Реферат: Signal Amplification By Reversible Exchange (SABRE) is a member of the family of Parahydrogen Induced Polarization (PHIP) techniques aimed to enhance tremendously weak NMR-signals of heteronuclei as 13C, 15N, exploiting non-equilibrium singlet spin order of parahydrogen. SABRE polarization transfer takes place in metalorganic complexes in which hydrogen nuclear spins are coupled with the spins of substrate while they are transiently bound to the complex. There are two general approaches to perform heteronuclear SABRE polarization transfer. The first one is to exploit ultralow (≤ 1 μT) magnetic fields in order for the spin system to become strongly coupled. In this regime, the difference in Larmor frequencies of nuclear spins is comparable with the spin-coupling constant. Therefore, SABRE polarization transfer from parahydrogen to the substrate occurs not inside an NMR-spectrometer, but rather inside a magnetic shield aimed to provide ultralow magnetic fields (SABRE-SHEATH, Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei) [1]. The second approach is to fulfill conditions for strong spin coupling inside the high field of NMR-spectrometer exploiting radiofrequency magnetic field pulses without an additional magnetic shield equipment. However, SABRE at high magnetic field requires RF-pulse sequence engineering. In contrast to the SABRE-SHEATH, SLIC-SABRE (SLIC, Spin Lock Induced Crossing) [2], which is the most efficient pulse sequence for heteronuclear SABRE hyperpolarization at high field, strongly depends on the symmetry of the SABRE polarization transfer complex demonstrated in fig. 1. Fig. 1. (a) Symmetrical SABRE-complex, where both equatorial ligands are identical; (b) Non-symmetrical SABRE complex, where equatorial ligands are different, S1≠ S2. The SABE complex is either symmetrical, i.e., the equatorial ligands of the complex are the same, therefore, hydride protons have identical resonant frequencies (fig. 1 a). The other situation is also possible, when the polarization transfer complex is non-symmetrical, i.e., it contains different equatorial ligands, therefore, hydride protons have significantly different resonant frequencies. SLIC-SABRE pulse sequence is efficient only for symmetrical SABRE-complexes [3]. Therefore, an efficient heteronuclear hyperpolarization takes place only for the substrate molecules which are able to form a symmetrical SABRE-complex, which leads to the strong restrictions on the samples. In our work, we focus on the design of new pulse-sequences which provide an efficient 15N hyperpolarization for non-symmetrical SABRE complexes. These pulse-sequences presented in fig. 2 are based on the simultaneous RF excitation of 1H and 15N nuclear spins in the SABRE complex. Fig. 2. Pulse sequence used for SABRE hyperpolarization at high magnetic fields for non-symmetrical complexes. One polarization cycle of the pulse sequence consists of pH2-bubbling the sample during the time tb, time-delay tw necessary for removing the bubbles from the sample and then simultaneous RF excitation of 1H and 15N spins during the time period tp. The polarization cycles is sequentially repeated n times. We found that exploiting weak RF magnetic fields allows one to efficiently hyperpolarize 15N nuclei, that is, 15N signal enhancement relative to the thermal signal acquired at 9.4 T was equal to 2500. Due to its high selectivity, we also consider proposed pulse sequences as a very precise method for obtaining heteronuclear 15N-1H correlations in the non-symmetrical SABRE complexes. We tested our method on the sample which consisted of two biologically relevant molecules (antibiotics) as a substrate on the natural isotopic abundance of 15N (0.36%): tinidazole (TNZ) and secnidazole (SCZ) in the presence of DMSO as a co-substrate. The structure of the substrates is demonstrated in fig. 3. Fig. 3. The structure of the substrates used for hyperpolarization at high field: secnidazole (left) and tinidazole (right). Using high selectivity of the proposed pulse sequence, we were able to detect the heteronuclear correlations with the difference in 30 Hz between the hydride protons resonant frequencies and 20 Hz difference between the complexes-bound 15N resonant frequencies, as demonstrated in fig. 4. Fig. 4. Dependence of 15N enhancement of SCZ on the frequency of 15N RF-field. 1H RF frequency was resonant towards (1) -22.277 hydride proton; (2) -22.35 hydride proton; (3) -22.415 hydride proton. From the comparison of calculations with the experiment we determined the following 15N-1H SCZ correlations: (198.52 ppm; -22.35 ppm) and (199.12 ppm; -22.28 ppm). While for TNZ we obtain (200.9 ppm; -22.42 ppm). The sample was consisted of 30 mM of SCZ and 37 mM of TNZ at natural 15N isotopic abundance. Experimental parameters: 1 = 1 = 8 Hz, = 0.5 s, = 0.5 s, = 0.5 s, n = 30. Acknowledgements This work is supported by the Russian Science Foundation (№ 23-73-10103).

Библиографическая ссылка: Markelov D.A. , Kozinenko V.P. , Kiryutin A.S. , Yurkovskaya A.V.
New methods for SABRE hyperpolarization at high magnetic field
21th International School-Conference MAGNETIC RESONANCE AND ITS APPLICATIONS Spinus-2024 01-05 Apr 2024