Generation of hyperpolarized substrates by secondary labeling with [1,1-13C] acetic anhydride

In this manuscript, the remarkable NMR signal enhancement that can be provided by dynamic nuclear polarization (DNP) was combined with the reactivity of acetic anhydride with amines to perform rapid, high SNR analyses of amino acid mixtures and to hyperpolarize new biomolecules of interest. [1,1-C-13] acetic anhydride is an excellent substrate for DNP hyperpolarization because it can be well polarized in only 30 min and has a relatively long T-1 relaxation time (33.9 s at 11.7 T and 37 degrees C). The secondary hyperpolarization approach developed in this project takes advantage of the preferential reaction of acetic anhydride with amine nucleophiles, which occurs much more rapidly than hydrolysis to produce hyperpolarized N-acetyl adducts. This new approach was used to reproducibly and near-quantitatively (mean yield - 89.8%) resolve a mixture of amino acids Gly, Ser, Val, Leu, and Ala in a single acquisition (3 s) with a signal enhancement of up to 1,400-fold as compared with thermal equilibrium. Secondary hyperpolarization was performed for several small peptides and N-acetylcysteine, a drug administered intravenously to treat acetaminophen overdose. Although, in general the T-1 of the N-acetyl adducts decreased with increasing molecular weight of the biomolecules, the T-1 values were still on the order of 10 s, and the correlation of T-1 with molecular weight was not exact suggesting the potential of secondarily polarizing relatively large biomolecules. This study demonstrates the feasibility of using prepolarized [1,1-C-13] acetic anhydride and rapid chemical reactions to provide high SNR NMR spectra of amino acid derivatives and other biomolecules.