.beta.-Carboline Photosensitizers. 3. Studies on Ground and Excited State Partitioning in AOT/Water/Cyclohexane Microemulsions

Abstract

The photophysical behavior of the P-carbolines norharmane, h m a n e , and harmine has been studied in waterin-oil (w/o) microemulsions in AOT/cyclohexane. With pure solvents, solubility and partitioning studies indicate that these molecules dissolve preferentially in the organic phase but that their solubility in the aqueous phase increases upon decreasing pH as a consequence of protonation. However, in w/o microemulsions these molecules appear to be located predominantly at the surfactant/water interface. Again, decreasing pH increases partitioning into the water region. From absorption and fluorescence spectral studies with the microemulsion system, differences are observed in the predominant species at any "pH' compared with that in pure water. From analysis of fluorescence decays, four kinetically distinguishable species have been observed in the excited state, namely, neutral species in cyclohexane, neutral species in the microemulsion droplets, cation, and zwitterion. For harmine, the effect of waterhfactant ratio (WO) on the photophysical behavior and distribution of these species was studied. Partitioning was observed between aqueous and organic subphases. A study has been carried out on the effect of pH on these processes. In agreement with other studies on "pH' in microemulsions, the cation is observed up to apparent pH values much higher than those for pure aqueous solutions. The results are interpreted in terms of the compartmentalization of the hydroxide ions between the water pools. Effects of changing wo on the decay of the various species are interpreted in terms of "quenching" of the neutral species in the cyclohexane subphase by its trapping within the microemulsion droplets. A kinetic model for this is presented. The effect of changing the polarity (by addition of methanol) upon the partitioning behavior has been studied, and it is shown that addition of methanol favors partitioning into the organic phase.

Protein Stabilization by Osmolytes from Hyperthermophiles

“…The combined effect of the two variations induces the observed decrease of both fluorescence quantum yield and lifetime from dioxane to water. A similar increase of k nr has been observed for other compounds in dioxane-water mixtures and has been attributed to the increase of the internal conversion rate constant upon increasing water content (31)(32)(33)(34)(35).…”

“…These solvent mixtures were used to represent the range of environment polarity sensed by tryptophan when the protein undergoes unfolding. Dioxane-water mixtures have been successfully used to mimic the microenvironment sensed by fluorescent probes in heterogeneous media, such as micelles (34) and micro-emulsions (35).…”

Fluorescence resonance energy transfer from tryptophan in human serum albumin to a bioactive indoloquinolizine system

“…are by now well established as bioactive molecules. [12][13][14][15][16] The single-step synthesis of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ) from 1-methyl-3,4-dihydro-β-carboline leads to the assumption that the molecule should have biological activity. 17 Through a series of experiments we have already established that the fluorophore molecule serves as an excellent fluorescent probe for biological systems.…”

Effect of nanocavity confinement on the rotational relaxation dynamics: 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine in micelles

“…11 Electron donor-acceptor molecules capable of photoinduced intramolecular charge transfer ͑ICT͒ have been employed as reporters of microenvironments in micelles, reverse micelles, and biological membranes. 5,[12][13][14][15][16][17] The fluorophore 3-acetyl-4-oxo-6,7-dihydro-12H indolo-͓2,3-a͔ quinolizine ͑AODIQ͒ used in the present experiment has recently been shown to be an excellent probe for biological systems. 12,18 In a recent work, 19 we reported that the fluorometric behavior of AODIQ with dual emission from a locally excited ͑LE͒ state and a charge transfer ͑CT͒ state is very much dependent on the solvent polarity.…”