeneous and heterogeneous catalysts. The characterization of immobilized metal complexes is performed on the FTIR spectrometer VERTEX 70 (Bruker). This device with a high-precision optical system and modern software allows registering infrared spectra in 8000 to 50 cm-1 range.
Gas chromatography-mass spectrometry
In addition to NMR spectroscopy, the analysis of catalytic reaction products mixture is highly desirable. In our laboratory, the product mixtures could be analyzed using an Agilent7820A gas-chromatography system coupled with an Agilent 5975 mass spectroscopy system.
For the manipulation with air- and/or moisture-sensitive compounds our laboratory is equipped with 2GB glove box (LabConco) coupled with AtmosPureTM Re-Gen gas purifier.