Photolabile precursors of glutamate: Synthesis, photochemical properties, and activation of glutamate receptors on a microsecond time scale

Newly synthesized photolabile derivatives of glutamate, caged glutamate, that release free glutamate on a microsecond time scale after a pulse of UV laser light are described. 2-Nitrobenzyl derivatives were attached to the amino or carboxyl groups of glutamate. Substitution with a -CO₂/^- group at the benzylic carbon accelerates the photolysis reaction when compared to -H and -CH₃ substituents. γ-O-(α-Carboxy-2-nitrobenzyl)glutamate is stable at neutral pH. In 100 mM phosphate buffer at pH 7.0, the compound is photolyzed at 308 nm with a quantum product yield of 0.14. The half-life of the major component of the photolytic reaction, as judged by the transient absorbance change at 430 nm, is 21 μs (≃90%); the half-life of a minor component (≃10%) is 0.2 ms. The amine-linked derivatives have half-lives in the millisecond region and a 4-fold lower quantum yield. The potential of the newly synthesized compound for use in rapid chemical kinetic investigations of glutamate receptors is demonstrated. (i) The caged glutamate at 1 mM concentration does not desensitize glutamate receptors in rat hippocampal neurons. (ii) Caged glutamate (1 mM) does not inhibit activation of the receptors by 50 μM glutamate. (iii) Photolysis of the compound induces rapid onset of transmembrane currents in rat hippocampal neurons.