The pH-Dependent primary photoreactions of Ochratoxin A

Steady-state and time-resolved spectroscopies are used to elucidate the primary photoprocesses following the excitation of ochratoxin A (OTA), its dechlorinated derivative ochratoxin B (OTB), and O-methyl ether of OTA (MOA). The excited-state dynamics of OTA and OTB depend on the protonation of the isocoumarin moiety. Fluorescence spectra of the protonated forms reveal anomalously large Stokes shifts that are attributed to the enol tautomer formed via an intramolecular excited-state proton transfer. No evidence for "normal" emission of the keto form of OTA and OTB is found even in aqueous solutions. MOA, which lacks a proton on the phenol moiety and exists, therefore, only in the keto form, exhibits weak fluorescence with a substantially smaller Stokes shift. The deprotonated species show relatively strong emission typical for phenolate anions. OTA decomposes slowly upon UV irradiation in aqueous solutions. The photoreaction quantum yield varies significantly with solution pH and O-2 concentration. The highest yield is observed for the deprotonated form of OTA in deoxygenated solutions. The corresponding hydroquinone (OHQ) is identified as a major photoproduct. Monophotonic photoionization of the fully deprotonated OTA in aqueous solution is demonstrated with nanosecond laser flash photolysis. In the absence of O-2 and other scavengers, hydrated electrons are trapped by OTA in the ground state with the diffusion-controlled rate constant. Photoirradiation of OTA in the presence of supercoiled plasmid DNA results in the formation of relaxed circular DNA (form II). The yield of form II correlates with the quantum yield of OTA photodecompositon in these solutions, because the photocleavage efficiency is higher in the absence Of O-2 and at basic pH.