22 1
J. phys. Chem. 1982, 86, 221-223
Photochemical Generation at Superoxide Ions from p-Superoxo-decacyanodicobalt(I I I ) Ions in Aqueous Solutions Mlklo Hoshlno,' Masao NakaJkna, Masaakl Takakubo, and Masashl Imamura The Institute of Fhyslcal and Chemlcai Research, Wako, Saitama 351, Japan (Received: June 8, 1981; I n Final F m : October 8, 1981)
Photochemical reactions of p-superoxo-decacyanodicobalt(II1)ions were studied in aqueous solutions. The quantum yields of the photochemical decomposition are dependent on the irradiation wavelength. From the quantum yield measurements at various pH's, the following initial reactions are proposed. For 257-nm irradiation, + 2H20 + hu ~ [ C O ( C N ) ~ H ~+O0]2 ~- , -and, for 313-nm irradiation, [(CN),Co[(CN)~CO-O~-CO(CN)~]" 02-Co(CN),Ik + H20 + hu [Co(CN),H20I2-+ [Co(CN),0213-followed by 2[Co(CN),O2I3- [(CN),Co02-Co(CN),16 + 0,: Superoxide ions produced upon irradiation were detected from the reduction of cytochrome(II1) c.
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Introduction (111) c (Cyt(II1)). The superoxide ions are concluded to be a precursor of hydrogen peroxide which is identified as The dioxygen-metal complexes have attracted much one of the final photodecomposition products of p-soc in attention from the viewpoint of a model compound for aqueous solution. ~ crystal structure dioxygen-binding proteins in v i ~ 0 . l The and ESR studies of these complexes have been made to Experimental Section explore the nature of the metal-oxygen binding~.~J Potassium salt of p-superoxo-decacyanodicobalt(III), For the p-superoxo-dicobalt complexes, the X-ray K5[ (CN)SCo-02-Co(CN),] (K,-p-soc), synthesized accordstructure studies revealed that the distance between the to Mori et 81.: was purified by recrystallizing it from ing two oxygen atoms is almost equal to that of the alkali metal a mixture of methanol and water. Horse-heart cytosuperoxides.& The ESR studies showed that the two chrome(II1) c purchased from Sigma Chemical Co. was cobalt atoms in each complex are equivalent and an unused without further purification. paired electron is predominantly located on the dioxygen The absorption spectrum of p-soc in aqueous solution These results indicate that the p-superoxo-dihas absorption peaks at 486 and 310 nm, whose absorption cobalt complexes have a structure of dinuclear cobalt(II1) coefficients were determined to be 7.73 X lo2 and 1.7 X bridged by a superoxide ion. 1 0 4 M-' cm-', respectively. The ESR spectrum of p-SOCin Photochemical reactions of p-superoxo-dicobalt comaqueous solution shows hyperfine structure due to interplexes in aqueous solutions were studied by Valentine and action of an unpaired electron with two equivalent cobalt Valentein, Jr.,9 who established the stoichiometry of the nuclei (I = 7/2).' reactions and found the wavelength dependence of the Monochromatic light was obtained with a 250-W USquantum yields of the decomposition. Among the several H-250D high-pressure mercury lamp and a Shimadzu complexes studied, p-superoxo-decacyanodicobalt(III)ions Bausch and Lomb monochromator (2700 grooves/mm). A (psoc) in aqueous solution containing 0.1 M HC10, unxenon lamp incorporated in a Hitachi MPF4 spectrodergo photodecomposition to produce [CO(CN)~H~O]~-, fluorimeter was used occasionally as a conventional hydrogen peroxide, and oxygen. The quantum yield was monochromatic-light source. Light intensities were meadetermined to be 0.2 for 320-nm irradiation. sured by chemical actinometry using aqueous potassium In 1975 Miskowski et al.l0 assigned the electronic abferrioxalate solutions." sorption bands of p-soc in aqueous solution and deterChanges in concentration of p-soc in photochemical remined the quantum yields for photodecomposition at pH actions were determined by monitoring the absorbance at 7 to be 0.06 f 0.05 and 0.094 f 0.01 for 366- and 313-nm 486 nm. pH was adjusted by using sulfuric acid and soirradiation, respectively. The difference in quantum yields dium hydroxide instead of using buffer solutions in order reported by the two groupsQJohas not yet been elucidated. to minimize the ionic strength effect on the photochemical In the present study, we determined quantum yields for reactions. Optical absorption and ESR spectra were rephotodecomposition of p-soc in aqueous solutions at varcorded on a Hitachi 200-20 spectrophotometer and a ious pHs using monochromatic light and found that the JES-FE 3AX spectrometer, respectively. quantum yields do depend on pH for 313- but not 257-nm irradiation. The photodecomposition produces superoxide Results ions, which were detected from reduction of cytochromeIrradiation of aqueous p-soc solutions gives rise to a decrease in absorbance over the wavelength range 240-600 (1) Valentine, J. S. Chem. Rev. 1973, 73, 235. nm. Prolonged irradiation results in a weak absorption (2) Vaska, L. Acc. Chem. Res. 1976.9, 175. centered around 313 nm, which is ascribed to [Co(3) Henrici-Olive, G.; Olive, S. Angew. Chem., Znt. Ed. Engl. 1974,13, (CN),H20I2-, a final product. Since quantum yields of the 29. photochemical decomposition were determined below 10% (4) Fronczak, K. R.; Schaefer, W. P.; Marsh, R. E. Znorg. Chem. 1975, 14, 611. conversion of p-soc, an inner filter effect due to final (5) Shaefer, W. P.; Marsh, R. E. Acta Crystallogr. 1966,21, 735. products can be disregarded. (6) Marsh, R. E.; Shaefer, W. P. Acta Crystallogr. 1968,24, 246. Figure 1shows the absorption spectrum of aqueous p-soc (7) McLendon, G.; Picknes, S. R.; Martell, A. E. Znorg. Chem. 1977, 16, 1551. solution at pH 7 before irradiation and the quantum yields ~
~
~
~
~~~
~
(8) Mori, M.; Weil, J. A.; Kkaird, J. K . J. Phys. Chem. 1967, 71, 103.
(9) Valentine, J. S.;Valentine, D., Jr. J. Am.Chem. SOC.1971,93,1111. (10)Miskowski, V. M.; Robins, J. S.;Treitel, I. M.; Gray, H. B. Znorg.
Chem. 1975,14, 2318.
(11) Hatchard, G. H.; Parker, C. A. h o c . R. 9oc. London, Ser. A 1956, 220, 104.
0 1982 American Chemical Society
222
The Journal of Physical Chemlstty, Vol. 86, No. 2, 1982
500
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-5
WAVELENGTA l n m ) 400 330
25c
I
I
\
Hoshino et al.
I
I
W
1
I - 1000 [
7 30
20
WAVENUMBER ( k K )
Flgure 1. Absorption spectrum (-) and the wavelengthdependent quantum ylelds for photodecomposition (-*-) of p-soc in aqueous solution at pH 7. Concentration of p-soc for photolysis: 1.28 X 10" M.
obtained with irradiated aerated aqueous solutions as a function of excitation wavelength. The same quantum yields were obtained with deaerated solutions. A detailed analysis of the electronic structure and the optical absorption spectrum of p-soc revealed that the absorption bands around 268 and 310 nm are ascribed to the ligand-to-metal charge transfer (LMCT) of the types 7r*h(02-) d+ .a and T*h(02-) d,l, respectively.1° The broad absorption band around 480 nm is due to the metal-to-ligand charge transfer (MLCT) of the type d a 7r*,(02-).The absorption shoulder around 373 nm is regarded as being due to the d-d transition ('Al lE). The quantum yields of photodecomposition were ded+z, 0.07 f termined to be 0.20 f 0.01 for T*h(02-) 0.003 for 7r*,,(02-) dz2, and 0.05 f 0.001 for d-d transitions. Irradiation at 480 nm (da r*,(02-))gave Final products are regarded quantum yields as low as as being identical irrespective of irradiation wavelength judging from the absorption spectra observed after prolonged irradiation. The aqueous solution of p-soc at room temperature ex~ hibits the same ESR spectrum as that r e p ~ r t e d .The intensity of the ESR signal gradually decreases on exposing the solution to UV light. An attempt was also made to detect reaction intermediates by a matrix isolation technique using concentrated LiCl solution a t 77 K; no reactions took place in the matrix, however. Figure 2 shows the variations of the quantum yields for 257- and 313-nm irradiation of aqueous p-soc solutions as a function of pH. The absorption spectrum of p-soc is invariant over the pH range 1-12. Thermal decomposition of p-soc is negligible under the present experimental conditions for the quantum yield measurements. The pH dependence of the quantum yield for 257-nm irradiation is very small, in contrast to that for 313-nm irradiation, as shown in Figure 2. In the latter case, the quantum yield of about 0.07 at pH > 4 abruptly increases to about 0.12 at pH < 2. This result implies that different primary photoreactions take place between 257- and 313-nm irradiation. Photochemical decomposition of p-soc in 0.1 M aqueous HC104solution is known to produce hydrogen p e r ~ x i d e . ~ W e considered that superoxide ions would be a precursor of hydrogen peroxide. In order to detect the superoxide ion production, a series of experiments were carried out with the aqueous p-soc solutions containing Cyt(III), since
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4
6
40
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8
1
0
!
2
1
4
PH
Figure 2. Quantum yields for photodecomposition of p-soc as a function of pH: (- -) 257- and (-) 313-nm irradiation. 1.01
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WAVELENGTH i nm
Figwe 3. Absorptlon spectra of aqueous solutions of (A) 6.2 X 10"' M p-soc, (6)4.1 X lo-' M Cyt(III),and (C)a mlxture of 6.2 X lo-' M p-SOC and 4.1 X 10-5 M Cyt(I I I).
superoxide ions are known to reduce Cyt(II1) in aqueous solution.12 Figure 3 shows the absorption spectra of p-soc, Cyt(III), and their mixture in aqueous solutions at pH 7. The absorption spectrum of the mixture is evidently the sum of each component, indicating no formation of molecular complexes. Figure 4 shows the spectral change observed on 257-nm irradiation for the 6.2 X lo4 M aerated aqueous solution of p-soc containing 4.1 X M Cyt(II1) at pH 7. With decreasing absorbance around 480 nm, new absorption peaks appear at 520 and 550 nm, which are ascribed to Cyt(II) formed by the reduction of Cyt(III) with superoxide ions. The relative initial yield of Cyt(I1) formed to that of p-soc decomposed was determined to be 0.7. Similar results were obtained for 313- and 485-nm irradiation. In the photolysis of the aerated aqueous solution containing only Cyt(II1) at 4.1 X M with 257-nm light, Cyt(II1) was found to decompose mostly; the quantum yield for Cyt(I1) formation is about times as low as that obtained with the solution containing p-SOC. (12) Butler, J.; Jayson, G . G.; Swallow, A. J. Biochim. Biophys. Acta 1976,418, 215.
The Journal of Physical Chemlstty, Vol. 88, No. 2, 1982 223
Photochemical Gieneration of Superoxide Ions
and for 313-nm irradiation hu
[(CN),CWO~-CO(CN)~]& + H2O [CO(CN)6H2OI2-+ [CO(CN)5 0 2 13- (3) +
-
followed by reaction 4, which was studied by McLendon 2[Co(CN)502I3-
[(CN)6Co-02-CO(CN)6]"
+ 02-
(4)
et aL7 Since the superoxide ions produced by reactions 2 and 4 form oxygen and hydrogen peroxide, the stoichiometry is in accord with reaction 1. The characteristic pH dependence of the quantum yields for 313-nm irradiation is explained in terms of reactions 5 and 6. Re-
I
0.21
2H02 0
500
600 WAVELENGTH ( n m l
Figuro 4. Spectral changes observed for a 6.2 X lo4 M aqueous solution of p-soc mtalnlng 4.1 X M Cyt(II1) for 257nm irradlatkn: (A) betore hedlatkn, (B) afbr 2mln ltwdation, (C)after 4-min irradiation, (D) after &mln Irradiation, (E)after &mln lrradlatlon. Absorbed light Intensity: 7.5 X lo-' M s-'.
Nitro blue tetrazolium (NBT2+)is also commonly used as a detection reagent for superoxide ions.lS However, a mixed aqueous solution of NBT2+and p-soc became turbid owing to the formation of hardly soluble microcrystallines, and NTB2+was not useful for detection of superoxide ions in the present system.
Discussion The overall photochemical reaction of p-soc in aqueous solution is given by Valentine and Valentine, Jr.? as
actions 5 and 6 predict that the quantum yields at low pHs are twice as high as those at higher pHs. This is the case as seen from Figure 2. On the other hand, no pH dependence is expected for reaction 2 assumed for 257-nm irradiation. The quantum yields obtained for excitation of transition bands show a pH the d-d and d?r ?r*,(02-) dependence similar to that for 313-nm excitation. The primary photoreactions may also be expressed by reactions 3 and 4 for these bands. The fact that no photochemical reaction of p-SOCwas observed to occur in the aqueous LiCl solution at 77 K indicates that the Co-0 bond cleavage is completely suppressed in such a rigid matrix. The photochemical reduction of Cyt(II1) observed for the aerated aqueous solutions at pH 7 on 257-, 313-, and 488-nm irradiation indicates that superoxide ions are produced by photochemical decomposition of p-SOC, irrespective of the excitation wavelength. Cyt(II1) is reduced by superoxide ions:
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hu
~ [ ( C N ) ~ C ~ - ~ ~ - C O+( C 2H+ N )+~ 4H20 ]~ 4[Co(CN)5H20I2-+ HzOz + 02 (1) The present study revealed that the quantum yields for the photodecomposition of p-soc in neutral aqueous solutions are strongly dependent on the electronic band excited. The yield obtained for excitation of the ?r*h(Oz-) d+? transition band is 3 times as high as that for exci& band. This result suggesta that tation of the +h(o,-) each electronic absorption band causes a different mode of photolysis. The pH dependence of the quantum yields observed for 332-nm excitation is also in contxast with that for 257-nm excitation, as shown in Figure 2. On the basis of these results and the overall reaction (reaction l),the following two different primary processes for the photodecomposition of psoc can be assumed. For 257-nm irradiation
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hv
[(CN)~CWO~CO(CN)S]" + 2Hz0 -C 2[Co(CN)6H20I2-+ 02- (2) (13) Bielski, B. H.J.; Shiue, G.G.;Bajuk, S.J. Phys. Chem. 1980,84, 830.
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[CO(CN)6H2012-+ HO2 (5) H2Oz + 0 2 (6)
[C0(CN),O2l3- + H+ + H2O
Cyt(II1)
+ 0 2 - k',Cyt(II1) + 0 2
(7)
Superoxide ions also decay as
20,
+ 2H20 A H202+ O2 + 20H-
(8)
The rate constants, k7 and kg,were determined to be 1.4 X lo6 (ref 12), and 1.7 X lo7-5.0 X lo7 M-' 5-l (ref 14), respectively, in aqueous solutions at pH 7. The steadystate concentration of the superoxide ions, [02-],,, was estimated for 257-nm irradiation to be 2.4 X
[email protected] X l@ M from the light intensity absorbed by p-SOC(7.5 X lo-' M-' s-'), the quantum yield of photodecomposition (0.20 f 0.01), the concentration of Cyt(II1) (4.46 X loa M), and is considerably the rate constants, k7 and ks. Since [02-],, lower than [Cyt(III)], reaction 7 is probably a principal pathway for the decay of superoxide ions in the present system. This argument is consistent with the fact that the relative yield of Cyt(I1) produced to that of p-soc photodecomposed is approximately unity (0.7). (14)Farhntaziz; Ross; A. B. Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) 1977, No. 59.
