Ф-order kinetics of photoreversible-drug reactions.

Background: Drug photodegradation data are usually treated by zero-, first- or second-order kinetic equations. Such treatments would lack reliability since the aforementioned equations have been originally developed for pure thermal reactions. In this respect, it has recently been shown that unimolecular photodegradations obey Ф-order kinetics (Maafi and Maafi, 2013). However, no similar information is, thus far, available for other reactions including photoreversible AB(2Ф) systems. This paper aims at filling this gap for AB(2Ф) kinetics.

Methods: Runge-Kutta numerical integration data for photoreversible reactions traces were combined with a template equation in order to derive an optimized (semi-empirical) integrated rate-law equation for AB(2Ф) reactions. The proposed model equation was test by examining its ability to fit synthetic Runge-Kutta data that have not been used for the optimization. The obtained fitting parameters are then compared to their theoretical counterparts.

Results: Both an integrated rate-law and an analytical equation for the overall reaction rate-constant were set for photoreversible drug reactions. The values of overall reaction rate-constant and initial velocity obtained theoretically correlated well with those obtained by fitting the kinetic traces of reactions with the derived integrated rate-law. AB(2Ф) photodegradation reactions have been proven to obey Ф-order kinetics. The equation proposed describes faithfully their kinetic behaviour in solution. The formula of the overall rate-constant involves both reagents' characteristics and experimental parameters. These equations facilitated the rationalisation and prediction of the individual effects of each reaction parameter. Specially, our results proved a self-photostabilisation with increasing initial drug-concentration and demonstrated the potential for actinometry of drugs obeying AB(2Ф) mechanism.