Caged nitric oxide. Stable organic molecules from which nitric oxide can be photoreleased.

We report the synthesis and testing of a series of "caged" nitric oxide compounds that are stable indefinitely in oxygen-containing solutions until photolyzed by ultraviolet irradiation, whereupon they release nitric oxide (NO) with quantum yields of delta 8% for 3a (CNO-1) and delta 2% for compounds 3b-e (CNO2-5). After a flash, NO release is complete within 5 ms, so that precise temporal control of NO release is possible. NO donor 3d (CNO-4) includes two carboxylate negative charges at physiological pH, which reduce membrane permeability and enable photolytic generation of NO to be selectively confined to either extracellular or intracellular compartments. Esterification of these carboxyls with acetoxymethyl groups produces 3e (CNO-5), which is membrane-permeant and intracellularly hydrolyzable. Therefore, large populations of intact cells can be conveniently intracellularly loaded with "caged" NO donor 3d by incubation with 3e (CNO-5). The biological efficacy of these NO donors and their absolute dependence on UV-irradiation was demonstrated by inhibition of thrombin-stimulated platelet aggregation. Extracellular hemoglobin blocked the effects of NO generated outside but not inside platelets, verifying the sidedness of the NO donors and the limited spatial range of NO action. These molecules should permit precise spatial, temporal, and concentration control of NO release for investigation of its important biological functions.