Effect of hydroxy groups on X-ray-induced reactions of azo benzene derivatives

Caged compounds whose chemical bonds are cleavable by specific stimuli are useful tools for life science research because they facilitate control of various biological activities spatiotemporally. Although caged compounds activatable by hard X-rays can be employed for control in deep tissue owing to the high bio-permeability of X-rays, chemical bond cleavage by ionizing radiation has not been investigated adequately. Previously, we demonstrated that an azo bond tethered to a rhodamine scaffold can be efficiently cleaved by hydrated electrons, which is one of the radiolysis products of water, to release rhodamine. In this study, we synthesized novel azo benzene derivatives, AZO1-4, which can release 3-aminobenzamide (3-ABA), a poly (ADP-ribose) polymerase (PARP) inhibitor, and hydroxy groups or amino groups were introduced into them in order to assess the substituent effect on azo bond cleavage. While the amount of 3-ABA was nearly the same for all the azo compounds, decomposition of azo compounds increased according to the number of hydroxy groups. Furthermore, a methoxyl-radical-adding product was detected from AZO2. These results suggested that the hydroxy group accelerates not azo bond cleavage but the other decomposition pathway.