Influence of charge transfer on the isomerisation of stilbene derivatives for application in cancer therapy

The photoisomerisation of non-toxic trans-combretastatin CA4 to its cytotoxic cis isomer demonstrates the high potential of this and similar compounds for localised cancer therapy. The introduction of intramolecular charge-transfer character by altering the substituents of combretastatin systems opens up possibilities to tailor these stilbene derivatives to the special demands of anticancer drugs. In this TDDFT study we explore how absorption wavelengths for both the trans and cis isomers can be red shifted to enable deeper light penetration into tissue and how the trans → cis and cis → trans isomerisations are affected by charge transfer effects to different degrees.

New Approaches to Photodynamic Therapy from Types I, II and III to Type IV Using One or More Photons

Photodynamic therapy (PDT) is an alternative cancer treatment to conventional surgery, radiotherapy and chemotherapy. It is based on activating a drug with light that triggers the generation of cytotoxic species that promote tumour cell killing. At present, PDT is mainly used in the treatment of wet age-related macular degeneration, for precancerous conditions of the skin (e.g. actinic keratosis) and in the palliative care of advanced cancers, for instance of the bladder or the oesophagus. PDT is still not used as a first line cancer treatment, which is surprising given the first clinical trials by Dougherty's group dating back to the 1970's. PDT has significant advantages over surgery or radiation therapy for low lying tumours due to better cosmetic outcome and localised treatment for the patients. However, despite these advantages and significant developments in optical technology that has enabled light penetration to deeper lying tumours, in excess of 5 cm, a lack of phase III clinical trials has slowed down the uptake of PDT by the healthcare sector as a frontline treatment in cancer. However research continues to demonstrate the potential benefits of PDT and the need to stimulate funding and uptake of clinical studies using next generation photosensitizers offering advanced targeted delivery, improved photodynamic dose combined with modern light delivery technologies. This review surveys the available PDT treatments and emerging novel developments in the field with a particular focus on two-photon techniques that are anticipated to improve the effectiveness of PDT in tissues at depth and on next generation drugs that work without the need of the presence of oxygen for photosensitization making them effective where hypoxia has taken hold.

Three-dimensional imaging and uptake of the anticancer drug combretastatin in cell spheroids and photoisomerization in gels with multiphoton excitation

The uptake of E -combretastatins, potential prodrugs of the anticancer Z -isomers, into multicellular spheroids has been imaged by intrinsic fluorescence in three dimensions using two-photon excited fluorescence lifetime imaging with 625-nm ultrafast femtosecond laser pulses. Uptake is initially observed at the spheroid periphery but extends to the spheroid core within 30 min. Using agarose gels as a three-dimensional model, the conversion of Z(trans)→E(cis) via two-photon photoisomerization is demonstrated and the location of this photochemical process may be precisely selected within the micron scale in all three dimensions at depths up to almost 2 mm. We discuss these results for enhanced tissue penetration at longer near-infrared wavelengths for cancer therapy and up to three-photon excitation and imaging using 930-nm laser pulses with suitable combretastatin analogs.

Anticancer phototherapy using activation of E-combretastatins by two-photon-induced isomerization

The photoisomerization of relatively nontoxic E-combretastatins to clinically active Z-isomers is shown to occur in solution through both one- and two-photon excitations at 340 and 625 nm, respectively. The photoisomerization is also demonstrated to induce mammalian cell death by a two-photon absorption process at 625 nm. Unlike conventional photodynamic therapy (PDT), the mechanism of photoisomerization is oxygen-independent and active in hypoxic environments such as in tumors. The use of red or near-infrared (NIR) light for two-photon excitation allows greater tissue penetration than conventional UV one-photon excitation. The results provide a baseline for the development of a novel phototherapy that overcomes nondiscriminative systemic toxicity of Z-combretastatins and the limitations of PDT drugs that require the presence of oxygen to promote their activity, with the added benefits of two-photon red or NIR excitation for deeper tissue penetration.

A series of flexible design adaptations to the Nikon E-C1 and E-C2 confocal microscope systems for UV, multiphoton and FLIM imaging

Multiphoton microscopy is widely employed in the life sciences using extrinsic fluorescence of low- and high-molecular weight labels with excitation and emission spectra in the visible and near infrared regions. For imaging of intrinsic and extrinsic fluorophores with excitation spectra in the ultraviolet region, multiphoton excitation with one- or two-colour lasers avoids the need for ultraviolet-transmitting excitation optics and has advantages in terms of optical penetration in the sample and reduced phototoxicity. Excitation and detection of ultraviolet emission around 300 nm and below in a typical inverted confocal microscope is more difficult and requires the use of expensive quartz optics including the objective. In this technical note we describe the adaptation of a commercial confocal microscope (Nikon, Japan E-C1 or E-C2) for versatile use with Ti-sapphire and OPO laser sources and the addition of a second detection channel that enables detection of ultraviolet fluorescence and increases detection sensitivity in a typical fluorescence lifetime imaging microscopy experiment. Results from some experiments with this setup illustrate the resulting capabilities.

Ultrafast vibrational spectroscopic studies on the photoionization of the α-tocopherol analogue trolox C

The initial events after photoexcitation and photoionization of α-tocopherol (vitamin E) and the analogue Trolox C have been studied by femtosecond stimulated Raman spectroscopy, transient absorption spectroscopy and time-resolved infrared spectroscopy. Using these techniques it was possible to follow the formation and decay of the excited state, neutral and radical cation radicals and the hydrated electron that are produced under the various conditions examined. α-Tocopherol and Trolox C in methanol solution appear to undergo efficient homolytic dissociation of the phenolic -OH bond to directly produce the tocopheroxyl radical. In contrast, Trolox C photochemistry in neutral aqueous solutions involves intermediate formation of a radical cation and the hydrated electron which undergo geminate recombination within 100 ps in competition with deprotonation of the radical cation. The results are discussed in relation to recently proposed mechanisms for the reaction of α-tocopherol with peroxyl radicals, which represents the best understood biological activity of this vitamin.

Fluorescence lifetime imaging of E-combretastatin uptake and distribution in live mammalian cells

To investigate within live mammalian cells the uptake and disposition of combretastatins, fluorescence lifetime imaging was used with two-photon excitation (2PE). Combretastatin A4 (CA4) and analogues are potential anticancer drugs due to their ability to inhibit angiogenesis. E(trans)-combretastatins are considerably less active than the Z(cis)-combretastatins proposed for clinical use. However the E-combretastatins exhibit stronger intrinsic fluorescence with quantum yields and lifetimes that depend markedly on solvent polarity and viscosity. It is proposed that 2PE in the red and near-infrared tissue window may allow in situ isomerization of E-combretastatins to the more active Z-isomer, offering spatial and temporal control of drug activation and constitute a novel form of photodynamic therapy. In the present work we have characterised 2PE of E-CA4 and have used fluorescence lifetime imaging with 2PE to study uptake and intracellular disposition of E-CA4 and an analogue. The results show that these molecules accumulate rapidly in cells and are located mainly in lipidic environments such as lipid droplets. Within the droplets the local concentrations may be up to two orders of magnitude higher than that of the drug in the surrounding medium.

Effect of antioxidant oxidation potential in the oxygen radical absorption capacity (ORAC) assay

The "oxygen radical absorption capacity" (ORAC) assay (Ou, B., Hampsch-Woodill, M., Prior, R.L. (2001). Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe. Journal of Agricultural and Food Chemistry 49, 4619-4626) is widely employed to determine antioxidant content of foods and uses fluorescein as a probe for oxidation by peroxyl radicals. Kinetic modeling of the ORAC assay suggests that the lag phase for loss of fluorescence results from equilibrium between antioxidant and fluorescein radicals and the value of the equilibrium constant determines the shape of the lag phase. For an efficient antioxidant this constitutes a "repair" reaction for fluoresceinyl radicals and produces a well defined lag phase. The lag phase becomes less marked with increasing oxidation potential of the antioxidant. Pulse radiolysis confirms that fluoresceinyl radicals are rapidly (k∼10(9)dm(3)mol(-1)s(-1)) reduced by Trolox C, a water soluble vitamin E analogue. ORAC assays of phenols with varying oxidation potentials suggest that it might be employed to obtain an estimate of the redox potential of antioxidants within food materials.

Near infrared multiphoton-induced generation and detection of hydroxyl radicals in a biochemical system

Solutions of tryptophan and N-hydroxypyridine-2-thione (mercaptopyridine-N-oxide, MPNO) were irradiated at 335 nm. Formation of 5-hydroxytryptophan was inferred from increased fluorescence at 334 nm on excitation at 315 nm, conditions chosen for selective detection of 5-hydroxytryptophan. Such experiments are complicated by overlapping absorption spectra in the region of 300–350 nm. Similar solutions were exposed to multiphoton excitation at 750 nm using 180 fs pulses from a titanium:sapphire laser. In solutions containing both tryptophan and MPNO strong emission at 500 nm was observed that was absent in solutions containing either MPNO or tryptophan only. This emission is ascribed to the characteristic fluorescence (‘hyperluminescence’) from 5-hydroxyindoles resulting from multiphoton photochemistry. The conclusion that MPNO generates hydroxyl radicals by 2-photon activation at 750 nm is confirmed by the scavenging effects of ethanol and kinetic analysis of the results. This method has potential applications in intracellular induction of oxidative stress using multiphoton near-infrared illumination, a technology that is gaining momentum as a research tool.

Formation of singlet oxygen from solutions of vitamin E

Vitamin E offers protection against oxidative stress and is an efficient quencher of singlet oxygen. A recent report suggests that photo-excitation of vitamin E results in the formation of a triplet state (Naqvi et al. Photochem Photobiol Sci 2, 381 (2003)). This leads to the possibility of the triplet state of vitamin E being able to sensitize singlet oxygen and if this is the case it would be counter productive in terms of the biological protective function of vitamin E. We report the production of singlet oxygen, detected by 1270 nm luminescence, from pulsed laser excitation (308 nm) of vitamin E and an analogue, 2,2,5,7,8-pentamethyl-6-hydroxy-chroman (PMHC), with quantum yields between ~0.1 and 0.2. The luminescence was identified as singlet oxygen from self-quenching by vitamin E with solvent-dependent rate constants similar to published values. Whilst the beneficial antioxidant aspects of vitamin E are well established, these results indicate that vitamin E when directly excited can sensitize singlet oxygen formation and may, therefore, be capable of inducing biochemical and biological damage. The results are discussed in relation to recent reports on the deleterious effects of vitamin E dietary supplementation and pro-oxidant effects of vitamin E.

Single- and multi-photon excited fluorescence from serotonin complexed with beta-cyclodextrin

The fluorescence of serotonin on binding with beta-cyclodextrin has been studied using both steady state and time-resolved methods. Steady state fluorescence intensity of serotonin at 340 nm showed approximately 30% increase in intensity on binding with K(A) approximately 60 dm(3) mol(-1) and the fluorescence lifetimes showed a corresponding increase. In contrast, the characteristic green fluorescence ('hyperluminescence') of serotonin observed upon multiphoton near-infrared excitation with sub-picosecond pulses was resolved into two lifetime components assigned to free and bound serotonin. The results are of interest in relation to selective imaging and detection of serotonin using the unusual hyperluminescence emission and in respect to recent determinations of serotonin by capillary electrophoresis in the presence of cyclodextrin. The results also suggest that hyperluminescence occurs from multiphoton excitation of a single isolated serotonin molecule.

Investigation of multiphoton-induced fluorescence from solutions of 5-hydroxytryptophan

It is reported (J. B. S. Shear, C. Xu and W. W. Webb, Photochem. Photobiol. 1997, 65, 931) that multiphoton near infrared excitation of 5-hydroxytryptophan results in a transient product with green fluorescence. Visible fluorescence from multiphoton excitation enables detection of 5-hydroxytryptophan with extremely high sensitivity and also has potential applications in imaging of biological systems and investigation of protein dynamics. The characteristic fluorescence at 500 nm has now also been observed in a two laser experiment whereby 308 nm photolysis of the solution is followed by an excitation step at 430 nm. Fluorescence was observed in aerated and deaerated solutions and in the presence of ascorbate. Enhancement of fluorescence was observed on addition of ethanol. Transient absorption experiments with 308 nm photolysis showed the formation of three transient species. In the presence of ascorbate the radical formed by photoionisation was quenched, revealing a long-lived species (tau > 1 ms) with a similar absorption spectrum, which is ascribed to the fluorescing species. Fluorescence induced by multiphoton excitation had a lifetime of 910 +/- 10 ps and was also unaffected by ascorbate. In the presence of organic solvents there was an increase in fluorescence lifetime, but a decrease in overall fluorescence intensity. The fluorescence intensity and fluorescence lifetime both decreased in acidic solution (pH < 3). The results indicate that the fluorescence does not originate from the 5-indoxyl radical as previously suggested but from one or more other transient products which require further characterisation.

One-electron oxidation of "photo-Fenton" reagents and inhibition of lipid peroxidation

The "photo-Fenton" reagent, 2-mercaptopyridine N-oxide (MPO), which releases a hydroxyl radical on ultraviolet irradiation, has been found to act as an antioxidant. In the peroxidation of linoleate initiated by a water-soluble azo-initiator, MPO has about one-third the activity of the water-soluble vitamin E analogue Trolox C. In contrast, the oxygen-containing analogue, 2-hydroxypyridine N-oxide (HPO), does not have measurable antioxidant activity in this system. Both reagents react with hydroxyl radical with second order rate constants very close to the diffusion-controlled limit. With the less oxidising dithiocyanate radical anion, MPO reacts approximately 50 times more rapidly than HPO at pH>7. The more reducing properties of MPO result in its activity as an antioxidant and make it less suitable than HPO as a source of hydroxyl radicals for investigation of oxidative stress in biological systems.

Structure of the radical from one-electron oxidation of 4-hydroxycinnamate

Radicals from one-electron oxidation of 4-hydroxycinnamate, ferulate and 3,4-dihydroxycinnamate have been formed by reaction with the oxidising triplet state of duroquinone. All three compounds react with triplet duroquinone with second order rate constants close to the diffusion-controlled limit. The identity of the resulting radicals is confirmed by observation of their characteristic visible absorption spectra. Time-resolved resonance Raman (TR3) spectra of the radical from 4-hydroxycinnamate were measured using a probe laser wavelength of 600 nm, to be in resonance with the long wavelength absorption band of the radical. The TR3 spectra contain prominent bands ascribed to the C-O and ring C-C stretching vibrations. The spectra are interpreted as indicating strong delocalisation of the radical site to the double bond in conjugation with the aromatic ring in 4-hydroxycinnamate. This contributes to the low reduction potential of the radical and the antioxidant properties of hydroxycinnamates.

Wavelength-programmed solute release from photosensitive liposomes

Liposomes of dipalmitoylphosphatidylcholine containing a photochromic lipid "Bis-Azo PC" release entrapped solutes on exposure to UV light. We have now demonstrated that on addition of cholesterol (up to 25 mol%) to the liposomal membrane the liposomes also release their contents in response to visible light in the region of 470 nm, to which liposomes lacking steroid are insensitive. In a mixed population of liposomes prepared with and without cholesterol, this enables wavelength-dependent release of entrapped solutes on sequential exposure to visible and UV light. Furthermore, the cholesterol-containing liposomes allow stepped partial release of entrapped solute following multiple periods of short visible illumination. It is suggested that the cholesterol-containing liposomes may be potentially useful for drug delivery and for "caging" of reagents.

Active uptake of drugs into photosensitive liposomes and rapid release on UV photolysis

Liposomes containing high concentrations of the anticancer drug doxorubicin, prepared by active-loading techniques, have been intensively investigated as potential agents for chemotherapy. The present study investigates the possibility of active uptake and photoinduced release of such solutes from liposomes incorporating a photoisomerizable lipid. The active loading of acridine orange and doxorubicin was investigated using liposomes containing entrapped ammonium sulfate. The liposomes were prepared with dipalmitoyl-L-alpha-phosphatidyl choline (DPPC) and a photochromic lipid, (1,2-(4'-n-butylphenyl)azo-4'-(gamma-phenylbutyroyl))-glycero-3- phosphocholine (Bis-Azo PC), which isomerizes on exposure to near-UV light with resulting changes in membrane permeability to solutes. The rate of loading of the vesicles below the phase transition temperature of DPPC was investigated as a function of Bis-Azo PC and cholesterol concentrations in the liposome. The rate of doxorubicin uptake was found to be greatly decreased in the presence of cholesterol, while below 30 degrees C the rate of acridine orange uptake was increased in the presence of cholesterol. On exposure to a single UV laser pulse, actively loaded acridine orange was rapidly released from liposomes containing Bis-Azo PC at a rate similar to that found for the indicator dye calcein. However while cholesterol had previously been shown to greatly enhance the rate of photo-induced calcein leakage, it had no significant effect on the rate of acridine orange release. After active loading into DPPC vesicles containing Bis-Azo PC, doxorubicin was also released after exposure to a single laser pulse, but at a rate slower than for acridine orange and calcein. The difference in behavior between these systems is ascribed to the interactions of acridine orange and doxorubicin with the liposome bilayer. Photoinduced release of pharmacologically active materials from sensitized liposomes might provide a useful adjunct or alternative to conventional photodynamic therapy.

Photosensitive liposomes as 'cages' for laser-triggered solute delivery: the effect of bilayer cholesterol on kinetics of solute release

Liposomes containing acyl chains incorporating azobenzene chromophores have been investigated as potential 'caging' agents for fast solute release. On photolysis, trapped marker dye can be released from gel-phase liposomes within milliseconds. Solute release is markedly sensitive to the presence of cholesterol in the bilayer. Phospholipids bearing one saturated acyl chain and an azobenzene-substituted chain are ineffective as sensitisers unless cholesterol is present, while doubly substituted phospholipids sensitise release in its absence. Cholesterol markedly affects the temperature profile of solute release depending on the host phospholipid chain length. Solute release is not seen for lipid hosts with unsaturated acyl chains.

Fast laser-induced solute release from liposomes sensitized with photochromic lipid: effects of temperature, lipid host, and sensitizer concentration

Liposomes of gel-phase phospholipid have been prepared containing a photochromic lipid sensitizer. A fast UV laser pulse isomerizes the sensitizer destabilizing the lipid bilayer structure and causing release of trapped solute. The kinetics of solute release have been investigated as a function of host lipid chain length, sensitizer concentration, and temperature, and the limits of liposome stability have been established. At low concentrations of sensitizer, pulsed laser irradiation induces some solute release when continuous UV illumination is ineffective. Although rates of solute release usually increase with temperature, at low sensitizer concentration in a rigid host, leakage at first increases but then decreases rapidly above a threshold temperature. The results presented are relevant to the design of photostimulated drug delivery systems and to potential applications of photosensitive liposomes as caging agents for biological effectors.

Antioxidant reactions of dihydrolipoic acid and lipoamide with triplet duroquinone

The oxidation of the antioxidants dihydrolipoate and lipoamide by triplet duroquinone (3DQ) has been studied by laser flash photolysis and time-resolved resonance Raman (TR3) spectroscopy. Reaction of 3DQ with lipoamide by electron transfer [k(H2O)/k(D2O approximately 1] was more rapid than with dihydrolipoate, in which a proton is also involved [k(H2O)/k(D2O approximately 2]. For dihydrolipoate at neutral pH the undeprotonated form was the major reactive species with k approximately 10(9) dm3 mol-1 s-1. At higher pH values the reaction of ionised (thiolate) forms was observed with k > or = 4 x 10(9) dm3 mol-1 s-1. The electron transfer mechanism of reaction between 3DQ and lipoamide was confirmed by TR3 spectra in which formation of the durosemiquinone radical anion and lipoamide disulfide radical cation (RSS+.) was observed.

The free radical site in pea seedling copper amine oxidase probed by resonance Raman spectroscopy and generated by photolysis of caged substrate

Resonance Raman spectra were obtained of the free radical site in substrate reduced anaerobic samples of pea seedling copper amine oxidase (PSAO). The spectra differ significantly from those reported previously for E. coli copper amine oxidase [Moenne-Loccoz et al. (1995) Biochemistry 34, 7020]. The spectra were found to be independent of substrate (benzylamine, spermidine or methylamine) used to reduce the TOPA quinone cofactor, however, several of the peaks in the Raman spectrum displayed small shifts on using [15N]benzylamine, proving incorporation of the substrate nitrogen atom onto the cofactor radical. Changes in the spectrum were also observed when measured in D2O solution indicating a strongly bound proton in the radical. The spectra were independent of pH values between 5 and 9 and are interpreted as showing that the radical exists as a semiiminoquinone radical monoanion. Benzylamine and phenethylamine have been caged with 2-nitrobenzaldehyde and shown by laser flash photolysis to uncage on a sub-millisecond timescale. Preliminary experiments have shown the formation of the enzyme radical intermediate on laser flash photolysis of 2-nitrobenzyl-caged benzylamine in the presence of enzyme. This should permit time-resolved resonance Raman spectral investigations of the catalytic cycle of copper amine oxidases.

Quenching of reactive oxidative species by probucol and comparison with other antioxidants

One-electron oxidation of the antiatherogenic and antiatherosclerotic drug probucol has been studied in relation to its activity as an antioxidant. Oxidation by triplet excited states of duroquinone and benzophenone, and by the inorganic radicals Br2.- and N3., lead to the formation of a transient absorption at 500 nm. This was identified by time-resolved resonance Raman spectroscopy as the phenoxyl radical from probucol, formed by hydrogen atom or electron plus proton loss from one of the phenolic groups of probucol. The reactivity of probucol with triplet duroquinone and triplet benzophenone, and as a quencher of singlet oxygen, was compared with the reactivities of other antioxidants (alpha-tocopherol, palmitoyl ascorbic acid, dihydrolipoic acid and N-acetyl cysteine). In quenching of the triplet states the reactivity of probucol was comparable with that of alpha-tocopherol, whereas as a quencher of singlet oxygen probucol (k < 10(6) M-1 s-1) was less effective than alpha-tocopherol (k = 2.0 x 10(8) M-1 s-1) by more than two orders of magnitude. This difference in reactivity may allow the contribution of singlet oxygen towards oxidative stress to be quantified separately.

Fast solute release from photosensitive liposomes: an alternative to 'caged' reagents for use in biological systems

The kinetics of release of soluble marker trapped in liposomes of gel phase phospholipid containing a photoisomerisable phospholipid analogue have been investigated. Marker release is triggered by UV laser flash photolysis at 355 nm. A markedly temperature-dependent release rate is seen, and above 25 degrees C millisecond release kinetics can be achieved. These results suggest that such liposomes might find application as an alternative to conventional 'caged' reagents for photo-triggered reagent release in biological research.

Reaction of ascorbate with the alpha-tocopheroxyl radical in micellar and bilayer membrane systems

The reaction by which the antioxidant capacity of alpha-tocopherol is sustained by recycling of the alpha-tocopheroxyl radical in membranes or lipoproteins by aqueous ascorbate has been studied by laser flash photolysis in model micellar and membrane systems. In bilayers of dimyristoylphosphatidylcholine at 35 degrees C the measured second-order rate constant was 3 x 10(5) M s-1, or about five times slower than previously reported in a solvent system. The rate of reaction was decreased on addition of negatively charged lipid (dipalmitoylphosphatidic acid) and increased by addition of positively charged lipid (didodecyldimethylammonium bromide). These effects of bilayer charge were suppressed by increasing the ionic strength of the aqueous medium. Micellar charge also had an effect on the pH dependence of the reaction rate. Arrhenius data showed that the enthalpy of activation was effectively zero for the reaction in solution between ascorbate and radicals of water-soluble tocopherol analogues, but was positive in membrane and micellar systems. In all cases the entropy of activation was strongly negative. The kinetic deuterium isotope ratio varied between 3 and 8. The data strongly support a concerted electron and proton transfer mechanism for the reaction between alpha-tocopheroxyl radical and ascorbate.

Control of pro-oxidant activity of cupric ions by entrapment in unilamellar lipid vesicles

As a demonstration of a potential means of delivering and controlling the biochemical and biological activity of metal ions, cupric ions have been trapped in unilamellar phospholipid vesicles. The activity of these cupric ion-containing vesicles as catalysts of the autoxidation of ascorbate and epinephrine has been investigated. A marked increase in autoxidation rate was observed on release of the cupric ion on addition of detergent. When an azobenzene-containing photochromic lipid was incorporated in the bilayer membrane of the vesicles, the release of cupric ions could be initiated by irradiation with ultraviolet light. In the dark, these vesicles remained stable for at least several weeks. Photo-controlled release of liposomally-entrapped species might find application in areas similar to those where 'caged' reagents are presently used.

Reactions of the alpha-tocopheroxyl radical in micellar solutions studied by nanosecond laser flash photolysis

Laser flash photolysis of alpha-tocopherol in methanol and in aqueous micellar solutions has been shown to produce the alpha-tocopheroxyl radical. The reaction between the alpha-tocopheroxyl radical and ascorbate in positively charged hexadecyltrimethylammonium chloride (HTAC) micelles occurred with a second order rate constant of 7.2 x 10(7) M-1.s-1, whereas in negatively charged sodium dodecyl sulphate (SDS) micelles the rats constant was only 3.8 x 10(4) M-1.s-1. The alpha-tocopheroxyl radical was found to be relatively long-lived in HTAC micelles (t1/2 greater than or equal to 5 min), allowing the slow disappearance of the alpha-tocopheroxyl radical by reaction with glutathione to be observed.

Reactions of a free radical intermediate in the oxidation of amodiaquine

One electron oxidation of the antimalarial drug amodiaquine by inorganic radicals was investigated by pulse radiolysis. A transient species was observed and identified as the semiiminoquinone radical, which has recently been implicated in the toxicity of amodiaquine. Pulse radiolysis was used to determine the reactivity of this radical. In the absence of other solutes it decays rapidly in a second order process. No reaction between the semiiminoquinone radical and oxygen could be observed. In the presence of ascorbate or a phenolic antioxidant (Trolox C) the semiiminoquinone radical was rapidly repaired. Similar reductants have been reported (Maggs JL et al., Biochem Pharmacol 37: 303-311, 1988) to inhibit irreverisble protein binding during amodiaquine autoxidation, and the present results support the involvement of the radical during these reactions.

Pulse radiolysis, free radicals and antiparasitic drugs

Free radicals have been proposed to be of significance in several important aspects of parasitology because of their generation during parasitic infection and chemotherapy, and their potential for causing cellular damage. Roger Bisby describes how radiation chemical methods, and pulse radiolysis in particular, may be used to generate and study free radicals that are of interest in parasitology.

Interactions of vitamin E with free radicals and membranes

alpha-Tocopherol performs an antioxidant role in biological membranes by acting as a one-electron reductant. In micellar solutions it has been observed by pulse radiolysis that the micellar charge has a pronounced effect on the rate constant for repair of organic free radicals by alpha-tocopherol. The interactions between alpha-tocopherol and model bilayer lipid membranes have been studied by fluorescence spectroscopy. Quenching of alpha-tocopherol fluorescence by acrylamide and some n-doxyl stearates shows the transverse distribution of alpha-tocopherol in membranes to be affected by the physical state of the membrane lipids and by the salt concentration in the aqueous phase. Time-resolved fluorescence depolarization measurements, with a diphenylhexatriene-phospholipid conjugate as probe, demonstrate an increase in bilayer order parameter on incorporation of alpha-tocopherol into a membrane.

Radiosensitization of E. coli B/r by arylhydrazonopropanedinitriles

Several arylhydrazonopropanedinitriles and an arylhydrazonopropane-diethyl ester (derivatives of well-known uncouplers of oxidative phosphorylation) have been studied with respect to their ability to radiosensitize E. coli B/r under oxic and hypoxic conditions. Of the compounds studied, 2-carboxyphenylhydrazono-propanedinitrile and 2-carboxyphenylhydrazonopropanediethylester were found to be the most efficient radiosensitizers under hypoxia, whilst the former compound was also found to provide radiosensitization under oxic conditions. Increased radiosensitization by 4-carboxyphenylhydrazonopropanedinitrile was observed on decreasing the pH of the irradiation incubation medium. The results are discussed with respect to the physicochemical properties of these compounds and their reactivity with thiols, for which data are presented.

The radiation-induced inactivation of external yeast invertase in dilute aqueous solution

The inactivation of external yeast invertase by irradiation in dilute aqueous solution has been investigated. The contributions of the individual radical species from water radiolysis to inactivation and amino acid degradation were estimated from the results of experiments in which solutions were saturated with nitrogen, nitrous oxide or oxygen, and on addition of hydroxyl radical scavengers. Under conditions where inactivation by hydroxyl radicals predominates, the rate of inactivation increased with increasing dose, indicating that in the initial stages of the radiolysis the mannose-rich oligosaccharide chains of the glycoprotein protect the polypeptide chain from radical attack. Amino acid analysis of the irradiated external invertase showed that there was significant destruction of tyrosine, phenylalanine, methionine and histidine residues. Destruction of methionine and histidine residues may be responsible for the free radical-induced inactivation of this enzyme.

A time-resolved fluorescence anisotropy study of bilayer membranes containing alpha-tocopherol

Rotational mobility in fluid phase dipalmitoylphosphatidylcholine unilamellar vesicles containing alpha-tocopherol has been studied by time-resolved anisotropy measurements of fluorescence from a diphenylhexatriene-phosphatidylcholine conjugate. The results are analysed using a simple wobbling-in-cone model. The diphenylhexatriene probe shows an increasing order parameter and more restricted wobbling with increasing alpha-tocopherol content of the membrane. The diffusional rate for wobbling was found not to change significantly.

Transverse distribution of alpha-tocopherol in bilayer membranes studied by fluorescence quenching

The fluorescence properties of alpha-tocopherol in a range of solvents and in micelles and membrane vesicles have been measured. In solvents the fluorescence decay was fitted by a single exponential. In bilayer membranes of dipalmitoylphosphatidylcholine or egg phosphatidylcholine the fluorescence decay was more accurately fitted as a double exponential. This may indicate that alpha-tocopherol occupies two or more sites in such membranes. Depth-dependent quenching of alpha-tocopherol fluorescence by acrylamide and some doxyl stearates has also been studied. The results confirm that in gel-phase lipid the chromanol group has a transverse distribution close to the head-group region of the lipid. In fluid phase lipid in the presence of buffer the results indicate there is more penetration of the chromanol group into the bilayer.

Qinghaosu does not affect the major thermotropic phase transition in model membranes of dipalmitoylphosphatidylcholine

The effect of the antimalarial agent qinghaosu (artemisinin) on the major thermotropic phase transition in bilayer membranes of dipalmitoylphosphatidylcholine has been studied by differential scanning calorimetry and by the use of fluorescent probes (pyrene excimer formation and depolarization of diphenylhexatriene fluorescence). Addition of up to 40 mol% qinghaosu to dipalmitoylphosphatidylcholine bilayer caused no observable effect on the temperature or enthalpy of the phase transition. The results suggest that the antimalarial action of qinghaosu is not due to a direct effect on the lipid structure of the parasite membrane.

Properties of the radicals formed by one-electron oxidation of acetaminophen--a pulse radiolysis study

The semi-iminoquinone radical of acetaminophen, which has previously been proposed as a possible hepatotoxic intermediate in the cytochrome P-450 catalysed oxidation of acetaminophen, has been generated and studied by pulse radiolysis. In the absence of other reactive solutes, the radical decays rapidly by second order kinetics with a rate constant (2k2) of (2.2 +/- 0.4) x 10(9) M-1 sec-1. In alkaline solutions the radical deprotonates with a pK of 11.1 +/- 0.1 to form a radical-anion, as confirmed by the effect of ionic strength on the rate of radical decay. The acetaminophen radical-anion reacts with resorcinol at high pH values, leading to the formation of a transient equilibrium from which the one-electron reduction potential of the semi-iminoquinone radical of acetaminophen is estimated to be +0.707 +/- 0.01 V at pH 7. This value predicts that acetaminophen should be oxidised by thiyl radicals. This was confirmed by pulse radiolysis experiments for reaction of the cysteinyl radical, for which rate constants of 7 x 10(6) M-1 sec-1 at pH 7 and 2.7 x 10(8) M-1 sec-1 at pH 11.3 were obtained. The reaction of O2 with the acetaminophen semi-iminoquinone radical could not be detected by pulse radiolysis, and alternative mechanisms for superoxide radical formation are discussed.

One-electron reduction of the antimalarial drug primaquine, studied by pulse radiolysis

One-electron reduction of the antiparasitic drug primaquine has been studied by pulse radiolysis. Primaquine is reduced by the hydrated electron at neutral pH with a rate constant of (2.47 +/- 0.1) x 10(10) dm3mol-1s-1. Reduction by formate and isopropanol radicals is relatively slow (less than or equal to 10(7) dm3mol-1s-1) at neutral pH, but increases in rate with decreasing pH on protonation of the quinoline moiety. The one-electron reduction product form reaction of the hydrated electron with primaquine at neutral pH reacts with O2, benzyl viologen and NAD+ with rates of (1-2.3) x 10(9) dm3mol-1s-1. The relevance of these observations to the mechanisms proposed by Thornalley et al. (Biochem. Pharmacol. 32, 357, (1983] for oxygen free radical generation in solutions of NADPH and primaquine and the antiparasitic action of the drug is discussed.

Radiolysis of human gastric glycopolypeptides in aqueous solution

The degradation of human gastric glycopolypeptides by hydroxyl radicals formed in irradiated N2O-saturated aqueous solution has been investigated. Gel exclusion chromatography shows the formation of lower molecular weight degradation products after irradiation and the appearance of unsaturated carbonyl-containing products which absorb in the ultra-violet. The radiation-induced destruction of individual monosaccharides in three human glycopolypeptides having different oligosaccharide chains has been measured. The results indicate that the structure of the oligosaccharide chain determines the extent of destruction of each type of monosaccharide present.

Free radical reactions with alpha-tocopherol and N-stearoyl tryptophan methyl ester in micellar solutions

The rate constants have been measured for one-electron oxidation by N3. and Br2-. of N-stearoyl tryptophan methyl ester (STME) and alpha-tocopherol (alpha-T) in micelles of sodium dodecyl sulphate (SDS) or tetradecyl trimethyl ammonium bromide (TTAB). Compared with analogous reactions of tryptophan and Trolox C in aqueous solution, the rate constants for oxidation in micellar solution by N3. are reduced by 30-70%. The micellar charge increased the rate of oxidation of STME by Br2-. in TTAB micelles by almost an order of magnitude, compared with the reaction of Br2-. with tryptophan in aqueous solution. In SDS micelles the rate of oxidation of STME by Br2-. was reduced more than 30-fold. Quenching of fluorescence from STME in micelles by acrylamide confirmed the accessibility of the indole ring to the aqueous solvent. The rate of repair of the neutral STME radical by alpha-T in TTAB micelles was found to be accelerated by a factor of at least 27, compared with the similar reaction between Trolox C and tryptophan radicals in aqueous solution.

Transverse location of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene in model lipid bilayer membrane systems by resonance excitation energy transfer

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 inactivation of papain by high LET radiations

The effect of varying LET over a wide range (0.2-1570 eV/nm) on the radiation-induced inactivation of the enzyme papain in dilute aqueous solution has been investigated. Measurements of total, reparable and non-reparable inactivation G values in oxygen, nitrous oxide and argon saturated solutions have allowed the contributions to inactivation from radicals and hydrogen peroxide to be evaluated. At high LET the results demonstrate an increasing component due to reaction of the superoxide radical, formed from oxygen produced in the track as a primary radiolysis product. This effect was not observed in our previous study with ribonuclease due to the insensitivity of ribonuclease to inactivation by superoxide and hydrogen peroxide. The results obtained with papain clearly demonstrate a maximum in G (H2O2) at an LET of approximately 140 eV/nm. Generation of O2 within the track as a primary radiolysis product at high LET now appears to be confirmed as an important mechanism leading to reduction in the oxygen enhancement ratio for cellular systems exposed to high LET radiations (Baverstock and Burns 1981).

Repair of amino acid radicals by a vitamin E analogue

Free radicals derived from one-electron oxidation of the amino acids tryptophan, tyrosine, methionine and histidine have been found to be rapidly (k = 10(7) -10(9) dm3 mol-1 s-1) and efficiently repaired by Trolox C, a vitamin E analogue. The reactions form a relatively stable phenoxyl radical of Trolox C (lambda max = 440 nm; epsilon = 5.4 X 10(3) mol dm-3 cm-1). The radical cation of tryptophan is more rapidly repaired than the neutral tryptophan radical. Repair of tryptophanyl radicals in the enzyme lysozyme has also been observed. The results suggest that a function of alpha-tocopherol in membranes may be the repair of radicals of integral membrane proteins.