title: Effect of Nicotinamide on the Photolysis of Riboflavin in Aqueous Solution.
abstract: The photolysis of riboflavin (RF) in aqueous solution in the presence of nicotinamide (NA) by visible light has been studied in the pH range 1.0-12.0 and the various photoproducts have been identified as known compounds. RF has been determined in degraded solutions by a specific multicomponent spectrometric method in the presence of its photoproducts and NA. The second-order rate constants (k 2) for the bimolecular interaction of RF and NA range from 0.54 (pH 1.0) to 9.66 M(-1) min(-1) (pH 12.0). The log k 2-pH profile for the photolysis reaction follows a sigmoid curve showing a gradual increase in the rate of pH due to a change in the ionization behavior of the molecule. The lower rate in the acid region is probably due to protonation of the molecule since the cationic form of RF is less susceptible to photolysis than the neutral form. Similarly, a slowing of the rate in the alkaline region is due to anion formation of the molecule. NA is involved as an electron acceptor during the sequence of reactions and thus enhances the rate of photolysis of RF. Absorption and fluorescence measurements did not provide evidence for the complex formation between the two compounds under the present conditions.
authors.affiliation: Baqai Institute of Pharmaceutical Sciences, Baqai Medical University, Toll Plaza, Super Highway, Gadap Road, Karachi 74600, Pakistan.
authors.forename: Iqbal
authors.initials: I
authors.lastname: Ahmad
authors.affiliation: Baqai Institute of Pharmaceutical Sciences, Baqai Medical University, Toll Plaza, Super Highway, Gadap Road, Karachi 74600, Pakistan.
authors.forename: Sofia
authors.initials: S
authors.lastname: Ahmed
authors.affiliation: Baqai Institute of Pharmaceutical Sciences, Baqai Medical University, Toll Plaza, Super Highway, Gadap Road, Karachi 74600, Pakistan.
authors.forename: Muhammad Ali
authors.initials: MA
authors.lastname: Sheraz
authors.affiliation: Baqai Institute of Pharmaceutical Sciences, Baqai Medical University, Toll Plaza, Super Highway, Gadap Road, Karachi 74600, Pakistan.
authors.forename: Zubair
authors.initials: Z
authors.lastname: Anwar
authors.affiliation: Baqai Institute of Pharmaceutical Sciences, Baqai Medical University, Toll Plaza, Super Highway, Gadap Road, Karachi 74600, Pakistan.
authors.forename: Kiran
authors.initials: K
authors.lastname: Qadeer
authors.affiliation: Department of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Karachi, Karachi 75270, Pakistan.
authors.forename: Adnan
authors.initials: A
authors.lastname: Noor
authors.affiliation: Department of Physics, Sevastopol State University, Universitetskaya str. 33, Sevastopol 299053, Russia; Department of Biological and Chemical Sciences, Belgorod National Research University, Pobeda str. 85, Belgorod 308015, Russia.
authors.forename: Maxim P
authors.initials: MP
authors.lastname: Evstigneev
keywords: Kinetics; Nicotinamide; Photolysis; Rate-pH profile; Riboflavin