DIFFERENT PHOTODIMERIZATION BEHAVIOR OF TRANILAST IN ALPHA-CYCLODEXTRIN, BETA-CYCLODEXTRIN AND GAMMA-CYCLODEXTRIN COMPLEXES - CAVITY-SIZE AND STOICHIOMETRY DEPENDENCE

The effects of alpha-, beta- and gamma-cyclodextrins (CDxs) on the photoisomerization and photodimerization of an antiallergic drug tranilast [1, N-(3,4-dimethoxycinnamoyl)anthranilic acid] have been kinetically investigated. Alpha- and beta-CDxs decelerate both the photoisomerization and photodimerization of 1, showing saturation kinetics, due to 1:1 complex formation. On the other hand, gamma-CDx accelerates the photodimerization of 1 at higher 1:gamma-CDx molar ratios, but decelerates it at lower 1:gamma-CDx molar ratios. The photoisomerization of 1 is decelerated over the gamma-CDx concentration range employed. The continuous variation plot of CDx-induced circular dichroism intensities of 1 and the solubility data indicate that 1 forms inclusion complexes with gamma-CDx in different stoichiometry, depending on the guest:host molar ratio. At higher 1:gamma-CDx molar ratios, 1 formed the 2:1 (guest:host) complex which accelerates by about 5500 times, the dimerization of 1. With increasing gamma-CDx concentration, 1:1 and 1:2 complexes are formed and the dimerization rate of 1 decreases markedly; in particular, the 1:2 complex decelerates it by 19 300 times. The results reveal clear evidence for the cavity-size- and stoichiometry-dependent dimerization of 1 in CDx complexes.