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Efficient ¹³C hyperpolarization of ketoisocaproate is demonstrated in natural isotopic abundance and [1-¹³C]enriched forms via SABRE-SHEATH (Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). Parahydrogen, as the source of nuclear spin order, and ketoisocaproate undergo simultaneous chemical exchange with an Ir-IMes-based hexacoordinate complex in CD₃OD. SABRE-SHEATH enables spontaneous polarization transfer from parahydrogen-derived hydrides to the ¹³C nucleus of transiently bound ketoisocaproate. ¹³C polarization values of up to 18% are achieved at the 1-¹³C site in 1 min in the liquid state at 30 mM substrate concentration. The efficient polarization build-up becomes possible due to favorable relaxation dynamics. Specifically, the exponential build-up time constant (14.3 ± 0.6 s) is substantially lower than the corresponding polarization decay time constant (22.8 ± 1.2 s) at the optimum polarization transfer field (0.4 microtesla) and temperature (10 °C). The experiments with natural abundance ketoisocaproate revealed polarization level on the ¹³C-2 site of less than 1%—i.e., one order of magnitude lower than that of the 1-¹³C site—which is only partially due to more-efficient relaxation dynamics in sub-microtesla fields. We rationalize the overall much lower ¹³C-2 polarization efficiency in part by less favorable catalyst-binding dynamics of the C-2 site. Pilot SABRE experiments at pH 4.0 (acidified sample) versus pH 6.1 (unaltered sodium [1-¹³C]ketoisocaproate) reveal substantial modulation of SABRE-SHEATH processes by pH, warranting future systematic pH titration studies of ketoisocaproate, as well as other structurally similar ketocarboxylate motifs including pyruvate and alpha-ketoglutarate, with the overarching goal of maximizing ¹³C polarization levels in these potent molecular probes. Finally, we also report on the pilot post-mortem use of HP [1-¹³C]ketoisocaproate in a euthanized mouse, demonstrating that SABRE-hyperpolarized ¹³C contrast agents hold promise for future metabolic studies. Full article

The present work investigates the potential for enhancing the NMR signals of DNA nucleobases by parahydrogen-based hyperpolarization. Signal amplification by reversible exchange (SABRE) and SABRE in Shield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) of selected DNA nucleobases is demonstrated with the enhancement (ε) of ¹H, ¹⁵N, and/or ¹³C spins in 3-methyladenine, cytosine, and 6-O-guanine. Solutions of the standard SABRE homogenous catalyst Ir(1,5-cyclooctadeine)(1,3-bis(2,4,6-trimethylphenyl)imidazolium)Cl (“IrIMes”) and a given nucleobase in deuterated ethanol/water solutions yielded low ¹H ε values (≤10), likely reflecting weak catalyst binding. However, we achieved natural-abundance enhancement of ¹⁵N signals for 3-methyladenine of ~3300 and ~1900 for the imidazole ring nitrogen atoms. ¹H and ¹⁵N 3-methyladenine studies revealed that methylation of adenine affords preferential binding of the imidazole ring over the pyrimidine ring. Interestingly, signal enhancements (ε~240) of both ¹⁵N atoms for doubly labelled cytosine reveal the preferential binding of specific tautomer(s), thus giving insight into the matching of polarization-transfer and tautomerization time scales. ¹³C enhancements of up to nearly 50-fold were also obtained for this cytosine isotopomer. These efforts may enable the future investigation of processes underlying cellular function and/or dysfunction, including how DNA nucleobase tautomerization influences mismatching in base-pairing. Full article