The hyperpolarization of biologically relevant molecules for signal-enhanced magnetic resonance detection holds great promise for studying biological processes in vitro and in vivo. Being important metabolites as well as the building blocks of peptides, the canonical amino acids are promising targets for hyperpolarization, and a number of parahydrogen based strategies have been devised to hyperpolarize amino acids[1-7]. For biochemical and biomedical applications, however, these approaches suffer from limited biocompatibility of the obtained products, due to the utilization of toxic solvents, such as methanol-d4, or because chemically modified (e.g. N-protected) amino acids are obtained.
Herein we present a PHIP strategy for hyperpolarizing proteinogenic amino acids under conditions of improved biocompatibility. We obtain chemically unmodified proteinogenic amino acids, which are directly produced in aqueous media. This is enabled through the use of a heterogeneous nanocatalyst, which allows for PHIP hyperpolarization of the N-unprotected amino acids in water and further provides the prospect of easy catalyst separation. Efficient coherent hyperpolarization transfer from 1H to 13C is achieved at high field, using the recently presented ESOTHERIC approach. This allows for polarization storage on the carbonyl-13C for tens of seconds.
Whereas the strategy presented is demonstrated here for the hyperpolarization of single amino acids, it is likely that the approach can be extended into a general strategy for 13C-hyperpolarizing small synthetic peptides in aqueous solutions.
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