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Fluorescence probes have been used to estimate Kamlet-Taft solvatochromic parameters alpha and pi* representing hydrogen-bond donation ability and dipolarity/polarizability, respectively, of sodium dodecyl sulfate (SDS)-Triton X 100 (TX100) mixed aggregates with varying compositions. The hydrogen-bond donation ability of the mixed aggregate has been found to increase with SDS composition, whereas the dipolarity/polarizability parameter decreases. The relative contribution of electrostatic and steric effect toward the total free energy of micellization have been calculated for the mixture. The solvatochromic parameters alpha and pi* depend linearly on the total free energy of micellization, indicating a correlation between aggregational and solvatochromic properties.

Proton affinities of the two N atoms of reserpine (methyl-11,17alpha-dimethoxy-18beta-[(3,4,5-trimethoxybenzoyl)oxy]-3beta,20alpha-yohimban-16beta-carboxylate) have been determined in two ways from the pH-dependent variation of the UV-vis absorption spectra (i) of reserpine itself and (ii) of the charge-transfer (CT) spectra of its complexes with o-chloranil, p-chloranil, and DDQ in aqueous medium (containing 0.1% ethanol v/v). For the second method, the CT absorption bands of the complexes were determined, their formation constants were estimated by a modified Benesi-Hildebrand equation, and variation of CT absorption spectra with a change in pH was noted. A necessary working formula for the second method was derived and utilized with the experimental data. The pKa values obtained by the two methods are well in agreement with each other within the limits of experimental error. To our knowledge, so far, this is the first report on determination of pKa from charge-transfer complex formation in aqueous solution using simple absorption spectroscopy in the UV-vis region. The results obtained were further checked by noting the variation of fluorescence intensity of reserpine upon addition of o-chloranil, acid, and base, and almost complete agreement with the absorption spectrometric result was observed.

Individual proton affinities of the four dissociable functional groups of (+/-)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7H-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid (commonly called "ofloxacin" and to be denoted henceforth as OflH), have been determined from the pH-dependent variation of the UV-vis absorption and fluorescence spectra of the compound itself and of its charge transfer complexes (CT) with p-bromanil and p-chloranil (in aqueous medium containing 0.1% ethanol, v/v). To utilize the CT spectra for determination of the proton affinity of the anilinic N, the CT absorption band of the ofloxacin-p-bromanil complex has been studied by changing the pH of the medium. Further, the effect of inclusion on the proton affinities of the four dissociable groups of OflH has been studied in presence of beta-cyclodextrin (beta-CD). Two pK(a) values corresponding to anilinic and tertiary N atoms change, whereas those corresponding to phenolic -OH and aromatic -COOH groups remain unchanged by the addition of beta-CD, a fact that indicates partial inclusion of the ofloxacin molecule in beta-CD. Formation constant and related thermodynamic parameters for the OflH(2)(+).beta-CD inclusion complex in aqueous solution have been determined from absorption intensities. A general relation between pK(a) values of guests having proton-releasing functional groups and formation constants of the inclusion complexes of the protonated and deprotonated forms with a host molecule has been utilized for determination of the formation constant of the OflH(3)(+2).beta-CD complex from the pK(a) values of OflH(3)(+2) in the presence and absence of beta-CD, along with the formation constant of the OflH(2)(+).beta-CD complex. Results of the present study reveal that the N-methylpiperazinyl moiety of ofloxacin is included in beta-CD, and the remaining part of the guest molecule remains outside. Also, in molecular interaction with quinone-type electron acceptors, charge transfer occurs from the aromatic part of the ofloxacin molecule, which is conjugated with the anilinic N atom of ofloxacin.

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