A photocaged, cyclopropene-containing analog of the amino acid neurotransmitter glutamate

Abstract

Substituted cyclopropenes serve as compact biorthogonal appendages that enable analysis of biomolecules in complex systems. Neurotransmitters, a chemically diverse group of biomolecules that control neuron excitation and inhibition, are not among the systems that have been studied using biorthogonal chemistry. Here we describe the synthesis of cyclopropene-containing analogs of the excitatory amino acid neurotransmitter glutamate starting from a Garner’s aldehyde-derived alkyne. The deprotected cyclopropene glutamate was stable in solution but decomposed upon concentration. Appending a light-cleavable group improved the stability of the cyclopropene while simultaneously caging the neurotransmitter. This strategy has the potential to permit deployment of cyclopropene-modified glutamate as a bioorthogonal probe of the neurotransmitter glutamate in vivo with spatiotemporal precision.

Caged Cyclopropenes with Improved Tetrazine Ligation Kinetics

“…Taking these factors into account in the kinetic analysis, we determined the second-order rate constants with tetrazine S17 and 3,6-di-2-pyridyl-1,2,4,5-tetrazine to be 0.11 and 0.05 M –1 s –1 , respectively (Figure F and Figures S6 and S7). The more sluggish kinetics with 3,6-di-2-pyridyl-1,2,4,5-tetrazine than with S17 was expected on the basis of literature reports of cyclopropene kinetics with this tetrazine. ,, This rate of uncaged ketone cyclopropene (0.05–0.11 M –1 s –1 ) is substantially higher than that of the caged version and 125–270-fold higher than that of the first-generation cyclopropene, thereby improving its potential for applications that require faster ligation kinetics, spatial and/or temporal control, and small bioorthogonal tags like cyclopropene. ,,, …”

Precise spatiotemporal control of voltage-gated sodium channels by photocaged saxitoxin

“…Photocaged molecules provide unique advantages over traditional pharmacologic agents and pro-drugs, namely precise temporal and spatial release, tunability, and reagentless deprotection. Neuroactive compounds including GABA 8 , glutamic acid 9,10 , dopamine 11 , serotonin 12 , Leu-enkephalin 13 , cAMP 14 , lipids 15 , 4aminopyridine [16][17][18] , and calcium 19 have been caged in order to study signaling mechanisms in cells, tissues, and whole organisms. Despite this impressive collection of photo-protected reagents, to our knowledge, no such compound has been designed to directly target action potential (AP) initiation in electrically excitable cells.…”

Precise spatiotemporal control of voltage-gated sodium channels by photocaged saxitoxin

“…Photocaged molecules provide unique advantages over traditional pharmacologic agents and pro-drugs, namely precise temporal and spatial release, tunability, and reagent-less deprotection. Neuroactive compounds including GABA 8 , glutamic acid 9,10 , dopamine 11 , serotonin 12 , Leu-enkephalin 13 , cAMP 14 , lipids 15 , and calcium 16 have been photocaged in order to study signaling mechanisms in cells, tissues, and whole organisms. Despite this impressive collection of photo-protected reagents, to our knowledge, no such compound has been designed to directly target action potentials (APs), the fundamental mode of communication in electrically excitable cells.…”