water interface

Bioelectrochemistry and Bioenergetics. 11 (1983)167-172 A section of J. Electroanal. Chem., and constituting Vol. 156 (1983)

Elsevier Sequoia S.A., Lausanne

167

Printed in The Netherlands

5 8 7 - - O X Y G E N E V O L U T I O N IN T H E P R E S E N C E O F C H L O R O P H Y L L

A D S O R B E D AT THE O C T A N E IWATER I N T E R F A C E

M.D, KANDELAKI Institute of Cybernetics, Tbilisi (U.S.S.R.)

A.G. VOLKOV Institute of Electrochemistry, Academy of Sciences of the USSR, Moscow (U.S.S,R.)

A.L. LEVIN All Union Research Institute for Medical Instrumentation, Moscow (U.S.S.R.)

L.I. BOGUSLAVSKY Institute of Electrochemistry, Academy of Sciences of the USSR, Moscow (U.S.S.R.)

(Revised manuscript received August 27th 1983)

SUMMARY Chlorophyll a adsorbed at the octane ]water interface catalyzes the reaction of water photooxidation. The potential drop generated in this case at the octane ]water interface is proportional to the amount of adsorbed chlorophyll and to the oxygen evolution rate, in agreement with the phenomenological model of catalytic reactions at the interface between two immiscible liquids. The spectrum of action of the oxygen evolution rate resembles the absorption spectrum of chlorophyll a in the octane Iwater system. The quantum yield as calculated per number of incident light quanta is equal to 0.6 % at the wavelength 660 nm.

INTRODUCTION P h o t o o x i d a t i o n of water in various model systems is of interest n o t only for photosynthesis, b u t also in view of the practical application of light energy conversion into other forms of energy [1]. Earlier experiments have shown that i l l u m i n a t i o n of the octane Iwater interface in the presence of chlorophyll a or metalloporphyrines, the acceptor of p r o t o n s in octane (dinitrophenol, p e n t a c h l o r o p h e n o l ) a n d electrons in water {K3[Fe(CN)6], N A D , N A D P ) , results in oxygen evolution from water [2-4]. This process was detected b y mass-spectrometry a n d polarography. I n this work we investigated water p h o t o o x i d a t i o n b y chlorophyll a adsorbed at the interface a n d recorded the spectrum of oxygen evolution reaction. We also c o m p a r e d the Volta-potential drop with the surface excess of chlorophyll at the o c t a n e Iwater interface. 0302-4598/83/$03.00

© 1983 Elsevier Sequoia S.A.

168 EXPERIMENTAL A system consisting of equal volumes of octane and water and containing dinitrophenol ( D N P H ) or pentachlorophenol (PCPH) was equilibrated in a separate vessel for 24 hours. Chlorophyll a and a water-soluble acceptor of electrons were introduced just before the beginning of the experiment. All the solutions were prepared with twice-distilled water; the salts were twice-recrystallized; D N P H , PCPH, and potassium ferrocyanide were three times recrystallized by the conventional techniques. The chemically pure (CP) grade or chromatographic analysis grade octane and the Reanal N A D were used in the experiments. Chlorophyll a was extracted from nettle in the laboratory of Prof. A.A. Krasnowsky by the procedure proposed in Ref. 5. The absorption spectrum of chlorophyll a is presented in Fig. 2. In all the experiments, where oxygen was determined, the cell was initially blown out with argon to remove all atmospheric oxygen, then the cell was tested for air tightness. The measurements were carried out at 20 °C. The octane water interface was illuminated by a QGM-300 W iodine incandescent lamp. The light passed through a water filter located in front of the cell. Interference filters (3,o. 5 = 12 nm) were used for measuring the action-spectra. The light-power falling onto the interface was measured with a thermoelement and, for greater reliability, also with a ferrioxalate actinometer at the wavelength 429 nm. The quantum yield was estimated from the energy of the incident light expressed in terms of the number of quanta. The oxygen concentration was measured polarographically by a dropping mercury electrode or a Clark electrode immersed into the water fraction of the complete octane ]water system. The Clark electrode was designed earlier at the Institute for Medical Instrumentation for the purpose of analyzing the acid-base equilibrium " B a r o - A K O R - 2 " [6]. The parameters of the Clark electrode were so chosen [7] that it operated in the p O 2 measurement mode. The electrode was calibrated with respect to oxygen produced in the same cell as a result of electrolysis in 1 N K O H at the electrodes fitted in the cell. Special experiments were performed to check whether the Clark electrode readings depended on light radiation. The characteristics of the electrode allowed an oxygen concentration as low as 10 -8 M to be detected. We note that special test experiments with a Clark electrode, as well as previous mass-spectrometric investigation [4], undoubtedly indicate that oxygen detected in the reaction can in no way be attributed to the thermal effect on the Clark electrode, as was pointed out by Tien [8] in the discussion of the paper quoted in Ref. 9. The volta-potential was measured by the vibrating electrode method in the chain: air

Au

octane PCPH chlorophyll

water electron acceptor

water I saturated Hg2C12, Hg with KC1

(1)

and recorded automatically by a KSP-4 recorder. The adsorption of chlorophyll, F, at the octane]water interface was found by the

169

Gibbs equation from the change in the interfacial tension measured by the drop weight method. RESULTS A N D DISCUSSION

Illumination of the octane Iwater system containing chlorophyll a , D N P H and K3[Fe(CN)6 ] with visible light results in oxygen evolution. Figure 1 shows the oxygen evolution rate v e r s u s the electron acceptor concentration in water measured on a dropping electrode. The modifications of the Volta-potential at the octane qwater interface v e r s u s the concentration of this acceptor under illumination are also plotted in Fig. 1. Every point in this figure is the average value from 4 individual measurements. Inspection of Fig. 1 shows that the rate of oxygen evolution correlates with the potential drop change at the octane Iwater interface for acceptor concentrations not exceeding 10 -2 M. This means that the process leading to oxygen evolution is accompanied by charge separation in the double electrical layer at the interface, in agreement with a previous model [10,11], according to which the potential drop A~ is directly proportional to the heterogeneous reaction rate v and inversely proportional to the capacitance of the electric-double layer, C, and to the c o n s t a n t k 3 of charge rejection into water: z.~ Ag~ = - ~ 3 v

(2)

Chlorophyll participation in the water photooxidation reaction is evidenced by the action-spectrum of the oxygen evolution rate, which represents the ratio of the number of oxygen molecules to the number of incident light quanta as a function of the incident wavelength (Fig. 2). Chlorophyll molecules are known to have a noticeable surface activity and, hence,

i50

r 10-3

[K3Fe{CN)6](M)

I 10-2

Fig. 1. The oxygen evolution rate (O) and the Volta-potential measured in chain (]) (~) versus the concentration of K3[Fe(CN)6 ] during illumination. Medium: 2 × 10 -6 M of chlorophyll, 2 x 10 -2 M of tris-HC1, pH 7.4 and 10 -3 M of DNPH.

170

07]

O6

~0

05

04-

03-

0.5 02-

019, (nm)

o

4;o

500

600 700 800

Fig. 2. The chlorophyll absorption spectrum in octane and the spectral dependence of the number of evolved oxygen molecules on the number of incident quanta (dots). The oxygen evolution rate was determined in the medium: waterroctane, 10 -3 M PCPH, 5 x 1 0 -6 M chlorophyll, 10 -3 M N A D , 2 X 1 0 -2 M tris-HCl; pH 7.5.

E

u

c,)JM I

I

I

I

1

2

3

4

Fig. 3. The chlorophyll adsorption isotherm at the octane [water interface.

171

90

60 - ~

30

0

I 1

P x 10 ~° ( m o l e / c m 2) I 2

Fig. 4. The Volta-potential measured in chain (1) 10 3 M PCPH, 10 -2 M K3[Fe(CN)6]; pH 7.4.

versus

i 3

the amount of adsorbed chlorophyll. Medium:

they considerably reduce the interfaciat tension during adsorption at the interface. This property was used to determine the surface excess of chlorophyll, F, according to Gibbs. Figure 3 shows the surface excess of chlorophyll concentration in octane. From the number of particles per cm 2 of the interface for coverage value 0 = 1 it was possible to calculate the projected area per one molecule, which was found to be equal to c a . 55 ~2 for chlorophyll. Comparison of this area with the area of the chlorophyll molecule itself allowed estimation of the inclination of the chlorophyll molecule plane at the octane Jwater interface. The angle of inclination is equal to c a . 50 ° , which is close to the value obtained by the optical methods both for chloroplast membranes and for chlorophyll built in the BLM [12,13]. When the octane ]water interface was illuminated in the presence of chlorophyll, the interracial tension did not change. Figure 4 shows the dependence of the potential, due to the illumination of the system, on the surface excess of chlorophyll at the octane]water interface. As is seen from Fig. 4, the potential drop is proportional to the amount of chlorophyll adsorbed at the interface. As the surface excess of chlorophyll increases, the dependence deviates from a linear one and exhibits saturation, indicative of mutual interaction of adsorbed chlorophyll molecules that may lead either to a change in their orientation on the surface or to growth of the size of aggregates in which molecule orientation remains unchanged. Thus, the data of Fig. 4 indicate that the water photooxidation reaction is catalyzed by the chlorophyll aggregates adsorbed at the octane t water interface. ACKNOWLEDGEMENTS

The authors express their deep gratitude to A.A. Krasnovsky, M.G. Kuz'min and Yu.A. Chizmadjev for valuable discussions.

172 REFERENCES 1 A.A. Krasnovsky, The Light Energy Conversion in Photosynthesis, Nauka Publishing House, Moscow, 1974 (in Russian). 2 L.I. Boguslavsky, A.G. Volkov, M.D. Kandelaki and E.A. Nizhnikovsky, Dokl. Akad. Nauk SSSR, 227 (1976) 727 (in Russian). 3 L.I. Boguslavsky, A.G. Volkov, M.D. Kandelaki, M.A. Bibikova and E.A. Nizhnikovsky, Biofizika, 22 (1977) 223 (in Russian). 4 L.I. Boguslavsky, L.T. Zhuravliev, M.D. Kandelaki and K.Ya. Shengeliya, Dokl. Akad. Nauk SSSR, 240 (1978) 1453 (in Russian). 5 K. Jrijama, N. Ogura and A. Takamiya, J. Biochem., 76 (1972) 1573. 6 A.L. Levin, A.V. Yushkin, M.Ya. Khodas, A.F. Albantov, T.N. Giorgobiani, N.V. Bochkaryev and L.R. Mosolova, Summaries of the 7th International Congress on Hyperbaric Medicine, Moscow, 1981, p. 445. 7 A.F. Albantov, A.L. Levin and A.L. Tverskoy, Summaries of reports presented at the Analytical Instrument Making Scientific-Technical Meeting, 1980, Tbilisi, p. 12. 8 H. Ti Tien in Topics in Photosynthesis, J. Barber (Editor), Elsevier, Amsterdam, 1979, Vol. 3, p. 115. 9 Y. Toyoshima, M. Marino, H. Motoki and M. Sukigara, Nature (London), 265 (19777 187. 10 Yu.l. Kharkats, A.G. Volkov and L.1. Boguslavsky, Dokl. Akad. Nauk SSSR, 220 (1975) 1441 (in Russian). 11 Yu.I. Kharkats, A.G. Volkov and L.I. Boguslavsky, J. Theor. Biol., 65 (1977) 179. 12 A.J. Hoff, Photochem. Photobiol., 19 (1974) 51. 13 J. Breton, M. Mishel-Villaz and G. Poullotin, Biochim. Biophys. Acta, 314 (1973) 42.