748
J. Phys. Chem. 1990, 94, 748-752
Oxidation Potential of Luminol. Is the Autoxidation of Singlet Organic Molecules an Outer-Sphere Electron Transfer? G. Merinyi,* J . Lind, X. Shen, and T. E. Eriksen Departments of Physical and Nuclear Chemistry, The RojJal Institute of Technology, S - 10044, Stockholm, Sweden (Receiued: June 7, 1989)
The one-electron reduction potential of the LH'/LH- couple was determined by pulse radiolysis to be 0.87 & 0.02 V vs NHE. The rate constant for the electron-transfer reaction between LH' and 02'-(k+,)was measured to be 3 X lo9 M-' s-l while (kT7)was estimated to be less than lo* M-I s-'. In combination with previously the corresponding rate between Lo- and 02'measured data the above values enabled us to quantify the kinetics and thermodynamics of luminol autoxidation. The data were also analyzed in terms of the Marcus-Hush theory. For comparison, a similar analysis was made on the dihydroflavin mononucleotide system (FMNH-). It was concluded that the autoxidation of both LH- and FMNH- and probably of most In contrast, the autoxidation singlet organic anions is initiated by an outer-sphere electron transfer between the anion and 302. of L'-. FMN'-, and other semiquinone radicals is suggested to involve inner-sphere electron transfer.
Introduction Bond formation between singlet ground-state organic molecules and triplet molecular oxygen is spin-forbidden. Autoxidation of such molecules has therefore been assumed to initiate from a one-electron transfer between the molecule and 02.While theoretically compelling this assumption has rarely been substantiated. Among the few studies addressing this issue we find the early works by Russell et al.' where the autoxidation of certain organic anions in dimethyl sulfoxide (DMSO) was shown to result in radical formation. I n aqueous solutions Bruice et a1.2 have provided evidence for electron transfer between 1,5-dihydroflavins and molecular oxygen. I n the literature on the autoxidation of dihydroflavins there are sufficient thermodynamic and kinetic data2-6 extant to warrant quantitative analysis in terms of electron-transfer theory. A similar analysis of luminol (LH-) requires knowledge of the redox potential of the LH'/LH- couple which will be determined in this work. Additional information needed is the rate of luminol autoxidation. While in DMSO oxygen reacts with the luminol dianion7 (L2-) with a rate of ca. SO M-' s-I autoxidation rates in water are too low to be measurable at any pH. For example, even at pH 14, air-saturated luminol solutions are indefinitely stable in the dark. In the present work we shall measure or estimate the rate of the assumed reverse reaction, Le., electron transfer between the luminol semiquinone radical (LH', Lo-) and 02"-. Previous analysis* of data on electron-transfer reactions involving redox couples (mostly inorganic metal complexes) and 0,/02'have resulted in an enormous spread of the apparent couple. Recently, we self-exchange rate ( k e J of the 02/02'determined directly9 k e x ( 0 2 / 0 2 *to - ) be 450 f 160 M-' SKI. In the present work the aqueous autoxidation will be analyzed of both LH- and FMVH- (1.5-dihydroflavin mononucleotide) as well as ( I ) (a) Russell, G. A.; Jansen, E. G.; Becher, H. D.; Smentowsky, F. J. J . Am. Chem. Sot. 1962, 84, 2652. (b) Russell, G. A,; Moye, A. J.; Nagpal, K . J . Am. Chem. Sot. 1962, 84. 4154. ( 2 ) Eberlein. G.;Bruice, T. C. J . Am. Chem. Sor. 1983. 104, 6685. (3) Favaudon, V . E u r . J . Biochem. 1977, 78, 293. (4) Draper, R. D.; Ingraham. L. L. Arch. Biochem. Biophys. 1968. 125,
802.
( 5 ) Anderson, R. F. Biochim. Biophys. Acta 1983. 722, 158. (6) Lind, J.; Merinyi, G. Photochem. Phofobio!., in press.
(7) (a) White, E. H.; Zafiriou, D.; Kagi, H. Hh.; Hill, J . H . M. J . Am. Chem. SOC.1963,86, 940. (b) Gorsuch, J. D.; Hercules, D. M . Photochem. Photobiol. 1972, I S , 567. (c) Seliger, H. H. In Liquid Scintillation Counting, Volume 2; Peng. C . T., Horrocks, D. L.. Alpen. E. L., Eds.: Academic Press: New York, 1980; p 296. (8) McDowell, M. S.: Espenson. J . H.; Bakac, A . Inorg. Chem. 1984. 23? 2232. in
( 9 ) Lind. J . ; Shen, X.; MerOnSi, G . ; Jonsson. B-0. J . Am. Chem. Sor.. press
0022-3654/90/2094-0748$02.S0/0
l.H
I
0 LO2
u7
P" 2
of the corresponding semiquinones L'- and FMN'-.
Experimental Section Sodium chlorite (Alpha) and luminol (Ega Chemie) were recrystallized prior to use. Phenol, 4-iodophenol (both Aldrich), KSCN, NaN,, and the various buffers (all Merck) were employed as received. Water was triple distilled in quartz. Before irradiation the solutions were purged with N 2 0 gas (Aga). Pulse radiolysis was carried out at room temperature with a 7-MeV microtron accelerator. Details of the setup'O and the computerized optical detection system1' have been described elsewhere. The length of the applied pulses was between IO-' and 2 X lO-'s corresponding to doses of 1-2 Gy. The concentration of radicals generated in such pulses was in the vicinity of mol/dm3. Dosimetry was performed by means of aerated aqueous solutions containing 10 m M KSCN. A Gt value of 2.2 X mZ/J for the (SCN),'radical at 500 nm was employed.12 Results The Redox Potential for the L H ' I L K Couple. Attempts have been made with electrochemical methods to determine the one electron oxidation potential of lumin01.l~ As the authors point ( I O ) Rosander, S . Thesis, The Royal Institute of Technology, Stockholm, TRITAEEP-74-16, 1914; p 28. ( 1 1 ) Eriksen, T. E.; Lind, J.; Reitberger, T. Chem. Scr. 1976, 10, 5 . ( 1 2 ) Fielden, E. M. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis; Baxendale, J. H., Busi, F., Eds.; Reidel: Dordrecht, Holland, 1982: Nata Advanced Study Institutes Series; pp 49-62. ( 1 3) Epstein, B.; Kuwana, T. Photochem. Photobiol. 1965, 4, 1 157.
0 1990 American Chemical Society
Oxidation Potential of Luminol
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 149
TABLE I: Pertinent Acid-Base and Redox Equilibria proton equilibrium
+
LH2 + LHHt LHL2- H+ LH' = L'Ht L02H2 LO2H- + H t L 0 2 H - + L022- + H + H202 + HO2- Ht 022- H+ H02H0; + 02*- Ht
+ +
+ + +
PK.
ref
6.7 15.1 7.7 10.4 -16 11.7 15.9 4.69
14
V vs N H E 0.24 -0.16 0.89 0.936 0.80 f 0.02
redox couple L/ L'0 2 / 0 2 ' - ( 1 M 02) 02*-/H202 (PH 7)
EO,
c102*/c102PhO'/PhO-
15 16 17 18 19 20 ref 21 20 20 22 23
-
1 0
out, the measured potential (ca. 0.67 V at p H 13) is not related
to the thermodynamic potential due to irreversible electrode reactions. In the present work this problem is overcome by having the reactants equilibrate faster than irreversible processes have time to occur. Determination of Eo(LH' f LH-) against the CIO,f C10,Couple. Table I collects data pertinent to the redox measurements. C102 oxidizes LH- with a moderate ratez4 of ca. IO6 M-' s-l, suggesting that Eo(LH'/LH-) is close to ca. 0.9 V. The pulse radiolytic experiments were performed at pH 8.1 in N20-saturated solutions containing I O mM luminol and 0.5 M NaN, with the NaCIOz concentration varying from 0.1 to 1 M. In such solutions the following reaction sequence (rates in M-' S-I) takes place: eaq-
+ CIOz-
H20,CIOi
C102
+ OCI- + 20H-
-
0
1mo
1
1 OD
Figure 1. The ratio [CIO;]/[LH-] as a function of the inverse transient optical density a t 450 nm. Conditions: [LH-] = 5 X 10-3-10-2 M, [C102] = 0.1-1.2 M, [N,-] = 0.5 M, pH = 8.1, N 2 0 saturation. The applied dose was 1.4 Gy/pulse. The measurements were made after equilibration.
2.5 X IO9 (ref 25)
+ N z O H20 OH' + N, + OH- 9.1 X lo9 (ref 25) OH' + C102 C102' + OH6.6 X lo9 (ref 25) OH' + N,N3* + OH1.2 X 1Olo (ref 25) N,' + CIOz- ClO,' + N31.9 X lo9 (this work) N3' + LHLH' + N,5.4 X lo9 (this work) OH' + LHvarious radicals (ref 25) 8.8 X IO9 eaq-
0 500
-
Figure 2. Ratio between the rate of equilibration and [LH-] as a function of the ratio [CIO
