The role of 4- and 5-aminosalicylic acids in a riboflavin-photosensitised process

A kinetic study on the effect of the therapeutic anti-oxidative drugs 4- and 5-aminosalicylic acids (ASA) in a Vitamin B2 (riboflavin (Rf))-photosensitised process was performed. Employing Rf 0.01–0.02 mM and ASA 0.5–1 mM both in the excited singlet and triplet states of the pigment are quenched by ASA with rate constant values slightly lower than the diffusional ones. From the latter states singlet molecular oxygen and Rf− are generated and subsequently scavenged with the concomitant degradation of ASA, mainly by means of superoxide radical anion. As a consequence, the photodegradation of the vitamin is impeded due to the electron transfer process from Rf− to ground state dissolved oxygen.