Noncovalent Binding of Anthracene and Ciprofloxacin with Molecular Pseudophase: Fluorescence and pH studies



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Pseudophase compounds are molecular or micellar self-assembled aggregates that form in many natural aquatic environments. Pseudophase chemistry offers distinct advantages in modeling phase distributions or complexation in water-colloid and water-surfactant systems where the phases or microemulsions cannot be physically separated to assess binding constants. The goal of the present study was to determine binding constants in preaggregate and postaggregate phases for the solutes ciprofloxacin (CPF) and anthracene, with selected pseudophase compounds. The pseudophase compounds tested included sodium dodecyl sulfate (SDS), DL-α-Tocopherol methoxypolyethylene glycol succinate (TPGS-750M), perfluorooctanesulfonic acid (PFOS), and myelin basic protein (MBP). In order to fully understand the pseudophase interactions, the key equations represented either fluorescence enhancement (FE) or fluorescence quenching (FQ). Equilibrium binding models for anthracene and ciprofloxacin with the pseudophase compounds were derived as: FQ: F0/FT = 1 + K11.S + (Kn1/n)(ST-cac) FE: (FT-F0)/ (CT . α) = [(kC + (k11. K11) S] + (kn1. Kn1/n) (ST – cac)] Where FT is the total fluorescence intensity while F0 is the initial fluorescence of solute. CT is the total concentration of solute and α is the fraction of the solute present at the corresponding pH. ST is the total surfactant concentration, S is the monomer surfactant concentration, n is the aggregation number and cac is the critical aggregation concentration. kC, k11 and kn1 are the fluorescence efficiencies of the free solute, bound solute in the preaggregate phase and bound solute in the postaggregate phase. K11 is the binding constant for the monomer-monomer complex and Kn1 is the binding constant for the monomer-pseudophase complex. K11, Kn1 and cac were extracted from the intercept and slope from the equations above. The fluorescence efficiencies, kC, k11 and kn1, were obtained directly from the experimental data. K11 and Kn1 were obtained for SDS, TPGS-750M and PFOS. The change in the fluorescence intensity was too low to measure reliable binding constants for the CPF-MBP system. The equations were evaluated experimentally using both static and continuous-flow fluorometric titrations. The influence of pH on CPF fluorescence was rigorously evaluated at pH 1-11 where CPF speciation was calculated based on CPF as a tetraprotic acid. It was determined that the interactions between CPF and the pseudophase compounds were dependent on properties such as pH, salt concentration, and quenching compounds like O2 radical and Cl-. FE was observed when CPF interacted with SDS while overall FQ was observed with PFOS and TPGS-750M. The binding of the anthracene-TPGS-750M system was significant where K11 was 292 ± 8 M-1 and Kn1 was (2.8 ± 0.05) x 104 M-1. The binding was higher when CPF is considered a tetraprotic acid compared to a triprotic acid with PFOS where Kn1 was 438.2 ± 56 M-1 and 175.7 ± 14 M-1 respectively for the zwitterionic species. The highest interactions occurred with CPF- SDS complexation in the postaggregate phase with a Kn1 of 1780 ± 71 M-1. The highest binding in the preaggregate phase occurred with the CPF-PFOS tetraprotic complex where K11 was 160 ± 3 μM-1. The most significant variables to consider are pH, and ionic strength. The highest binding occurred at neutral pH and the lowest in acidic pH. These factors significantly influence the interactions or lack of interactions that occur with the molecular pseudophase. By fully understanding the interactions between the preceding compounds, this in turn provides a greater understanding in the fate and effects and potential removal from the environment.