New Photoremovable Protecting Groups for Carboxylic Acids with High Photolytic Efficiencies at Near‐UV Irradiation. Application to the Photocontrolled Release of L ‐Glutamate

We report here the syntheses and the photolytic properties of 3‐(4,5‐dimethoxy‐2‐nitrophenyl)‐2‐butyl (DMNPB) esters as new photoremovable groups for carboxylic acids, and their use for the caging of L ‐glutamate. A high‐yielding synthesis of the DMNPB esters led to a 4:1 threo / erythro diastereomeric mixture, which could be separated by HPLC. While these esters were stable in neutral buffer, photolysis at 364 nm induced a ≥95 % release of the carboxylic acid, with a 0.26 quantum yield for L ‐glutamate formation. L ‐Glutamate release was also possible by two‐photon photolysis with an action cross section of 0.17 GM at 720 nm. Laser photolysis at 350 nm generated a transient species at around 410 nm, attributed to a quinonoid aci‐nitro intermediate that decayed in the submillisecond time range ( t 1/2 =0.53 ms) for the faster γ ‐ L ‐glutamyl threo ‐esters. Given the absorbance of these esters ( λ max =350 nm; ε =4500), the threo DMNPB esters represent new caging groups that can be efficiently photolyzed at near‐UV wavelengths. An efficient and rapid photolytic release of L ‐glutamate has been demonstrated on hippocampal neurons in primary culture.