An antenna-sensitized nitroindoline precursor to enable photorelease of L-glutamate in high concentrations

1-Acyl-7-nitroindolines are useful reagents for rapid release of carboxylates upon flash photolysis in aqueous solution and have been particularly effective for rapid (submicrosecond) release of neuroactive amino acids such as l-glutamate in biological experiments. In model systems the efficiency of photorelease has been shown to be greatly improved by attachment of a benzophenone triplet-sensitizing antenna. The present work describes synthesis and initial biological evaluation of the l-glutamate precursor 3. A significant improvement in the overall synthesis uses double Boc protection of the glutamate amino group to avoid side reactions during introduction of the nitro group. To accommodate the multiple functionalities in 3, linkage of the nitroindoline and benzophenone moieties is carried out late in the synthesis. Photolysis of 3 occurs with near-quantitative stoichiometry and the released l-glutamate efficiently activates neuronal glutamate ion channels.