Breaching the Fortress: Photochemistry of DNA-Caged Ag.

A DNA strand can encapsulate a silver molecule to create a nanoscale, aqueous stable chromophore. A protected cluster that strongly fluoresces can also be weakly photolabile, and we describe the laser-driven photochemistry of the green fluorophore CACTCGT/Ag. The embedded cluster is selectively photoexcited at 490 nm and then bleached, and we describe how the efficiency, products, and route of this photochemical reaction are controlled by the DNA cage. With irradiation at 496.5 nm, the cluster absorption progressively drops to give a photodestruction quantum yield of 1.5 (±0.2) × 10, ∼10× less efficient than fluorescence. A new λ = 335 nm chromophore develops because the precursor with 4 Ag is converted into a group of clusters with 2 Ag - Ag, Ag, Ag, and Ag. The 4-7 Ag in this series are chemically distinct from the 2 Ag because they are selectively etched by iodide. This halide precipitates silver to favor only the smallest Ag cluster, but the larger clusters re-develop when the precipitated Ag ions are replenished. DNA-bound Ag decomposes because it is electronically excited and then reacts with oxygen. This two-step process may be state-specific because O quenches the red luminescence from Ag. However, the rate constant of 2.3 (±0.2) × 10 M s is relatively small, which suggests that the surrounding DNA matrix hinders O diffusion. On the basis of analogous photoproducts with methylene blue, we propose that a reactive oxygen species is produced and then oxidizes Ag to leave behind a loose Ag-DNA skeleton. These findings underscore the ability of DNA scaffolds to not only tune the spectra but also guide the reactions of their molecular silver adducts.