(13)C/(15)N‐Enriched l‐Dopa as a Triple‐Resonance NMR Probe to Monitor Neurotransmitter Dopamine in the Brain and Liver Extracts of Mice

In an attempt to monitor μm‐level trace constituents, we applied here (1)H‐{(13)C‐(15)N} triple‐resonance nuclear magnetic resonance (NMR) to (13)C/(15)N‐enriched l‐Dopa as the inevitable precursor of the neurotransmitter dopamine in the brain. The perfect selectivity (to render endogenous components silent) and μm‐level sensitivity (700 MHz spectrometer equipped with a cryogenic probe) of triple‐resonance allowed the unambiguous and quantitative metabolic and pharmacokinetic analyses of administered l‐Dopa/dopamine in the brain and liver of mice. The level of dopamine generated in the brain (within the range 7–76 μm, which covers the typical stimulated level of ∼30 μm) could be clearly monitored ex vivo, but was slightly short of the detection limit of a 7 T MR machine for small animals. This work suggests that μm‐level trace constituents are potential targets of ex vivo monitoring as long as they contain N atom(s) and their appropriate (13)C/(15)N‐enrichment is synthetically accessible.