Electron-Transfer Reactions in Micelles: Dynamics of Psoralen and Coumarin Radical Cations

Synergistic dynamics of photoionization and photoinduced electron transfer probed by laser flash photolysis and ultrafast fluorescence spectroscopy

Photoionization (PI) and photoinduced electron transfer (PET) dynamics of coumarin 450 (C450) in micelles were investigated in the time domains of micro to femtoseconds using steady-state and time-resolved absorption and fluorescence spectroscopy. The PI of C450 occurs inside the micelles leads to the formation of C450 cation radical (CR) and hydrated electron, which is characterized by the respective transient absorption. The PI of C450 is monophotonic in nature and the yield is dependent on the charge of the micelles. The observation of amine CR in the transient absorption confirms the PET from amine to the excited state of C450 in micelles, which results in the quenching of both fluorescence intensity and lifetime. The decrease in femtosecond fluorescent decay of C450 and the absence of transient C450 radical anion in the presence of amine implies that the concerted ultrafast PET promoted PI and PET to the C450 CR-electron pair. The decrease in the time constant for the formation of relaxed state in the presence of amines is due to the ultrafast PET to the C450 CR-electron pair, which prevents the formation of a relaxed state through recombination at a longer time scale. In the present investigation, the recombination dynamics of the CR-electron pair is justified as one of the origins of the slow solvation in micelles. The influence of amine concentration on the decay of C450 CR indicates ET reaction between C450 CR and amine, which is further confirmed by the bleach recovery of C450 ground state in the presence of amine.

Tuning of electron transfer reactions in pluronic-surfactant supramolecular assemblies

Photoinduced electron transfer (ET) reaction between an anionic acceptor, coumarin-343 (C343), and a neutral donor, N,N-dimethylaniline (DMAN), has been investigated in composite supramolecular assemblies (mixed micelles) comprised of a pluronic copolymer (P123: EO20-PO70-EO20 or F88: EO103-PO39-EO103 where EO: ethylene oxide and PO: propylene oxide) and a cationic surfactant (CTAC: cetyltrimethylammonium chloride), following fluorescence quenching studies. Systematic increase in the quenching rates for the studied donor-acceptor system with the increasing CTAC to pluronic molar ratio in the mixed micelles demonstrates a large modulation in the ET rates. The mixed micellar systems in the present cases are formed through the incorporation of the hydrocarbon chains of CTAC into the poly-PO core of the pluronic micelles whereby the cationic head groups of CTAC are placed at the periphery of the micellar core, protruded into the hydrated poly-EO corona region, leading to the formation of a positively charged layer deep inside these mixed micelles. Thus, the anionic C343 dye, initially dissolved at the micelle-water interface, experiences a gradually increasing electrostatic attraction and is therefore systematically dragged deeper inside the micellar corona, as the CTAC composition is increased in the mixed micellar systems. Consequently, the ET rate of the C343-DMAN pair undergoes a large enhancement in the studied mixed micellar systems with the increasing CTAC to pluronic molar ratio. The present strategy of modulating ET reactions using such composite supramolecular assemblies can find applications in areas where bimolecular ET is an integral reaction step.

Lignans from the heartwood of Streblus asper and their inhibiting activities to hepatitis B virus

Three new lignans, erythro-strebluslignanol (1), threo-7'-methoxyl strebluslignanol (2) and erythro-7'-methoxyl strebluslignanol (3), together with twelve known compounds were isolated from the n-butanol and chloroform fractions of the heartwood of Streblus asper. Their structures were elucidated through extensive spectroscopic methods, including MS and 2D NMR experiments (HMQC and HMBC). The stereochemistry at the chiral center was determined using CD spectra, as well as analysis of coupling constants and optical rotation data, respectively. Primary bioassays showed that 6-hydroxyl-7-methoxyl-coumarin (5) and ursolic acid (10) showed anti-HBV activities, with IC(50) values of 29.60 μM and 89.91 μM for HBsAg at no cytotoxicity, and IC(50) values of 46.41 μM and 97.61 μM for HBeAg at no cytotoxicity, respectively.

Photoionization of psoralen derivatives in micelles: Imperatorin and alloimperatorin

The fluorescence properties of psoralen derivatives, 8-methoxypsoralen (8-MOP), imperatorin (IMP) and alloimperatorin (ALLOI), were investigated in various solvent and micellar solutions. The variation in intensity and maxima of the fluorescence in micellar solutions suggest that psoralens are located in the micelle-water interface region. Radical cations and hydrated electrons were generated by photoionization in micellar solution upon excitation at 266 nm. A nonlinear relationship between transient yield and photon fluency was obtained for each compound, indicating that a two-photon mechanism is predominant in the photoionization of the sensitizers. The photoionization efficiencies are significantly higher in anionic sodium dodecyl sulfate (SDS) than in cationic cetyltrimethylammonium bromide (CTAB) micelles, reflecting the influence of micelle charge on the efficiency of the separation of the photoproduced charge carriers. The photoionization efficiencies of IMP and ALLOI are similar.

Emission under hypoxia: one-electron reduction and fluorescence characteristics of an indolequinone-coumarin conjugate

A characteristic feature of the reactivity of indolequinone derivatives, substituents of which can be removed by one-electron reduction under hypoxic conditions, was applied to the development of a new class of fluorescent probes for disease-relevant hypoxia. A reducing indolequinone parent molecule conjugated with fluorescent coumarin chromophores could suppress efficiently the fluorescence emission of the coumarin moieties by an intramolecular electron-transfer quenching mechanism and a conventional internal-filter effect. Under hypoxic conditions, however, the conjugate, denoted IQ-Cou, underwent a one-electron reduction triggered by X irradiation or the action of a reduction enzyme to release a fluorescent coumarin chromophore, whereupon an intense fluorescence emission with a maximum intensity at 420 nm was observed. The one-electron reduction of IQ-Cou was suppressed by molecular oxygen under aerobic conditions. IQ-Cou also showed intense fluorescence in a hypoxia-selective manner upon incubation with a cell lysate of the human fibrosarcoma cell line HT-1080. The IQ-Cou conjugate has several unique properties that are favorable for a fluorescent probe of hypoxia-specific imaging.

Psoralen and coumarin photochemistry in HSA complexes and DMPC vesicles

The photochemistry and photophysics of several psoralens and coumarins have been examined in human serum albumin (HSA) complexes and dimyristoylphosphatidylcholine (DMPC) vesicles. Fluorescence spectroscopy indicates that there are multiple binding sites with polarities that are intermediate between those of acetonitrile and water for the substrates complexed to HSA. In the case of the 6,7-dimethoxycoumarin-HSA complex, laser flash photolysis experiments provide evidence for the formation of radical cation in addition to triplet. Radical cations are not detected for other coumarin-HSA complexes, either due to a lower yield of formation or to rapid reaction of an initial radical cation with adjacent amino acids. Fluorescence spectra for coumarins indicate that they are primarily solubilized in the polar headgroup region in DMPC vesicles. Consistent with this, radical cations generated by photoionization are detected in transient experiments. For dimethoxycoumarins the radical cation is long-lived, indicating rapid exit from the vesicle and decay in the aqueous phase. However, 4,5',8-trimethylpsoralen and 7-ethoxy-4-hexadecylcoumarin radical cations are much shorter-lived, presumably due to rapid decay by electron recombination in the vesicle. The results for both HSA complexes and vesicles indicate that radical ions may play a role in psoralen and coumarin photochemistry in a cellular environment.

Intermolecular electron transfer between coumarin dyes and aromatic amines in Triton-X-100 micellar solutions: evidence for Marcus inverted region

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amines has been investigated in Triton-X-100 micellar solutions and the results have been compared with those observed earlier in homogeneous medium. Significant static quenching of the coumarin fluorescence due to the presence of high concentration of amines around the coumarin fluorophore in the micelles has been observed in steady-state fluorescence studies. Time-resolved studies with nanosecond resolutions mostly show the dynamic part of the quenching for the excited coumarin dyes by the amine quenchers. A correlation of the quenching rate constants, estimated from the time-resolved measurements, with the free energy changes (DeltaG0) of the ET reactions shows the typical bell shaped curve as predicted by Marcus outer-sphere ET theory. The inversion in the ET rates for the present systems occurs at an exergonicity (-DeltaG0) of approximately 0.7-0.8 eV, which is unusually low considering the polarity of the Palisade layer of the micelles where the reactants reside. Present results have been rationalized on the basis of the two dimensional ET model assuming that the solvent relaxation in micellar media is much slower than the rate of the ET process. Detailed analysis of the experimental data shows that the diffusional model of the bimolecular quenching kinetics is not applicable for the ET reactions in the micellar solutions. In the present systems, the reactions can be better visualized as equivalent to intramolecular electron transfer processes, with statistical distribution of the donors and acceptors in the micelles. A low electron coupling (Vel) parameter is estimated from the correlation of the experimentally observed and the theoretically calculated ET rates, which indicates that the average donor--acceptor separation in the micellar ET reactions is substantially larger than for the donor--acceptor contact distance. Comparison of the Vel values in the micellar solution and in the donor--acceptor close contact suggests that there is an intervention of a surfactant chain between the interacting donor and acceptor in the micellar ET reaction.

Ultraviolet photoionization of the photosensitizers khellin and visnagin in aqueous solution and in micelles: one-photon ionization is a minor process

One-photon ionization, leading to formation of hydrated electrons and radical cations, has been proposed as a possible mechanism of action of some sensitizers in photobiology. In this contribution, we have investigated this proposal for the compounds khellin and visnagin, used in photomedical applications. Nanosecond transient absorption spectroscopy covering a wide range of laser pulse energies was employed to measure the formation of radical cations and hydrated electrons in aqueous solution and in cationic (CTAB) as well as anionic (SDS) micellar solutions. A model allowing for simultaneous one- and two-photon processes and fully accounting for the nonlinearity of the pulse energy dependence was used to simulate the data. The results did not support the hypothesis of a significant role of one-photon ionization, the upper limits of the quantum yields of radical cation formation being phi < 0.01 for visnagin and phi < 0.004 for khellin.

Nature of the Water Molecules in the Palisade Layer of a Triton X-100 Micelle in the Presence of Added Salts: A Solvation Dynamics Study

The effect of added electrolytes on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation dynamics studies of C153 dye in the presence of different concentrations of NaCl, KCl, and CsCl salts. In all of the cases, the solvation dynamics is found to be biexponential in nature. It is seen that in the presence of added salts the solvation dynamics becomes slower. As previously reported (Charlton et al. J. Phys. Chem. B 2000, 104, 8327; Molina-Bolivar et al. J. Phys. Chem. B 2002, 106, 870), the presence of salt increases micellar hydration (and also size) for TX-100, mainly due to enhancement in the mechanically trapped water content in the palisade layer. Under normal circumstances, increased micellar hydration was expected to cause faster solvation dynamics (Kumbhakar et al. J. Phys. Chem. B 2004, 108, 19246), though in the present work, a reverse trend is in fact observed with the added salts. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microviscosity for the palisade layer of the TX-100 micelle with the added salts. The present results have been rationalized assuming that the ions reside in the palisade layer, and due to the hydration of the ions, especially the cations, the water molecules in the palisade layer undergo a kind of clustering, causing the microviscosity to in fact increase rather than decrease as expected due to increased micellar hydration. A partial collapse of the surfactant chains due to their dehydration as caused by the hydration of the ions in the palisade layer may also add to the increase in microviscosity and the consequent retardation in relaxation dynamics in the presence of salts.