Kinetics of the hydrolysis of cytotoxic pyronins in micellar systems

Abstract

As part of an investigation into the biological applications of cationic dyes of the xanthene class, the micellar-enhanced kinetics of pyronin Y (PY) and B (PB) were studied in two types of enriched solution of surfactant. The alkaline hydrolysis which transforms the R + pyronin species into neutral ROH (the so-called fading of red pyronins into colourless xanthydrols) was studied in cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulphate (SDS) micelles. The critical micelle concentration (cmc) at pH7 was measured by the spectral-change technique using rhodamine 123 as the fluorescent probe. In addition, the effective cmcs of the surfactants in strong alkaline solutions (cmc CTAB , cmc SDS ) were determined using the ROH species of PY as the fluorescer. The λ Fluo, max (R + ) values were unaffected by either CTAB or SDS micellization, but the λ Fluo, max values of the ROH forms were blue shifted by CTAB and SDS micellization when compared with the values obtained in neat water solutions of similar pH. Changes in the equilibrium quotient [R + ]/ [ROH] in favour of ROH and R + respectively were observed in the presence of CTAB and SDS micelles (up to 2 pH units) with respect to p K a (S 1 )=p K a (S 0 =11.4 measured for both pyronins in water. The micelles strongly affect the kinetics of the hydrolysis. As determined by fluorometric titration, the pseudo-first-order rate constants k obs (min −1 of R + hydrolysis were substantially enhanced by the positive charges of CTAB micelles but hindered by the negative charges of SDS micelles. The hydrolysis was completely inhibited by SDS micelles when [SDS] &>; cmc SDS , but was speeded up by CTAB micelles when [CTAB]&>;cmc CTAB , as demonstrated by the three-fold higher rate measued in the presence of CTAB micelle when compared with the values obtained in neat water. These results were rationalized in terms of elecrostatic interactions between the R + substrate and the hydroxide ion reactant. The reaction was catalysed by CTAB micelles when the reactants were both concentrated in the interfacial microenvironment (the micellar pseudophase) of the micelles. Conversely, the hydrolysis was inhibited k obs =0) by anionic micelles of SDS which were depleted in OH − . According to Menger's model, the values of the hydrolysis rate constant K m in the micellar phase were obtained by plotting the variation in k obs vs . the micellized surfactant concentration [D m ]. k m (min −1 ) values were three orders of magnitude higher than the pseudo-first-order rate constant k ′2 (min −1 = k 2 [OH − ] measured in water in the same conditions of alkaline pH.