Effect of pH on the fluorescence and absorption spectra of hypericin in reverse micelles

The well-characterized, monodispersed nature of reverse micelles formed by sodium bis(2-ethylhexyl)sulfosuccinate/heptane and their usefulness in appr oximating a membrane-like environment have been exploited to investigate th e effect of pH and water pool size on the photophysical properties of hyper icin (Hyp), Our measurements reveal two titratable groups of pK(a) similar to1.5 and similar to 12.5. These are assigned to the HypH(+)/Hyp equilibriu m (the deprotonation of a carbonyl group) and the Hyp(-)/Hyp(2-) equilibriu m (the deprotonation of a peri hydroxyl group). The low-energy absorbance m axima of HypH(+), of Hyp and Hyp(-) and of Hyp(2-) are 583, 594 and 613 nm, respectively. Neither at pH 13 nor at 1 M HCl is the system entirely in th e Hyp(2-) or the HypN(+) forms. Ours is the first study of Hyp in reverse m icelles as well as the first time-resolved study of Hyp as a function of pH .