• Corpus ID: 237485312

Selectively pulsed spin order transfer increases parahydrogen-induced NMR amplification of insensitive nuclei and makes polarization transfer more robust

@inproceedings{Schmidt2021SelectivelyPS,
  title={Selectively pulsed spin order transfer increases parahydrogen-induced NMR amplification of insensitive nuclei and makes polarization transfer more robust},
  author={Andreas B. Schmidt and Arne Brahms and Frowin Ellermann and Stephan Berner and J{\"u}rgen Hennig and Dominik von Elverfeldt and Rainer Herges and Jan Hovener and Andrey N. Pravdivtsev},
  year={2021}
}

• A. Schmidt, A. Brahms, +6 authors A. Pravdivtsev
• Published 10 September 2021
• Physics

We describe a new method for pulsed spin order transfer (SOT) of parahydrogen induced polarization (PHIP) that enables close to 100 % polarization in incompletely H-labeled molecules by exciting only the desired protons in a frequency-selective manner. While a selective pulse (SP) on H at the beginning of pulsed SOT had been considered before, using SPs during the SOT suppresses undesired indirect spin-spin interactions. As a result, we achieved a more robust SOT for the SP variants of the phINEPT+ sequence that we refer to as phSPINEPT+. Thereby, for the first time, we report a sequence that is effective for all weakly coupled spin systems. Our simulations show that the method converts close to 100 % of the parahydrogen-derived spin order into C hyperpolarization in weakly coupled three-spin systems and partially or fully H-labeled molecules if relaxation is neglected. Experimentally we demonstrate high hyperpolarization of C with 15.8 % for 1-C-hydroxyethyl propionate-d3 and 12.6 % for 1C-ethyl acetate-d6, which corresponds to ≈47 % and ≈38 % if the enrichment of parahydrogen had been 100 %. Even in nonH-labeled molecules, a remarkable C polarization is achieved, e.g. up to 20 % were simulated for 100 % pH2, and 1.25 % were obtained experimentally for 1-C-ethyl pyruvate and 50 % pH2, which can be further improved by a faster hydrogenation. As a result, full deuterium labeling may no longer be required e.g., when new PHIP agents are investigated, the synthesis of fully deuterated molecules is too complex, or when a kinetic isotope effect regarding the metabolic conversion rate of an agent is to be avoided. Using SPs during SOT seems very promising and may be extended to other sequences in the context of PHIP and beyond to make them less prone to experimental imperfections or real molecular environments. Nuclear magnetic resonance (NMR) has become indispensable in chemistry and medical diagnostics (magnetic resonance imaging; MRI), although it effectively detects only a millionth of the present nuclear spins. The intrinsically low thermal polarization of nuclear spins, which is of the order of parts per million, often limits magnetic resonance to detect only the most abundant substances and nuclei – for MRI, typically hydrogen atoms in water and lipids. Hence, the quest for an efficient hyperpolarization and spin order transfer (SOT) has been ongoing for several decades. Hyperpolarization has already enabled several exciting applications e.g., in analytical and catalytic chemistry, clinical in vivo gas imaging, as well as observation of metabolism in real time, early-stage cancer diagnostics, and longitudinal monitoring of treatment and recovery. Among current techniques, the versatile parahydrogen induced polarization (PHIP), also referred to as parahydrogen and synthesis allows a dramatically enhanced nuclear alignment (PASADENA), and signal amplification by reversible exchange (SABRE) are very promising, because they are cost and time efficient. PHIP and SABRE provided the basis for liquid state polarization of more than 50 % for C and over 20 % for N. Both techniques utilize the highly ordered spin alignment of parahydrogen (pH2), i.e. of molecular hydrogen (H2) enriched in the singlet nuclear spin isomer. PHIP incorporates the spin order by a pairwise addition of pH2 to an unsaturated compound (with C=C or C≡C bond), which results in an H hyperpolarized product (Fig. 1a). SABRE exploits reversible chemical exchange and interaction of pH2 and the target molecule. Hence, SABRE does not change the chemical structure of the compound and has enabled continuous hyperpolarization. However, for MR-applications, a heteronuclear polarization transfer e.g., to C or N is often needed, because (a) the lifetime of their polarization is typically longer than for H, (b) the background signal of non-hyperpolarized tissue in vivo is negligible, and (c) monitoring of metabolic conversion is easier because the chemical shift spread is larger. To transfer the spin order of pH2 to X-nuclei (nuclei other than H), two 2 approaches are commonly used: “spontaneous” transfer by evolution at specific external magnetic fields – i.e. magnetic field cycling (MFC) – or specific manipulations of the spin system by RF pulses. The latter features some interesting advantages, which are: (a) hyperpolarized agents can be prepared directly at high field in the MR system and thus, no external “polarizer” is needed; (b) high-quality, commercially available hardware can be used to conduct the SOT (i.e. NMR spectrometers or MRI systems); (c) a typically higher theoretical polarization (i.e., if the molecule consists of more than the three coupling nuclear spins from pH2 and C in MFC polarization transfer is often limited, e.g. P13C = 30 % for ethyl acetate); 34 (d) fully deuterium (H) labeled precursors can be polarized effectively with the advantage that the lifetime of hyperpolarization is often longer, which is favorable for (bio-) medical monitoring of hyperpolarized agents in vivo. Despite the nuclear spin-1 of the H atom the H-H and H-C J-coupling interactions are effectively suppressed with high-field pulsed SOT because no H refocusing pulses are applied. Although, deuteration is advantages for the pulsed polarization transfer it can cause a kinetic isotope effect (e.g. altered metabolic rates or changes of the pharmokinetics). However, in current high-field pulsed methods polarization transfer can suffer, if more H than the two stemming from pH2 are present. This is because undesired HH J-coupling interactions were effective throughout the SOT(see SI). Here, we propose and demonstrate a method that partially solves this problem physically instead of chemically with H-labeling: pH2 and the use of Selective Pulses enabled effective and robust Insensitive Nuclei Enhancement by Polarization Transfer (phSPINEPT+, in accordance to the predecessor – the nonselective phINEPT+ sequence; 21 Fig. 1b). Compared to the phINEPT+ sequence, the use of frequency-selective excitation and refocusing pulses holds fundamental benefits. First, the maximum polarization yield is more than doubled, allowing up to ~100 % C-polarization for fully or partially H-labeled molecules (Fig. 2c,d). Experimentally, C-polarizations up to ~16 % were achieved, corresponding to P13C = 47 % if 100 % parahydrogen had been used. Second, even when no H labeling is present, a robust and remarkably high polarization is possible e.g., close to 20% for 1-C-ethyl pyruvate (SI, Fig. S6). Here, the theoretical maximum is reduced from 100 % to 20 % only, because the selective pulses also excite the H of the original vinyl-CH or vinyl-CH2 group and J-couplings are not fully suppressed. Thus, selectively pulsed SOT improves the polarization for (partially) protonated molecules, e.g. for the research and investigation of new PHIP agents for which isotope labeling is synthetically difficult, too expensive or undesirable. Additionally, for such molecules our phSPINEPT+ variants are more robust to uncertainties or experimental errors compared to phINEPT+ (Fig. 2c,d) and other non-selective SOTs (i.e. the evolution timings τ1 and τ2 are a function of all J-coupling constants between excited nuclei of the molecule; see SI). Known SOT sequences for PASADENA (and SABRE) are usually designed for strongly coupled (i.e. where proton-proton indirect spin coupling is much larger than the chemical shift difference between protons HI and HS, JHIHS » Δδ) or weakly coupled spin systems (JHIHS « Δδ). All of the latter are modifications of the INEPT NMR sequence, which account for the very different spin order at the beginning of the sequence. For a thermally polarized spin only a single-spin order (Iz) is present; for two hydrogens I and S of pH2 after hydrogenation – if the added H are weakly coupled – two-spin order IzSz is given. 40 Among the sequences for weakly coupled spin systems, phSPINEPT+ and phINEPT+ sequences carry the advantage of consisting only of two free evolution intervals (2τ1 and 2τ2, Fig. 1). In contrast, the two other currently known SOT sequences for weakly coupled spin systems, namely the efficient spin order transfer to heteronuclei via relayed INEPT chains (ESOTHERIC) and the selective excitation of polarization using PASADENA (SEPP) -INEPT sequence need three intervals (SI, Fig. S4) and 1-2 more RF-pulses. Note that the concept of exploiting a first frequency-selective pulse to excite the IzSz spin order has indeed been introduced with SEPP. For high-field SABRE selective X-nuclear pulses have been used to re-polarize and accumulate polarization by repeating SABREINEPT. However, the selective H refocusing and excitation during the SOT of pH2 spin order is a new concept that enabled above-mentioned advantages of selective polarization transfer. FIGURE 1. Hydrogenation and spin order transfer (SOT) scheme (a) and phINEPT+ and phSPINEPT+ sequences (b). After hydrogenation with pH2, spin order of hyperpolarized spins is transferred to the selected C nuclei using an SOT sequence of choice (polarized sites are indicated in blue). phINEPT+ and several variants of phSPINEPT+ sequences are shown, which all consist of radiofrequency pulses (with flip angles 45, 90, or 180° and phases x or y) resonant on H or C and time intervals of free