FORMATION OF 2,5-DIHYDROXYBENZOIC ACID DURING THE REACTION BETWEEN 1O2 AND SALICYCLIC ACID - ANALYSIS BY ESR OXIMETRY AND HPLC WITH ELECTROCHEMICAL DETECTION

The oxygen consumption, vis-a-vis O-1(2) production during irradiation of dyes such as rose bengal, merocyanine-540, and aluminum phthalocyaninetetrasulfonate, in the presence of salicylic acid was measured by electron spin resonance (ESR) oximetry. Concomitantly, formation of 2,5-dihydroxybenzoic acid (2,5-DHBA) in the same sample was analyzed by HPLC-EC. Both O2 consumption and 2,5-DHBA formation were stimulated by D2O, quenched by azide, unaffected in the presence of catalase, superoxide dismutase, and hydroxyl radical scavengers (ethanol, formate, etc.), and vastly diminished under N2. The stoichiometry between O-1(2) consumption and 2,5-DHBA formation was determined to be ca. 0.5. On the basis of experiments using histidine, the chemical rate constant for the reaction between O-1(2) and salicylic acid was determined to be 0.20 X 10(6) M-1 s-1. Furthermore, O-1(2) generated from the thermal decomposition of the water-soluble endoperoxide of 3,3'-(1,4-naphthylidene)dipropionate (NDPO2) was shown to react with salicylic acid to form 2,5-DHBA as the major product. We conclude that exclusive formation of 2,5-DHBA is highly diagnostic of O-1(2) intermediacy in photochemical systems and in biochemical systems lacking metabolic activity. HPLC-EC is thus a valuable adjunct to ESR oximetry in the characterization of O-1(2) and may, on the basis of the selectivity of this reaction, provide a sensitive analytical method for detecting O-1(2) intermediacy.