Inorganic Semiconductors as Photosensitizers in Biochemical Redox

Membr. Cell Biol, 1998, Vol.12 (5), pp. 755-769 Reprints available directly from the publisher Photocopying permitted by license only

© 1998 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint, part of The Gordon and Breach Publishing Group Printed in India

Inorganic Semiconductors as Photosensitizers in Biochemical Redox Reactions V. V. Nikandrov Bakh Institute of Biochemistry, Russian Academy of Sciences, 33 Leninsky Prospect, Moscow, 117071, Russia tel: (095) 954-4008 The results of studies of biochemical redox reactions photosensitized by inorganic semiconductor particles are reviewed. The mechanisms of hydrogen photoproduction, NAD+ or NADP4" photoreduction, C0 2 photofixation and photosynthesis of organic and amino acids under the coupled action of Ti0 2 , ZnO, CdS, ZnS and enzymes or bacterial cells are considered. Studies on the photocatalytic activity of ferritin, a protein containing microcrystals of hydrous ferric oxide, are described. The data on biosynthesis of cadmium sulfide by microorganisms and plants are analyzed. The possibility of the participation of inorganic semiconductors in photoprocesses in vivo is discussed. Most works by academician A. A. Krasnovsky were devoted to the elucidation of the mechanism of light energy conversion in photosynthesis. Chlorophyll was the main object of his studies. The results of A. A. Krasnovsky's investigations in photochemistry of chlorophyll and its analogs in vitro and in vivo are broadly known. However, A. A. Krasnovsky had one more favourite area of investigations - inorganic semiconductors. A. A. Krasnovsky's PhD thesis was devoted to the photochemical properties of titanium dioxide, and he was always interested in inorganic semiconductors. The studies of his laboratory on the photochemistry of inorganic semiconductors are not so well known. However, now it is clear that those studies have marked the beginning of active research in a new area - photocatalysis under the coupled action of inorganic semiconductors and enzymes - and became especially significant after the biosynthesis of an inorganic semiconductor, CdS, by microorganisms and plants was revealed, and the photochemical activity of ferritin - a protein containing ferric oxide microcrystals - was shown.

755

756

V. V. NIKANDROV

It can be assumed that to-date there are data on the possible participation of inorganic semiconductors in the photoprocesses occurring in plant, bacterial and animal cells. This review considers the data on the coupled action of inorganic semiconductors and enzymes, on the photochemical properties of ferritin, and biosynthesis of inorganic semiconductors. First we will consider the history of the problem and analyze the studies performed under the supervision of A. A. Krasnovsky on the photochemical reactions sensitized by inorganic semiconductors. Krasnovsky had first proposed to use inorganic semiconductors as photosensitizers of the photochemical reactions modelling photosynthetic processes. In 1962, the model of Hill's reaction was created, where Ti0 2 , ZnO, and W 0 3 photosensitized water oxidation by ferricyanide [1]. The fact that oxygen evolved in this reaction belonged to the "water molecules was confirmed by massspectrometric measurements [2]. Later the model of Meller's reaction was obtained: semiconductor-photosensitized oxygen absorption to form hydrogen peroxide [3]; and the ability of inorganic semiconductors was demonstrated to sensitize photoreduction of the electron acceptors having redox potential close to that of hydrogen electrode [4, 5]. Subsequently, the results of those investigations were further developed in the studies on the coupling of enzymes with semiconductors. Photocatalysis under the Coupled Action of Semiconductor and Enzyme Systems In 1975, A. A. Krasnovsky suggested that the author of this survey use inorganic semiconductors in the photoproduction of molecular hydrogen. We showed hydrogen photoevolution from water suspensions of semiconductors (Ti0 2 , ZnO) [3]. For this purpose we employed hydrogenase, the enzyme catalyzing the reaction 2e~ + 2H+ = H2. Hydrogen photoproduction by the inorganic semiconductor-hydrogenase system occurs as a result of semiconductor-photosensitized oxidation of the electron donor and enzyme-catalyzed reduction of the substrate H+, using electrons photogenerated in the conductivity zone of the semiconductor. In our first work, the enzyme itself served as electron donor, thus, the reaction proceeded for no longer than 2 h. Later, the introduction of electron donors thiocompounds, organic acids, etc. was shown to lead to a considerable activation of the process and to prevent enzyme inactivation [5-7]. With hydrogen photoproduction, the possibility of using an enzyme as a catalyst of the redox process, sensitized by inorganic semiconductor, was shown. Later the mechanism of hydrogen photoproduction in such systems was studied in detail [8-17]. NAD+ and NADP+ photoreduction [18-21], C0 2 photofixation [22-24], photosynthesis of the organic acids and amino

INORGANIC SEMICONDUCTORS AS PHOTOSENSITIZERS

757

Figure 1. Photocatalysis under a mediatorless coupled action of inorganic semiconductor and enzyme. D is electron donor, S is enzyme substrate. acids under the coupled action of enzymes and inorganic semiconductors were also realized [18, 19]. A particle of inorganic semiconductor, the surface of which is bound to an enzyme, may be regarded as a catalyst, capable of reducing the substrate of the enzyme under the action of light, using various compounds as the electron donor (Fig.l). In the works of Swiss researchers, different methods of hydrogenase immobilization on Ti0 2 and CdS particles were studied in order to obtain a photocatalyst for hydrogen production [8-10]. Enzyme immobilization by adsorption or by a covalent binding with silanized semiconductor led to a partial inactivation of the enzyme. The highest extent of binding, with the enzyme activity preserved, was achieved in the case of covalent binding of the protein. Although, the pre-treatment of the semiconductor surface influenced its photochemical properties. The highest rates of H2 photoproduction were shown with adsorbed hydrogenase. The rate of H2 photoproduction sensitized by Ti0 2 and catalyzed by immobilized hydrogenase in the presence of electron donors and an electron carrier, methyl viologen, was comparable with the rate of hydrogen production by a similar system containing platinum, instead of hydrogenase. A simultaneous immobilization of the enzyme and of the covalently bound viologen on Ti0 2 surface caused a deterioration of the system functioning, because in the course of the photoreaction the mediator was photooxidized. Willner and co-authors proposed a system of NADH and NADPH photochemical regeneration using enzymes, and Ti0 2 and CdS as photosensitizers [18, 19]. In this system, methyl viologen is reduced in a suspension of semiconductors in the presence of electron donor (mercaptoethanol or formate) under illumination. Methyl viologen is then used in the subsequent reactions of NAD or NADP reduction catalyzed by lipoamide dehydrogenase or by ferredoxin-NADP-reductase, respectively. The cycle of co-enzyme reduction

758

V. V. NIKANDROV

here was coupled with various reactions of synthesis of the organic compounds: oxaloacetate reduction to malate in the presence of malate dehydrogenase; or pyruvate amination with alanine formation in the presence of alanine dehydrogenase. The limiting stage is the oxidation of the electron donor (formate) by the holes photogenerated in the semiconductor. The quantum yield of this reaction reached 1%. A group of Japanese scientists reported C0 2 fixation with participation of CdS, ZnS and enzymes [22-24]. In their first paper [22] they used isocitrate dehydrogenase that accepted electrons from methyl viologen reduced by the electrons generated in CdS. Isocitrate dehydrogenase catalyzed C0 2 addition to oxoglutaric acid to form isocitrate. Based on the analysis of the registered values of Michaelis' constants, an assumption was made that the limiting stage was oxoglutarate carboxylation. In [23] a similar system was described containing malate dehydrogenase, catalyzing C0 2 addition to pyruvate to produce malate. It was an interesting idea to use lactic acid as electron donor; during the oxidation by semiconductor holes it formed the substrate of the enzymatic reaction, pyruvate. However, a decrease of the enzyme activity was observed in the presence of lactic acid. In the paper [24] a complex system was described where ZnS photosensitized reduction of C 0 2 to methanol proceeds with the involvement of methanol dehydrogenase and pyrroloquinoline quinone (electron carrier). The reaction may be divided into two stages. At the first stage ZnS-photosensitized reduction of C 0 2 to formate takes place with the oxidation of the electron donor, isopropanol. At the second stage, enzyme-catalyzed formate reduction to methanol occurs at the account of the oxidation of the reduced electron carrier. The total quantum yield reached 5.9%, which was, as the authors reported, the highest value for C0 2 photoreduction to methanol. A drawback of the system is that on accumulation of a definite amount of methanol, its formation ceases due to the back enzymatic reaction and alcohol oxidation by the holes photogenerated in the semiconductor. An indispensable condition of the enzyme participation in the photoreaction sensitized by a semiconductor is the transfer of the electron, photogenerated in a semiconductor particle, to the active center of the enzyme. In the majority of works on the coupled action of semiconductors and enzymes, electron carriers such as methyl viologen, rhodium bipyridyl complex, etc., were employed. However, even in the first publications, a direct electron transfer from the semiconductor to the enzyme was shown [3, 6, 7]. Realization of the enzymatic reaction at a direct transfer of the electrons, photogenerated in the conductivity zone of the semiconductor, towards the reaction center of the enzyme is especially interesting: it implies the possibility of creating photobiocatalysts on the basis of inorganic semiconductors. Photocatalysis at mediatorless coupled action of inorganic semiconductor and enzyme presents a process of several stages. The stages that directly


759

influence the efficiency of the overall photoreaction are light absorption by semiconductor particles; oxidation of the electron donor by the semiconductor, which determines free electron concentration in the conductivity zone; enzyme binding to the surface of a semiconductor particle, providing the possibility of a direct transfer of the photogenerated electrons; reduction of the enzyme substrate with a formation of reaction product. Studies of hydrogen photoproduction at the coupled action of inorganic semiconductors and hydrogenase have shown that the indispensable condition of the effective use of the photogenerated electrons in the enzymatic reaction is enzyme binding to semiconductor surface, with enzyme activity preserved [11-16, 19, 20]. The character of interaction of the adsorbed enzyme with the semiconductor determines the possibility and efficiency of the direct electron transfer from the semiconductor to the enzyme active centre and further utilization of the electrons in the enzymatic reaction. The stage limiting photoreaction may vary depending on the conditions of enzyme sorption on the semiconductor. Studies of the interrelation of the conditions of hydrogenase sorption on the semiconductors and the efficiency of mediatorless H2 photoproduction allowed one to clarify how the character of the enzyme-semiconductor interaction determined the efficacy of the direct transfer of electrons and their utilization by the adsorbed enzyme. Adsorption of hydrogenase from Thiocapsa roseopersicina on the oxide semiconductors, Ti0 2 , ZnO (the surface charge of which depends on pH, due to the amphoteric properties of surface hydroxyl groups) occurs mainly through the electrostatic interaction of the enzyme with semiconductor [12-14]. Cations of alkali-earth metals, the adsorption of which on the surface of semiconductor particles leads to a change of the surface charge, influence enzyme binding with Ti0 2 . On Ti0 2 surface, the binding centres of the hydrogenase molecules negatively charged at pH > 4.2 may be cations of Ca, Cd, Zn or organic cations. In the region of acid pH, hydrogenase sorption may take place on the positively charged Ti0 2 centres formed as a result of protonation of the surface hydroxyl groups. Under conditions of electrostatic binding of hydrogenase with Ti0 2 surface via Ca cations (Fig. 2), H2 photoproduction was shown with quantum efficiency reaching 20%, at a direct electron transfer from the semiconductor particles [14]. Under those conditions the specific activity of the enzyme, utilizing the electrons photogenerated in semiconductor as the reductive agent, is close to that of hydrogenase, which uses the soluble substrate reduced methyl viologen - as the electron donor; this indicates that the process is limited by the enzyme functioning. The interaction of hydrogenase with cadmium sulfide has a different character, CdS is an inorganic semiconductor different from oxide semi-

760

V, V. NIKANDROV

Figure 2. A direct photoinduced electron transfer from Ti02 to the reaction center of hydrogenase bound with semiconductor surface by calcium cations. conductors in the surface and energetic properties, it is capable to absorb light in the visible region (X < 515 nm). Cadmium sulfide effectively adsorbs enzymes of various nature. We showed molecular hydrogen photoformation and photoreduction of nicotinamide co-enzyme NAD+ at the mediatorless coupled action of CdS and enzymes [14-17, 19, 20]. Hydrogenase is adsorbed on CdS in various buffer solutions: phosphate, Tris, MES, HEPES and in water. With an increase of the solution pH the degree of sorption is decreased. In the phosphate buffer, the dependence of hydrogenase sorption on CdS upon pH in the pH region 5-7 is close to linear. The bivalent cations of Ca, Cd, Zn activated hydrogenase sorption in Ti0 2 , but produced practically no effect on hydrogenase sorption on CdS. On the surface of CdS in water solution, there are two types of functional centres: Bransted centres and Lewis' centers; for them the potentials are determined by H+ and OH - ions or Cd2+ and S 2_ , respectively. The number of Bransted centres on CdS surface is not great, although, it is possible that under pH decrease their role in the enzyme sorption becomes significant. Hydrogenase desorption at pH increase indicates that hydrogenase is reversibly absorbed on the positively charged centres of CdS. However, data on hydrogenase sorption at pH 6-7 in different buffer solutions and in water, where the number of the positively charged centres on the semiconductor surface is low (for CdS the isoelectric point is 6), allowed one to assume that not only the


1

\S e / /e ©

' Conduction band

^ H

Cd-OH

i^vcd©

© ^

761

2

;

\/

Ni)

^-H*',

, ''c S—r- Fc

'/i 0 Fe_L-S I v --^ / S-H Rr-/: e s ! CdS i © l0S Fe 1 ; 1

e l/ 1/ ,

Reduced electron donor

Valence band

/S-H Cd-OH

: i

!

s

Ve

I

'©

Oxidized electron donor

\ Figure 3. Hydrogen photoproduction at a direct electron transfer from CdS to the bacterial hydrogenase. protonated centres provide the sorption of hydrogenase on CdS. In aqueous suspensions of CdS, like in those of Ti0 2 and ZnO, the photoformation of H2 is observed at a direct electron transfer from CdS to the enzyme (Fig. 3) in the presence of electron donors: dithiothreitol, cystein, glycerol, methanol, EDTA, ascorbate [14-16]. However, the quantum efficiency of the reaction is considerably lower (1-2%) than that with the oxide semiconductors; the electron carrier, methyl viologen, markedly activates H2 photoproduction in suspensions of CdS. These observations and the linear dependence of H2 photoformation on light intensity in CdS suspensions allowed an assumption that the limiting stage of the mediatorless H2 photoformation by CdS and hydrogenase is the electron transfer from the semiconductor to the enzyme. The cause may be a random or unfavourable orientation of hydrogenase adsorbed on CdS surface; in this case the electron transfer to the protein redox-center is hampered and proceeds at a low rate. A peculiar property of the inorganic semiconductors as photosensitizers is their ability to bind photoreaction products on their surface. In particular, photoproduction of metals on the surfaces of some inorganic semiconductors due to the photoreduction of metallic cations was shown. We studied the influence of metals on the photoreaction efficiency at the coupled action of the semiconductors and enzymes [15, 16, 19-21]. It was found that application of metals on the surface of semiconductor particles led to a marked increase in the rate of the release of the enzymatic reactions products (hydrogen, NADH, NADPH). In the course of those studies, dehydrogenases and hydrogenases were shown to use metals (cadmium, lead), photogenerated on the semiconductor surface, as the substrates - electron donors in the enzymatic reaction.

762

V. V. NIKANDROV

Figure 4. Participation of the metallic cadmium in CdS-sensitized H2 photoproduction and in NAD+ photoreduction as the regenerated enzyme substrate, a, Semiconductor-photosensitized reduction of cadmium ion; b, catalyzed by hydrogenase and NAD-dependent hydrogenase metal oxidation in the dark reaction. v It was shown that in metal photoformation, the process of NAD+ photoreduction or H2 photoproduction at the mediatorless coupled action of CdS and NAD-dependent hydrogenase from Alcaligenes eutrophus or hydrogenase from Thiocapsa roseopersicina can be divided into a light and a dark stage [16, 17, 20, 21]. During the light stage the metal is photoproduced, in the dark stage the enzyme-catalyzed metal oxidation occurs to form the product of enzymatic reaction (Fig. 4). Silver, the metal not oxidized by the enzymes used in our experiments, also activated hydrogen photogeneration. However, in this case, the photoreaction terminated with the end of illumination. Thus, metal can activate the photoreaction at the coupled action of inorganic semiconductor and enzyme by the activation of the transfer of photoelectrons from the conductivity zone to the enzyme reaction centers or due to metal participation in the enzymatic reaction as substrate - electron donor. In the electron donor-semiconductor-enzyme system, the enzyme, without direct participation in photooxidation of the electron donor, provides the utilization of the photogenerated electrons for the formation of a specific product (NADH, H2, etc.) that as a rule, is not formed in the absence of the enzyme. However, as it was shown in the studies on the influence of hydrogenase on CdS-sensitized formate photodestruction, addition of the enzyme may influence the photoreaction normally proceeding without the enzyme [17]. Addition of hydrogenase was shown to cause a change in the ratio of the release of the products formed in CdS-sensitized anaerobic destruction of formate (C0 2 , CO, H2) and an increase in the reaction quantum yield. These results are accounted for by the combined action of the metallic cadmium formed in the photoreductive corrosion of CdS and hydrogenase, as catalysts of hydrogen production and by the influence of the enzyme on the ratelimiting stage of the photoreaction. The scheme in Fig. 5 illustrates reactions proceeding during formate photodestruction in CdS suspension containing hydrogenase.


763

Figure 5. The scheme summarizing reactions proceeding in formate photodecomposition sensitized by CdS particles coated by metallic cadmium, in the presence of hydrogenase. hv

Semiconductor

H2

Clostridium cell

Figure 6. Hydrogen photoproduction under coupling of inoganic semiconductor with bacterial cells. D is electron donor, MV is methyl viologen.

Coupling of Inorganic Semiconductors with Bacterial Cells In 1985 the light energy conversion at semiconductor coupling with bacterial cells was demonstrated [25, 26]. Under illumination, in water suspensions of titanium oxides, zinc oxides or cadmium sulfide containing bacteria Clostridium butyricum, electron donors and methyl viologen, hydrogen production was observed. The reaction proceeds as a result of the coupled action of bacteria and semiconductors: under the action of light the semiconductors oxidize electron donors and reduce methyl viologen, which due to the ability of penetration lipid membranes, carries electrons from the semiconductor particles to the intracellular hydrogenase catalyzing molecular hydrogen formation (Fig. 6). In some cases an effective hydrogen photoproduction was observed in the absence of electron carrier, i.e., at a direct electron transfer from semiconductor to the enzyme located inside the bacteria. In that study we did not consider the possibility of such processes with the participation of biosynthesized semiconductors (the data on CdS biosynthesis were obtained later); only an assumption was made that in the

764

V. V. NIKANDROV

Ferritin

Cytochrome

Figure 7. Ferritin-photosensitized reduction of cytochrome c. course of biological evolution inorganic semiconductors, capable to convert the energy of light into the chemical, could maintain the primitive metabolism of probionts and primary microorganisms. It is proposed that inorganic semiconductors could provide the primary microorganisms with active reductive agents (electrons) generated under illumination on the semiconductor surface and capable of penetrating inside the bacterial cell directly or with electron carrier. Photochemical Activity of Ferritin One of the principal forms in which iron is contained in living organisms are microparticles of the hydrated iron oxide bound to ferritin. Iron-containing proteins, ferritins, are found in virtually all types of living organisms [27, 28]. Although they vary in structural details, ferritins have similar molecular architectonics: 24 protein subunits form an almost spherical shell of 7-8 nm in internal diameter which contains the iron core consisting of one or more microparticles of hydrous ferric oxide. The main function of ferritins is to store iron and supply it to the cells. They also participate in detoxification of bivalent iron and in redox processes in the cell. Particles of ferric hydroxide are formed inside the protein shell of ferritin during the protein-catalyzed oxidation of Fe2+. The processes of ferritin core formation and dissolution are intensively investigated, their mechanism remains unknown so far. It is not clear how the redox agents dissolved in the cytoplasm sustain the redox reactions in ferritin; several studies and hypotheses have been reported on the subject, though. Iron oxides and hydroxides have semiconductor properties and are photochemically active [29]. Ferritin containing ferric hydroxide also exhibits photochemical activity. Illumination by near UV-light of ferritin isolated from pea seeds or horse spleen was shown to lead to the reduction of endogenous iron [30, 31].


765

In co-studies with the Swiss Federal Institute of Technology we have shown mammalian ferritin to be a photocatalyst of the redox reactions with a transfer of the reductive equivalents photogenerated in the ferritin core through the protein shell towards the electron acceptor: cytochrome c or viologens [32]. As the object of investigation ferritin isolated from horse spleen was used whose ultrastructure was studied the most. Microparticles of ferric oxide making the core of this ferritin have the crystalline structure of the natural mineral ferrihydrite. Illumination of cytochrome c-containing ferritin aqueous solutions results in typical changes in the spectrum of cytochrome c which indicate its reduction. At ferritin concentration of 1 uM, 32 uM cytochrome is fully reduced, which shows the photocatalytic character of the reaction. The rate of cytochrome photoreduction depends on the intensity of the light, ferritin concentration and the electron acceptor. We used the light in various spectral regions and found that the light in the wavelength range of absorption of the ferritin mineral core was photoactive. Cytochrome photoreduction was observed both under aerobic and anaerobic conditions. Oxalate, citrate and malonate activated cytochrome photoreduction. In the presence of glutathion, cystein and sulfite a dark reduction of cytochrome c was observed; its rate was enhanced under illumination. Ferritin also sensitizes photoreduction of viologens: dimer viologen DV4+ with the redox potential E0 = 0.0 V and propylviologen sulfonate PVS° (E0 = -0.340 V). In contrast to cytochrome photoreduction, ferritincatalyzed reduction of DV4+ and PVS° is observed exclusively under anaerobic conditions. Photoreduction of DV4+ occurs in the absence of the exogenous electron donor, although the maximal obtained concentration of photoreduced DV4+ in this case does not exceed 1 jiM. Addition of tartrate markedly enhanced the rate and depth of DV4+ reduction. Using the laser pulse photolysis, we obtained kinetic curves for PVS° photoreduction within the microsecond time interval. Analysis of these kinetic curves showed PVS° photoreduction to take place 4 us after the laser flash (kobs = 106 s _1 ). The photochemically active element of ferritin is the mineral core which allows light absorption in the visible spectral region. Ferrihydrite microcrystals forming the ferritin core, as other iron oxides, apparently, have semiconductor properties. Absorption of photons whose energy exceeds the width of the semiconductor bandgap causes the generation of electron-hole pairs energetic and long-lived enough to be involved in redox reactions with the components of the surrounding medium. The valence band holes oxidize the electron donors or, in their absence, the ferritin protein shell. The electrons photogenerated in the valence band are involved in the interfacial electron transport reactions with electron acceptors, cytochrome c or viologens. Since the ferritin mineral core is surrounded by a 2.5 nm-thick protein shell possessing intrasubunit channels of 0.4-0.5 nm diameter, the photoreduction

766

V. V. NIKANDROV

of cytochrome and DV4+ (their molecules being much larger than the pore size) is accounted for by the transport of the photogenerated electrons through the protein shell (Fig. 7). As an alternative mechanism, the participation in the electron transport of iron ions bound with the polypeptide ferritin shell or formed during the photoinduced dissolution of ferrihydrite microcrystals was proposed. To elucidate the possibility of Fe2+ participation in the reaction, ferritin photoreduction was studied. Illumination of ferritin solutions containing electron donor, under anaerobic conditions, leads to a complete reduction of Fe3+ included in ferritin. Formed during the reductive ferritin photodissociation, Fe2+ are capable to reduce cytochrome and DV4+. However, ferritin photocorrosion and ferritin-photocatalyzed reduction of electron acceptors have markedly different pH dependencies. The photocorrosion rate is diminished with pH increase, at pH higher than 7.5 photocorrosion is no longer observed. The rate of cytochrome c photoreduction in the absence of exogenous donors is constant at pH 7-9 and decreases at the acidification of the medium. In the presence of tartrate it does not depend on the acidity of the medium in the pH range 5.5-8.5. Ferritin-photocatalyzed PVS° reduction is observed only at pH > 9 under conditions where the photocorrosion of iron oxide is thermodynamically impossible. From these observations the conclusion can be drawn that cytochrome and viologens are reduced principally by the electrons photogenerated in the ferritin mineral core, although, the possibility cannot be excluded of the photoinduced electron transport with participation of iron ions at pH < 7.0. In addition, under aerobic conditions, the superoxide-radical, whose formation is possible in the ferritin-catalyzed oxygen reduction, may act as the cytochrome reductant. Thus, iron-containing protein ferritin can act as the photocatalyst of redox reactions providing the photoreduction of electron acceptors with the redox potential of-+0.240 V (of cytochrome c) to -0.340 V (of PVS°). Cadmium Sulfide Biosynthesis. The Possibility of Participation of Inorganic Semiconductors in the Photoprocesses Occurring in the Cells The main goal of the studies carried out in A. A. Krasnovsky's and other laboratories on the coupled action of inorganic semiconductors and enzymatic systems was to elucidate the mechanism of photoreactions under conditions of the interfacial transfer of semiconductor particles to the enzymes and development of the photobiotechnological systems capable to convert light energy into the chemical. An assumption was made that those studies would clarify the variants of the involvement of the inorganic semiconductors - components of the Earth core in the energy supply of chemical and biological evolution.


767

However, in our opinion, the data on CdS biosynthesis by plants and microorganisms and its photochemical activity show the possibility of inorganic semiconductors participation in the photoprocesses in vivo. Formation of CdS in the cultures of sulfate-reducing bacteria and some yeast due to the interaction of cadmium ions with H2S produced by microorganisms has been long known [see reviews 33-35]. But only recently some microorganisms were shown to form CdS on their surface or inside the cells. Klebsiella pneumonia and Clostridium thermoaceticum detoxicate cadmium ions by forming CdS crystallites on the cell surface [36-42]. Cells of Klebsiella pneumonia grown with cadmium ions due to the presence of CdS on their surface acquire color absorbing light of the wavelength up to 470 nm [39, 40]. Illumination with near UV-light of the bacteria containing CdS on the cell surface leads to the reduction of cadmium ions of the semiconductor crystalline lattice with the formation of metallic cadmium [40]. The photochemical activity of semiconductor particles formed in Klebsiella aerogenes culture was shown by the ESR method [42]. The possibility of intracellular CdS synthesis was first demonstrated for the yeasts Schizosaccharomyces pombe and Candida glabrata [43-45]. Yeast cells in the presence of cadmium ions in the medium synthesize CdS crystallites of 20 A in diameter, coated with peptides [44, 45]. Those crystallites isolated from yeast cells emitted luminescence and sensitized methyl viologen reduction. Crystallites of CdS with similar properties were isolated from leaves and roots of tomato [46] and mustard [47]. It is proposed that CdS formation may present the mechanism of cadmium ion detoxification in plants and microorganisms. From the preceding it is clear that the data on the photoreactions in vivo involving CdS are not numerous. However, taking into consideration the high photochemical activity of inorganic semiconductors, it may be said that the most likely CdS is involved in the constructive and destructive photochemical reactions proceeding in the plant and microorganism cells containing this semiconductor. It is possible that photocatalytic redox reactions under a coupled action of CdS and enzymes, similar to the above described, take place in vivo. With the shown ability of the ferritin mineral core to sensitize cytochrome photoreduction and oxidation of various organic compounds, the possibility cannot be excluded of similar redox reactions involving ferritin in plants and bacteria. The probability of light excitation of ferritin in the mammalian cells is lower, though light may be absorbed by ferritin contained in blood and skin. In paper [3] based on the probability of light absorption by ferritin of dermal fibroplasts and macrophages, an assumption was made that ferritin-sensitized photodestructive reactions may lead to development of the processes of skin aging and cancerogenesis. Thus, it may be concluded that the photoreactions involving inorganic

768

V. V. NIKANDROV

semiconductors can be one of the ways of bioconversion of solar energy. The author is grateful to the Russian Foundation for Basic Research (grant No 960449805) whose support allowed him to continue the studies of photoreactions involving inorganic semiconductors begun by Academician A. A. Krasnovsky. REFERENCES 1. A. A. Krasnovsky and G. P. Brin, Dokl. Akad. NaukSSSR 147:656-659 (1962) (in Russian). 2. G. V. Fomin, G. P. Brin, M. V. Genkin, A. K. Ljubimova, L. A. Blumenfeld and A. A. Krasnovsky, Dokl. Akad. Nauk SSSR 212:424-430 (1973) (in Russian). 3. A. A. Krasnovsky, G. P. Brin and V. V. Nikandrov; Dokl. Akad. Nauk SSSR 229:990-993 (1976) (in Russian). 4. A. A. Krasnovsky and G. P. Brin, Dokl. Akad. Nauk SSSR 213:1463-1435 (1973) (in Russian). 5. A. A. Krasnovsky, G. P. Brin, A. N. Luganskaya and V. V. Nikandrov, Dokl. Akad. Nauk SSSR 249:896-899 (1979) (in Russian). 6. V. V. Nikandrov, G. P. Brin and A. A. Krasnovsky, Dokl. Akad. Nauk SSSR 256:1249-1253 (1981) (in Russian). 7. V. V. Nikandrov, G. P. Brin and A. A. Krasnovsky, Photobiochem. Photobiophys. 6:101-107(1983). 8. P. Cuendet, M. Gratzel and M. L. Pelaprat, J. Electroanal. Chem. 181:173-185 (1984). 9. P. Cuendet, M. Gratzel, K. K. Rao and D. O. Hall, Photobiochem. Photobiophys. 7:331-340(1984). 10. P. Cuendet, K. K. Rao, M. Gratzel and D. O. Hall, Biochimie 68:217-221, N2:101-107(1986). 11. V. V. Nikandrov, M. A. Shlyk, N. A. Zorin, I. N. Gogotov and A. A. Krasnovsky, Dokl. Akad. Nauk SSSR 300:990-994 (1988) (in Russian). 12. V. V. Nikandrov, M. A. Shlyk, N. A. Zorin, I. N. Gogotov and A. A. Krasnovsky, FEBSLett. 234:111-114. 13. ML A. Shlyk, V. V. Nikandrov, N. A. Zorin and A. A. Krasnovsky, Biokhim. 54:1598-1606 (1989) (in Russian). 14. V. V. Nikandrov, A. I. Arystarkhov, M. A. Shlyk and A.A. Krasnovsky, Dokl. Akad. Nauk SSSR 319:242-245 (1991) (in Russian). 15. V. V. Nikandrov, I. A. Shumilin, A. I. Nedoluzhko, N. A. Zorin, V. O. Popov and A. A. Krasnovsky, Dokl. Akad. Nauk 335: 802-805 (1994) (in Russian). 16. A. I. Nedoluzhko, I. A. Shumilin and V. V. Nikandrov, J. Phys. Chem. 100:17544-17550(1996). 17.1. Willner, D. Mandler and R. Maidan, Nouv. J. Chim. 11:109-121 (1987). 18. Z. Goren, N. Lapidot and I. Willner, J. Mol. Cat. 47:21-32 (1988). 19.1. A. Shumilin, V. V. Nikandrov, V. V. Popov and A. A. Krasnovsky, FEBS Lett. 306:125-128(1992). 20.1. A. Shumilin, V. V. Nikandrov, A. A. Krasnovsky and V. O. Popov, FEBSLett. 328:189-192(1993). 21. A. I. Nedoluzhko, A. N. Semenova and V. V. Nikandrov, / Russ. Conference of


769

Photobiol. pp. 35-36 (Pushchino, 1996). 22. H. Inoue, Y. Kubo and H. Yoneyama, J. Chem. Soc. Faraday Trans. 87:553 (1991). 23. H. Inoue, M. Yamachika and H. Yoneyama, J. Chem. Soc. Faraday Trans. 88:2215 (1992). 24. S. Kuwabata, K. Nishida, R. Tsuda, H. Inoue and H. Yoneyama, J. Electrochem. Soc. 141:(1994). 25. A. A. Krasnovsky, V. V. Nikandrov and S. A. Nikiforova, Dokl. Akad. Nauk SSSR 285:1467- 1471 (1985) (in Russian). 26. A. A. Krasnovsky and V. V. Nikandrov, FEBSLett. 219:93-96 (1987). 27. E. C. Theil, Adv. Enzymol. 63:421-449 (1990). 28. H. Massover, Micron 24:389-486 (1993). 29. K. Leland and A. J. Bard, J. Phys. Chem. 91:5076-5083 (1987). 30. J.-P. Laulhere, A.-M. Laboure and J.-F. Briat, Biochem. J. 269:79-84 (1990). 31. M. Aubailly, S. Salmon, J. Haigle, M. Bazin, J. C. Maziere and R. Santus, Photochem. Photobiol. 26: 185-191 (1994). 32. V. V. Nikandrov, C. Gratzel, J.-E. Moser and M. Gratzel, J. Photochem. Photobiol. B: Biol. 41:83-89 (1997). 33. G. M. Gadd and A. J. Griffiths, Microbiol. Ecol 303-317 (1978). 34. S. F. Minney and A. V. Quirk, Microbios. 42:37-44 (1985). 35. O. Ya. Sentzova and V. N. Maksimov, Usp. Mikrob. 20:227-252 (1985) (in Russian). 36. H. Aiking, K. Kok, H. van Heerikhuizen and J. van Reit, Appl. Environ. Microbiol. 44:938-944 (1982). 37. H. Aiking, H. Govers and J. van't Reit, Appl. Environ. Microbiol. 50:1262-1267 (1985). 38. D. P. Cunningham and L. L. Lundie, Appl. Environ. Microbiol. 59:7-14 (1993). 39. J. D. Holmes, P. R. Smith, R. Evans-Gowing, D. J. Richardson, D. A. Russel and J. R.Sodeau, Arch. Microbiol. 163:143-147 (1995). 40. J. D. Holmes, P. R. Smith, R. Ewans-Gowing and D. J. Richardson, Photochem. Photobiol. 62:1022-1026(1995). 41. J. D. Holmes, D. J. Richardson, S. Saed, R. Ewans-Gowing, D.A. Russel and J. R. Sodeau, Microbiol. 143:2521-2530 (1997). 42. J. D. Holmes, J. A. Farrar, D. J. Richardson, D. A. Russel and J. R. Sodeau, Photochem. Photobiol. 65: 811-817 (1997). 43. R. N. Reese and D. R. Winge, J. Biol. Chem. 263:12832-12836 (1988). 44. C. Dameron, B. R. Smith and D. R. Winge, J. Biol Chem. 264:17355-17360 (1989). 45. C. Dameron, R. N. Reese, R. K. Mehra, A. R. Kortan, P. J. Carroll, M. L. Steigerwald, L. E. Brus and D. R. Winge, Nature 338:596-597 (1989). 46. R. N. Reese, C. A. White and D. R. Winge, Plant Physiol. 98:225-229 (1992). 47. D. M. Speiser, S. L. Abrahamson, G. Banuelos and D. W. Ow, Plant Physiol. 99:817-821 (1992).


769

Photobiol. pp. 35-36 (Pushchino, 1996). 22. H. Inoue, Y. Kubo and H. Yoneyama, J. Chem. Soc. Faraday Trans. 87:553 (1991). 23. H. Inoue, M. Yamachika and H. Yoneyama, J. Chem. Soc. Faraday Trans. 88:2215 (1992). 24. S. Kuwabata, K. Nishida, R. Tsuda, H. Inoue and H. Yoneyama, J. Electrochem. Soc. 141:(1994). 25. A. A. Krasnovsky, V. V. Nikandrov and S. A. Nikiforova, Dokl. Akad. Nauk SSSR 285:1467- 1471 (1985) (in Russian). 26. A. A. Krasnovsky and V. V. Nikandrov, FEBSLett. 219:93-96 (1987). 27. E. C. Theil, Adv. Enzymol. 63:421-449 (1990). 28. H. Massover, Micron 24:389-486 (1993). 29. K. Leland and A. J. Bard, J. Phys. Chem. 91:5076-5083 (1987). 30. J.-P. Laulhere, A.-M. Laboure and J.-F. Briat, Biochem. J. 269:79-84 (1990). 31. M. Aubailly, S. Salmon, J. Haigle, M. Bazin, J. C. Maziere and R. Santus, Photochem. Photobiol. 26: 185-191 (1994). 32. V. V. Nikandrov, C. Gratzel, J.-E. Moser and M. Gratzel, J. Photochem. Photobiol. B: Biol. 41:83-89 (1997). 33. G. M. Gadd and A. J. Griffiths, Microbiol. Ecol. 303-317 (1978). 34. S. F. Minney and A. V. Quirk, Microbios. 42:37-44 (1985). 35. O. Ya. Sentzova and V. N. Maksimov, Usp. Mikrob. 20:227-252 (1985) (in Russian). 36. H. Aiking, K. Kok, H. van Heerikhuizen and J. van Reit, Appl. Environ. Microbiol. 44:938-944 (1982). 37. H. Aiking, H. Govers and J. van't Reit, Appl. Environ. Microbiol. 50:1262-1267 (1985). 38. D. P. Cunningham and L. L. Lundie, Appl. Environ. Microbiol. 59:7-14 (1993). 39. J. D. Holmes, P. R. Smith, R. Evans-Gowing, D. J. Richardson, D. A. Russel and J. R.Sodeau, Arch. Microbiol. 163:143-147 (1995). 40. J. D. Holmes, P. R. Smith, R. Ewans-Gowing and D. J. Richardson, Photochem. Photobiol. 62:1022-1026 (1995). 41. J. D. Holmes, D. J. Richardson, S. Saed, R. Ewans-Gowing, D.A. Russel and J. R. Sodeau, Microbiol. 143:2521-2530 (1997). 42. J. D. Holmes, J. A. Farrar, D. J. Richardson, D. A. Russel and J. R. Sodeau, Photochem. Photobiol. 65: 811-817 (1997). 43. R. N. Reese and D. R. Winge, J. Biol. Chem. 263:12832-12836 (1988). 44. C. Dameron, B. R. Smith and D. R. Winge, J. Biol Chem. 264:17355-17360 (1989). 45. C. Dameron, R. N. Reese, R. K. Mehra, A. R. Kortan, P. J. Carroll, M. L. Steigerwald, L. E. Brus and D. R. Winge, Nature 338:596-597 (1989). 46. R. N. Reese, C. A. White and D. R. Winge, Plant Physiol. 98:225-229 (1992). 47. D. M. Speiser, S. L. Abrahamson, G. Banuelos and D. W. Ow, Plant Physiol. 99:817-821 (1992).