Improved Photobactericidal Activity of Ultraviolet-A Light in Combination with Isomerizable p-Coumaric Acid Derivatives

In order to improve the photobactericidal activity of ultraviolet-A (UV-A) mediated by reactive oxygen species (ROS), the present study focused on trans-coumaric acid (trans-CA), which is isomerized by UV-A. Generation of ROS was expected during the isomerization of trans-CA. Trans-CA derivatives, in which the carboxyl group was modified with a methyl, n-butyl or phenyl group, thereby changing the interaction with the cellular membrane by quenching the anionic properties of the carboxyl group and changing the UV adsorption properties, were used. The photobactericidal activities of trans-CA derivatives were evaluated by using UV-A light (wavelength 350 to 385 nm). The number of surviving Escherichia coli NBRC12713 was determined by colony-forming assay. Derivative 4c, which was esterified with a phenyl group, reduced survival by more than 5.0-log at a dose of 7.4 J/cm(2) and by 3.2-log at a dose of 4.9 J/cm(2). This synergistic activity may have been caused by the absorption of photon energy from UV-A, which is attributable to the UV spectrum of 4c. The photobactericidal activity was comparable to that of riboflavin, a known photo-activated agent. Isomerized molecules serve as a promising lead for improving the photobactericidal activity of UV-A by activating molecule-mediated ROS generation.