Cast

Overview

The first Cast-like nitrobenzyl photocages were developed for Ca²⁺ by Roger Tsien in the 1980s.[1] The modulation of metal-binding affinity depends on the conversion of a benzhydrol substituent on an aniline ring into a benzophenone. The carbonyl introduces an electron-withdrawing group onto the position para to the aniline nitrogen atom. The resonance delocalization of the aniline lone pair onto the carbonyl oxygen makes the aniline nitrogen a poorer metal-binding ligand, which weakens the metal ion affinity of the chelator. Cast complexes exhibit up to a 400-fold decrease in metal binding affinity upon uncaging.

ZinCast, FerriCast and CrownCast photocages have been developed by the Burdette Group. The suffix Cast is evocative of the metal ion being cast off upon photolysis of the photocaging ligand. The prefix refers to either the kind of ligand utilized or the metal ion being caged.

Uncaging in Cast photocages

ZinCast Photocages

ZinCast photocages are a family of Cast compounds designed to have an enhanced selectivity for Zn²⁺ over other divalent metal ions. The prefix is derived from the Zinc selectivity. ZinCast-1 was the first member of this series of photocages. ZinCast-2 and ZinCast-3 systematically change the length of the linkage between the aniline nitrogen atom and the pyridyl ligands. ZinCast-4 and ZinCast-5 are structural isomers that vary the position of the aniline group in relationship to the nitrobenzyl group.

CrownCast Photocages

CrownCast photocages are a class of ligands that are designed to release M²⁺ ions upon exposure to light. CrownCast ligands utilize a crown ether metal ion receptor. CrownCast-1, CrownCast-2 and CrownCast-3 photocages have been developed.

FerriCast Photocages

FerriCast photocages are a class of ligands that are designed to release Fe³⁺ upon exposure to light. The name derives from the ferric (Fe³⁺) oxidation state. FerriCast ligands selectively bind Fe³⁺ in solvents like CH₃CN and THF owing to the incorporation of a unique macrocyclic receptor. FerriCast-1 and related FerriCast Analogues with different nitrobenzyl derivatrives have been studied.

Other Cast Photocages

ArgenCast photocages resemble CrownCast photocages, but utilize non-macrocyclic receptrs ArgenCast-1 utilizes a dimethoxynitrobenzyl chromophore, and ArgenCast Analogues were prepared to investigate nitronaphthyl photocaging groups.