Interaction of prothrombin with a phospholipid surface: evidence for a membrane-induced conformational change

Prothrombin interacts with phosphatidylserine containing platelet membranes via its N-terminal, gamma-carboxyglutamate (gla) residue-rich domain. Once bound it is cleaved to form the active protease, thrombin (factor IIa). Human prothrombin was cleaved with cathepsin G in the absence of calcium and magnesium ions. Under these conditions, the gla domain was removed. Phospholipid protected the protein from this proteolytic event, and this suggests that a conformational change may be induced by interaction with phospholipids. Binding of prothrombin to a surface containing 20% phosphatidylserine/80% phosphatidylcholine was detected by surface plasmon resonance, whereas no interaction with gla-domainless prothrombin was observed. Binding of intact prothrombin in the presence of calcium ions showed complex association kinetics, suggesting multiple modes of initial interaction with the surface. The kinetics of the dissociation phase could be fitted to a two-phase, exponential decay. This implies that there are at least two forms of the protein on the surface one of which dissociates tenfold more slowly than the other. Taken together, these data suggest that, on binding to a membrane surface, prothrombin undergoes a conformational change to a form which binds more tightly to the membrane.

Novel photocatalyst-based colourimetric indicator for oxygen: Use of a platinum catalyst for controlling response times

The preparation and characterisation of a novel, UV-activated, solvent-based, colourimetric indicator for oxygen is described, comprising a redox dye (methylene blue, MB), semiconductor photocatalyst (Pt-TiO2), and a sacrificial electron donor (SED = glycerol), all dispersed/dissolved in a polymer medium (sulfonated polystyrene. SPS). Upon exposure to UVA light, the Pt-TiO2/MB/glycerol/SPS oxygen indicator is readily photobleached as the MB is converted into its oxygen-sensitive, leuco form, LMB. In contrast to its non-platinised TiO2 counterpart (TiO2/MB/glycerol/SPS oxygen indicator), the recovery of the original colour is faster (ca. 1.5 days cf. 5 days at 21 degrees C). This is due to the catalytic action of the 0.38 wt% platinum loaded onto the semiconductor photocatalyst. TiO2, on the oxidation of the photogenerated LMB by ambient O-2. Furthermore, by increasing the level of platinum loading, recovery times can be decreased further; e.g. a Pt-TiO2/MB/glycerol/SPS oxygen indicator with platinum level of 1.52 wt% recovers fully within 12 h. A study of the kinetics of recovery as a function of film thickness revealed the recovery step is not controlled by the diffusion of O-2 through the film, but instead dependent upon the slow rate of oxidation of LMB to MB by O-2 in the low dielectric polymer encapsulation medium. Other work showed this recovery is only moderately dependant upon temperatures above -10 degrees C and very sensitive to relative humidity above 30% RH. Potential uses of this UV light activated indicator are discussed briefly. (C) 2011 Elsevier B.V. All rights reserved.

A rapid method of assessing the photocatalytic activity of thin TiO2 films using an ink based on the redox dye 2,6-dichloroindophenol

An indicator ink based on the redox dye 2,6-dichloroindophenol ( DCIP) is described, which allows the rapid assessment of the activity of thin, commercial photocatalytic films, such as Activ. The ink works via a photoreductive mechanism, DCIP being reduced to dihydro-DCIP within ca. 7.5 minutes exposure to UVA irradiation of moderate intensity ( ca. 4.8mW cm(-2)). The kinetics of photoreduction are found to be independent of the level of dye present in the ink formulation, but are highly sensitive to the level of glycerol. This latter observation may be associated with a solvatochromic effect, whereby the microenvironment in which the dye finds itself and, as a consequence, its reactivity is altered significantly by small changes in the glycerol content. The kinetics of photoreduction also appear linearly dependent on the UVA light intensity with an observed quantum efficiency of ca. 1.8 x 10(-3). Copyright (C) 2008.

A study of new photocatalyst indicator inks

This article reports the behaviour of three photocatalyst indicator inks, based on the redox dyes: methylene blue (NIB), resorufin (Rf) and 2,6-dichloroindophenol (DCIP), and assess their performance in comparison to the pioneering resazurin (Rz)-based ink for the rapid assessment of the activity of very thin, photocatalyst films, such as Activ (TM) self-cleaning glass. From a commercial 'demonstrator of photocatalysis' perspective, all three redox dyes appear more attractive compared to Rz since all generate colourless products in the ink formulation when photoreduced on Activ (TM) under anaerobic conditions, whereas, the reduced product from Rz, the redox dye resorufin, Rf. is pink in colour. However, the ink based on Rf is far too slow to effect the rapid measurement of photocatalytic activity even in the absence of oxygen, and in the presence of oxygen the latter inhibits the overall kinetics of photoreduction by re-oxidising the reduced product, dihydroresorufin, HRf, back to Rf. Similarly, despite the attractive rapid rate of photobleaching for NIB under anaerobic conditions, compared to the other redox dyes, the reduced product of the MB-based ink. leuco-MB, is so oxygen-sensitive that the ink cannot be photoreduced under aerobic conditions, thus rendering the ink unsuitable for use in the field. The DCIP-based ink is slightly less easy to photoreduce under both anaerobic and ambient atmospheric conditions compared to the Rz-based ink. However. in addition to its more attractive colour change, the DCIP-based ink is unaffected by the ambient level of oxygen present (%O-2) and the relative humidity (%RH), whereas, for the Rz-based ink, both parameters effect the photoreduction kinetics. By incorporating the DCIP ink into a felt-tipped pen, the ink is suitable for use in the laboratory and field to perform not only a qualitative test, but also to allow a semi-quantitative analysis of photocatalytic activity by eye. (C) 2007 Elsevier B.V. All rights reserved.

Method of rapid assessment of photocatalytic activities of self-cleaning films

An ink, comprising the redox dye resazurin (Rz) and the sacrificial electron donor glycerol, is shown to be capable of the rapid assessment of the photocatalytic activities of self-cleaning films. In the key initial stage of photocatalysis the ink changes from blue to pink. Prolonged irradiation bleaches the ink and eventually mineralizes it. The kinetics of the initial photoinduced color change is studied as a function of UV irradiance, [glycerol], [Rz], and temperature. The results reveal an apparent approximate quantum yield of 3.5 x 10(-3) and an initial rate, r(i), which increases with [glycerol] and decreases with [Rz]. It is proposed that the reduction of Rz, dispersed throughout the thick (ca. 590 nm) indicator film, may take place either via the diffusion of the dye molecules in the ink film to the surface of the underlying semiconductor layer and their subsequent reaction with photogenerated electrons and/or via the diffusion of alpha-hydroxyalkyl radicals, produced by the oxidation of the glycerol by photogenerated holes, or hydroxy radicals, away from the surface of the semiconductor into the ink film and their subsequent reaction with the dye molecules therein. The decrease in r(i) with [Rz] appears to be due to dimer formation, with the latter impeding the reduction process. The activation energy for the initial color-change process is low, ca. 9.1 +/- 0.1 kJ mol(-1) and not unlike many other photocatalytic processes. The initial rate of dye reduction appears to be directly related to the rate of destruction of stearic acid. The ink can be applied by spin-coating, stamping, or writing, using a felt-tip pen. The efficacy of such an ink for assessing the photocatalytic activity of any photocatalytic film, including those employed on commercial self-cleaning glasses, tiles, and paving stones, is discussed briefly.

The photo-oxidation of water by sodium persulfate, and other electron acceptors, sensitised by TiO2

A number of different electron acceptors are tested for efficacy in the oxidation of water to oxygen, photocatalysed by titanium dioxide. The highly UV-absorbing metal ion electron acceptors, Ce4+ and Fe3+, appear ineffective at high concentration (10(-2) M), due to UV-screening, but more effective at lower concentrations (10(-3) M). The metal-depositing electron acceptor, Ag+, is initially effective, but loses activity upon prolonged irradiation due to metal deposition which promotes electron-hole recombination as well as UV-screening the titania particles. Most striking of the electron acceptors tested is persulfate, particularly in alkaline solution (0.1 M NaOH). The kinetics of the photo-oxidation of water by persulfate, photocatalysed by titania are studied as a function of pH, [S2O82-] and incident light intensity (I). The initial rate of water oxidation increases with pH, is directly proportional to the concentration of persulfate present and depends upon I-0.6. The TiO2/alkaline persulfate photosystem is robust and shows very little evidence of photochemical wear upon repeated irradiation. The results of this work are discussed with regard to previous work in this area and current mechanistic thinking. The formal quantum efficiency of the TiO2/alkaline persulfate photosystem was estimated as ca. 2%. (C) 2004 Elsevier B.V. All rights reserved.

Detoxification of water by semiconductor photocatalysis

An overview of the use of semiconductor photocatalysis for water purification is given. The basic principles of semiconductor photocatalysis are described along with the current understanding of the underlying reaction mechanism(s) and how it fits in with the major features of the observed Langmuir-Hinshelwood-type kinetics of pollutant destruction. These features are illustrated based on literature on the destruction of aqueous solutions of 4-chlorophenol as a pollutant, using titanium dioxide as the photocatalyst. The range of organic and inorganic pollutants that can be destroyed by semiconductor photocatalysis are reported and discussed. The basic considerations that need to be made when designing a reactor for semiconductor photocatalysis are considered. These include: the nature of the reactor glass, the type of illumination source, and the nature and type of semiconductor photocatalyst. The key basic photoreactor designs are reported and discussed, including external illumination, annular, and circular photoreactors. Actual designs that have been used for fixed and thin falling film semiconductor photocatalyst reactors are illustrated and their different features discussed. Basic non-concentrating and concentrating solar photoreactors for semiconductor photocatalysis are also reported. The design features of the major commercial photocatalytic reactor systems for water purification are reported and illustrated. Several case studies involving commercial photocatalytic reactors for water purification are reported. An attempt is made briefly to compare the efficacy of semiconductor photocatalysis for water purification with that of other, more popular and prevalent water purification processes. The future of semiconductor photocatalysis as a method of purifying water is considered.

Alkaline hydrolysis of trinitrotoluene, TNT

The kinetics of the alkaline hydrolysis of trinitrotoluene, TNT, in an aqueous solution is a possible approach to destroying the active agent in unwanted munitions. The kinetics are shown to have a rapid initial step, step A, in which a highly coloured species, X (lambda(max) = 450 nm) is formed via an equilibrium reaction: TNT + OH- double left right arrow X. The bimolecular rate constant for the forward part of this equilibrium process, k(1), is: 0.099 +/- 0.004, 0.32 +/- 0.02 and 1.27 +/- 0.05 dm(3) mol(-1) s(-1), at 25, 40 and 60degreesC, respectively. The activation energy for the forward process is 60 kJ mol(-1). The first-order rate constant for the reverse of this process, k(-1), is: (5.3 +/- 2.6) x 10(-4), (1.2 +/- 1.0) x 10(-3) and (7.7 +/- 2.9) x 10(-3) s(-1) at 25, 40 and 60degreesC, respectively. The activation energy for the overall equilibrium process (k(1)/k(-1)) is ca. -5 kJ mol(-1). The subsequent alkaline hydrolysis of X to form the final product P, i.e. step B, is much slower than step A and appears to comprise two processes coupled in series, i.e. steps B1 (X +2OH(-) --> Z) and B2 (Z+OH- --> P). At 25degreesC, Step B1 appears rate determining throughout the decay process. At 45 degreesC and, more so, at 60degreesC, step B appears increasingly biphasic with increasing alkaline concentrations, as step B2 begins to compete with step B1 for position as the rate determining step. The trimolecular rate constant for step B1 is: 0.017 +/- 0.001, 0.0085 +/- 0.0002 and 0.0011 +/- 0.0001 dm(6) mol(-2) s(-1) at 25, 40 and 60degreesC, respectively, and the process has an activation energy of 64 kJ mol(-1). The transition from uniform kinetics, described by step B1, to mixed kinetics, described by steps B1 and B2, as the reaction temperature and alkali concentration are increased most likely occurs because (a) step B2 has a lower activation energy than B1, although it was not possible to measure the former parameter, and (b) step B2 has a lower (1st) order dependence upon [OH-] compared with that of step B1 (2nd). The bimolecular rate constant for step B2 is 0.0035 +/- 0.03 dm(3) mol(-1) s(-1) at 60degreesC. A brief NMR study of the initial hydrolysis product in water, acetone and chloroform, coupled with UV/visible spectra, provides evidence that species X is a Meisenheimer complex.

Characterisation of the photocatalyst Pilkington Activ (TM): a reference film photocatalyst?

Pilkington Glass Activ(TM) represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ(TM) is a glass product with a clear, colourless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ(TM) vastly superior mechanical stability, very reproducible activity and widespread commercial availability makes it highly attractive as a reference photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ(TM) are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7 X 10(-5) and 10.2 x 10(-5) molecules per photon when irradiated with light of 365 +/- 20 and 254 nm, respectively; the latter appears also to be the quantum yield for Activ(TM) at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like saturation type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type constant for oxygen of 0.45 +/- 0.16 kPa(-1) and an activation energy of 19 +/- 1 Kj mol(-1). A study of the PSH properties of Activ(TM) reveals a high water contact angle (67) before ultra-bandgap irradiation reduced to 0degrees after prolonged irradiation. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ(TM) appears a very suitable reference material for semiconductor film photocatalysis. (C) 2003 Elsevier Science B.V All rights reserved.

Photodecomposition of ozone sensitised by a film of titanium dioxide on glass

High levels of ozone (typically 850 ppm) are readily decomposed by semiconductor photocatalysis, using a thin film of the semiconductor titanium dioxide (Degussa P25 TiO2) cast on a glass tube, and UVA light, i.e. light of energy greater than that of the bandgap of the semiconductor (ultra-bandgap light); in the absence of this light the thermal decomposition of ozone is relatively slow. The semiconductor films show no evidence of chemical or photochemical wear with repeated use. At high levels of ozone, i.e. 100 ppm less than or equal to [O-3] less than or equal to 1400 ppm, the initial rate of ozone decomposition by semiconductor photocatalysis is independent of [O-3], whereas, at lower ozone concentrations, i.e. 5 ppm less than or equal to [O-3] less than or equal to 100 ppm, the initial rate of ozone photodestruction decreases in a smooth, but non-linear, manner with decreasing [O-3]. The kinetics of ozone photodecomposition fit a Langmuir-Hinshelwood type kinetic equation and the possible mechanistic implications of these results are briefly discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Response characteristics of optical sensors for oxygen: models based on a distribution in tau(o) or kappa(q)

The features of two popular models used to describe the observed response characteristics of typical oxygen optical sensors based on luminescence quenching are examined critically. The models are the 'two-site' and 'Gaussian distribution in natural lifetime, tau(o),' models. These models are used to characterise the response features of typical optical oxygen sensors; features which include: downward curving Stern-Volmer plots and increasingly non-first order luminescence decay kinetics with increasing partial pressures of oxygen, pO(2). Neither model appears able to unite these latter features, let alone the observed disparate array of response features exhibited by the myriad optical oxygen sensors reported in the literature, and still maintain any level of physical plausibility. A model based on a Gaussian distribution in quenching rate constant, k(q), is developed and, although flawed by a limited breadth in distribution, rho, does produce Stern-Volmer plots which would cover the range in curvature seen with real optical oxygen sensors. A new 'log-Gaussian distribution in tau(o) or k(q)' model is introduced which has the advantage over a Gaussian distribution model of placing no limitation on the value of rho. Work on a 'log-Gaussian distribution in tau(o)' model reveals that the Stern-Volmer quenching plots would show little degree in curvature, even at large rho values and the luminescence decays would become increasingly first order with increasing pO(2). In fact, with real optical oxygen sensors, the opposite is observed and thus the model appears of little value. In contrast, a 'log-Gaussian distribution in k(o)' model does produce the trends observed with real optical oxygen sensors; although it is technically restricted in use to those in which the kinetics of luminescence decay are good first order in the absence of oxygen. The latter model gives a good fit to the major response features of sensors which show the latter feature, most notably the [Ru(dpp)(3)(2+)(Ph4B-)(2)] in cellulose optical oxygen sensors. The scope of a log-Gaussian model for further expansion and, therefore, application to optical oxygen sensors, by combining both a log-Gaussian distribution in k(o) with one in tau(o) is briefly discussed.

Photomineralisation of 4-chlorophenol sensitised by TiO2 thin films

The results of a study of the oxidative mineralisation of 4-CP by oxygen, sensitised by thin films of Degussa P25 TiO2, are reported. The films are used under conditions in which the kinetics of photomineralisation are independent of mass transfer effects and stable towards repeated irradiation. Using a TiO2 film, the process goes through the same mechanism as a TiO2 dispersion, generating the same intermediates, namely: 4-chlorocatechol and hydroquinone. The kinetics of photomineralisation show clear differences between a TiO2 film and a dispersion. With TiO2 films the initial rate of photomineralisation is strongly dependent upon photocatalyst loading, (units; g dm(-3)) reaching a distinct maximum, which appears to be associated with the formation of a monolayer of aggregated particles - the diameter of the aggregated particles is estimated as 0.44 mu m. A simple 2D model is presented to help illustrate the features of such a system. With TiO2 dispersions the rate usually reaches a plateau at ca. 0.5 g dm(-3) of TiO2. For TiO2 films the initial rate depends directly upon the incident light intensity, implying that the photocatalytically active particles are under low illumination conditions, partially shielded by the other particles making up each aggregated particle. In contrast, with TiO2 dispersions R-i depends upon I-0.64, implying that the different light intensities used spanned both the high (R(i)proportional to I-1/2) and low (R(i)proportional to I) intensity kinetic regions. The kinetics of photomineralisation of 4-CP, sensitised by TiO2 films obey the same Langmuir-Hinshelwood expressions as found in most semiconductor photocatalyst work conducted with TiO2 dispersions. However, in a study of the variation R-i as a function of [4-CP] and [O-2] the values for the maximum rates were larger, and those for the apparent Langmuir adsorption coefficients were smaller, than those found for TiO2 dispersions. (C) 1998 Elsevier Science S.A. All rights reserved.

PHOTOMINERALIZATION OF SALICYLIC-ACID - A KINETIC-STUDY

The kinetics of the photomineralization of salicylic acid (SA) sensitized by Degussa P25 titanium dioxide (TiO2) dispersions in oxygenated aqueous solution are reported as a function of the following experimental parameters: [TiO2], percentage of O-2, [SA], temperature (T) and light intensity (I). The kinetics of SA photomineralization conform to a Langmuir-Hinshelwood kinetic scheme with SA and O-2 adsorbed at different sites with apparent Langmuir adsorption coefficients of (6.1 +/- 1.2) x 10(4) mol(-1) dm(3) and 0.061 +/- 0.007 kPa(-1) respectively. The overall activation energy for the system was determined as 4.6 +/- 0.2 kJ mol(-1). Two major stable reaction intermediates are identified (dihydroxybenzoic acids (DHBA) and catechol (C)) and the existence of a further pathway involving one or more very unstable and, as yet, unidentified reaction intermediates is proposed. A kinetic model is presented which describes the temporal behaviour of the concentrations of SA, CO2 and the major photogenerated intermediates (DHBA and C). This model is used to predict successfully the temporal behaviour of the major intermediates in the photomineralization of SA under non-standard conditions.

OXIDATIVE DISSOLUTION OF CHROMIUM(III) OXIDES BY METAPERIODATE IONS IN PERCHLORIC-ACID

The kinetics of oxidative dissolution of a number of different samples of chromium(III) oxide by periodate ions in 1 mol dm-3 HClO4 solution have been studied and the results interpreted using the inverse-cubic rate law. The metaperiodate acts as a two-electron oxidant and the overall reaction stoichiometry involves the reaction of 3 mol of periodate with 1 mol of Cr(III) oxide. From a detailed study of the kinetics of dissolution the rate-determining step appears to be the reaction between an adsorbed periodate ion and its associated Cr(III) oxide surface site, with inhibition by one of the reaction products, iodate, through competitive adsorption. Analysis of the kinetic data generates values for the Langmuir adsorption coefficients for periodate and iodate ions on highly hydrated Cr(III) oxide of 84 +/- 8 and 2600 +/- 370 dm3 mol-1, respectively. The Cr(III) oxide-periodate reaction has a high overall activation energy, 82 +/- 6 kJ mol-1. The kinetics of dissolution of highly hydrated Cr(III) oxide under conditions in which the simple inverse-cubic rate law function does not apply can be successfully predicted using a simple kinetic model.

OXIDATION OF RUTHENIUM(II) TRIS(2,2'-BIPYRIDINE) IONS BY THALLIC IONS IN NITRIC-ACID, MEDIATED BY RUTHENIUM DIOXIDE HYDRATE - AN EXAMPLE OF REVERSIBLE HETEROGENEOUS REDOX CATALYSIS

The kinetics of the oxidation of Ru(bpy)32+ to Ru(bpy)33+ by T13+ ions, catalyzed by a dispersion of RuO2-xH2O in 3 mol dm-3 HNO3, are reported as a function of [Ru(bpy)32+], [Tl3+], [Tl+], [RuO2.xH2O], and temperature. The kinetics of Ru(bpy)32+ oxidation fit an electrochemical model of redox catalysis involving electron transfer between the two electrochemically reversible redox couples, i.e. Ru(bpy)33+/Ru(bpy)32+ and Tl3+/Tl+, mediated by the dispersion of microelectrode particles of RuO2.xH2O. In this model, the rate of reaction is assumed to be controlled by the diffusion of Ru(bpy)32+ toward, and Ru(bpy)33+ away from, the catalyst particles. The Arrhenius activation energy for the catalyzed reaction is 25.9 +/- 0.7 kJ mol-1, and the changes in enthalpy and entropy for the reaction are 36 +/- 2 kJ mol-1 and 127 +/- 6 J mol-1 K-1, respectively. This work describes a rare example of reversible heterogeneous redox catalysis.