Bis(quaternary ammonium halide) surfactants (gemini surfactants) having, variously, diethyl ether, monohydroxypropyl, and dihydroxybutyl spacer groups have been investigated by surface tension, interfacial tension, and steady-state fluorescence techniques. The critical micelle concentration (cmc) and area per molecule (A min) are shown to deviate from the expected patterns of behavior as the number of carbon atoms in the alkyl chain (n) increases beyond a certain maximum. This aberrant behavior is observed at the hydrocarbon/water as well as the aqueous/air interface. The unexpected values of the physicochemical parameters at long alkyl chain length have been interpreted on the basis of a concentration region in which submicellar or multilayer structures are forming. Fluorescence measurements provide confirmation of cmc values by an alternative technique. Comparison of the fluorescence emission maxima profiles of the gemini surfactants with those of their monoquaternary analogues demonstrates that there is a continuously changing shape with change in n for the geminis, whereas the profiles for nongeminis are invariant. Long-chain geminis exhibit a gradually sloping sigmoidal profile, indicating a variety of environments experienced by pyrene-3-carboxaldehyde between the totally aqueous environment at low surfactant concentration and the hydrophobic (entirely micellar) environment at high surfactant concentration. The large variation in the polarity of the probe environment between these two extremes may be attributed to the formation of submicellar structures.

A fluorescent phospholipid derivative, the fluoresceinthiocarbamyl adduct of a natural phosphatidylethanolamine, has been synthesized and incorporated into sonicated single-bilayer vesicles of egg lecithin and dipalmitoyllecithin. The surface location of this probe has been confirmed by using extrinsic fluorescence quenching studies together with steady-state emission anisotropy measurements. Electronic excitation energy transfer between 1,6-diphenyl-1,3,5-hexatriene incorporated within the hydrophobic core of the bilayer and the novel derivative has been investigated to estimate the depth within the bilayer at which the former is located. Efficiencies have been measured for two different phospholipids, egg lecithin and dipalmitoyllecithin, in the latter case both above and below the phospholipid phase transition, with and without added cholesterol. The observed dependence of the transfer efficiency on the acceptor concentration was compared with that calculated according to Förster theory applied to random two-dimensional distributions of donor and acceptor molecules in parallel planes for various interplanar separations, taking into account orientational effects. The Förster R0 of about 45 A for this donor-acceptor pair is particularly well suited to such studies since it is of the order of the width of the bilayer. The experiments showed that energy-transfer spectroscopy can provide useful quantitative information as to the transverse location of diphenylhexatriene in homogeneous phospholipid bilayers and may also reflect lateral partitioning of donor or of both donor and acceptor into different phases in systems exhibiting phase separations.

The aggregation behavior of four series of bis(quaternary ammonium halide) surfactants (gemini surfactants) having diethyl ether, dihydroxybutyl, monohydroxypropyl, and dimethylene phenylene spacer groups has been studied using steady-state and time-resolved fluorescence spectroscopy. Aggregation numbers were determined using the time-resolved single photon counting method with pyrene as the probe. At certain surfactant concentrations, aggregation numbers of 2, expressed as gemini molecules per micelle, were obtained in all four series when the number of carbon atoms in the alkyl chain length (n) increased beyond a maximum. These long-chain geminis also have a critical micelle concentration (cmc) greater than expected on the basis of plots of log cmc vs n for the shorter chain homologues. This deviation has been attributed to the formation of premicellar aggregates in the surfactant concentration region between the observed and the expected cmc values. The aggregation numbers obtained here indeed point to the existence of dimers in this region. Steady-state fluorescence measurements of I1/I3 values, which are indicative of the polarity of the probe environment, are followed as a function of surfactant concentration. These ratios are used to compare cmc values obtained by surface tension methods as well as to confirm the more hydrophilic nature of systems where dimers are thought to be present.

Three analogues of the alpha-mating factor pheromone of Saccharomyces cerevisiae containing the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group were synthesized that had high binding affinity to the receptor and retained biological activity. The fluorescence emission maximum of the NBD group in [K7(NBD),Nle(12)]-alpha-factor was blue shifted by 35 nm compared to buffer when the pheromone bound to its receptor. Fluorescence quenching experiments revealed that the NBD group in [K7(NBD),Nle(12)]-alpha-factor bound to the receptor was shielded from collision with iodide anion when in aqueous buffer. In contrast, the emission maximum of NBD in [K7(ahNBD),Nle(12)]-alpha-factor or [Orn7(NBD),Nle(12)]-alpha-factor was not significantly shifted and iodide anion efficiently quenched the fluorescence of these derivatives when they were bound to receptor. The fluorescence investigation suggests that when the alpha-factor is bound to its receptor, K7 resides in an environment that has both hydrophobic and hydrophilic groups within a few angstroms of each other.

The two-state excited-state proton-transfer process for d-equilenin [d-3-hydroxyestra-1,3,-5(10),6,8-pentaen-17-one] and dihydroequilenin is found to depend both on pH and on proton acceptor concentration. Both the protonated and deprotonated forms of the excited molecule are fluorescent. As is the case for 2-naphthol, the excited-state pKa (pKa*) is substantially lower than the ground-state pKa. Fluorescence decay studies have been performed as a function of emission wavelength in aqueous solutions at pH 6.9 in the presence of acetate anion (0.1 M). At this pH, both back-reaction from the excited-state and ground-state heterogeneity are minimal. A monoexponential decay is found in the blue region of the spectrum and a biexponential decay on the red edge. The lifetimes measured across both regions are constant, with a negative preexponential term, characteristic of an excited-state reaction, evident at longer wavelengths. Decay-associated spectra (DAS), the preexponential terms associated with the measured lifetimes, have been acquired for these aqueous solutions. Equilenin and dihydroequilenin are found to adsorb to dimyristoyllecithin (DML) vesicles. Rates for excited-state proton transfer are greatly reduced when dihydroequilenin adsorbs to vesicles. The accessibility of the bound probe to acetate as a proton acceptor depends on the cholesterol content of the vesicles.

The mechanism of antimalarial action of the ruthenium-chloroquine complex [RuCl 2 (CQ)] 2 (1), previously shown by us to be active in vitro against CQ-resistant strains of Plasmodium falciparum and in vivo against P. berghei, has been investigated. The complex is rapidly hydrolyzed in aqueous solution to [RuCl(OH 2 ) 3 (CQ)] 2 [Cl] 2 , which is probably the active species. This compound binds to hematin in solution and inhibits aggregation to β-hematin at pH ∼ 5 to a slightly lower extent than chloroquine diphosphate; more importantly, the heme aggregation inhibition activity of complex 1 is significantly higher than that of CQ when measured at the interface of noctanol-aqueous acetate buffer mixtures under acidic conditions modeling the food vacuole of the parasite. Partition coefficient measurements confirmed that complex 1 is considerably more lipophilic than CQ in n-octanol-water mixtures at pH ∼ 5. This suggests that the principal target of complex 1 is the heme aggregation process, which has recently been reported to be fast and spontaneous at or near water-lipid interfaces. The enhanced antimalarial activity of complex 1 is thus probably due to a higher effective concentration of the drug at or near the interface compared with that of CQ, which accumulates strongly in the aqueous regions of the vacuole under those conditions. Furthermore, the activity of complex 1 against CQ-resistant strains of P. falciparum is probably related to its greater lipophilicity, in line with previous reports indicating a lowered ability of the mutated transmembrane transporter PfCRT to promote the efflux of highly lipophilic drugs. The metal complex also interacts with DNA by intercalation, to a comparable extent and in a similar manner to uncomplexed CQ and therefore DNA binding does not appear to be an important part of the mechanism of antimalarial action in this case.

We have constructed a fiber optic device that internally flows triplet oxygen and externally produces singlet oxygen, causing a reaction at the (Z)-1,2-dialkoxyethene spacer group, freeing a pheophorbide sensitizer upon the fragmentation of a reactive dioxetane intermediate. The device can be operated and sensitizer photorelease observed using absorption and fluorescence spectroscopy. We demonstrate the preference of sensitizer photorelease when the probe tip is in contact with octanol or lipophilic media. A first-order photocleavage rate constant of 1.13 h−1 was measured in octanol where dye desorption was not accompanied by readsorption. When the probe tip contacts aqueous solution, the photorelease was inefficient because most of the dye adsorbed on the probe tip, even after the covalent ethene spacer bonds have been broken. The observed stability of the free sensitizer in lipophilic media is reasonable even though it is a pyropheophorbide-a derivative that carries a p-formylbenzylic alcohol substituent at the carboxylic acid group. In octanol or lipid systems, we found that the dye was not susceptible to hydrolysis to pyropheophorbide-a, otherwise a pH effect was observed in a binary methanol-water system (9:1) at pH below 2 or above 8.

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