Department of Chemistry, University of Maryland Baltimore County, ... drives this value up to about 0.7 V, which is now very close to the upper limit which ..... 3 P. George in Oxidases and Related Redox Systems, T.E. King, H.S. Mason and M.
Bioelectrochemistv and Bioenergetics, 18 (1987) 3-11 A section of J. Electroanal. Chem., and constituting Vol. 232 (1987) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
995 - THERMODYNAMICS OF REACTIONS AND HYDROGEN PEROXIDE *
W.H. KOPPENOL Department
l
INVOLVING
OXYRADICALS
*
of Chemistry,
University of Maryland Baltimore County, Catonsville,
MD 21228 (U.S.A.)
SUMMARY
Gibbs energy changes (at pH 7) are calculated for reactions of 0- and H-containing species (H,O,, 0; and OH’) with metal complexes and metalloproteins. Catalysis of superoxide dismutation is possible by any metal complex for which -0.33 V< Va’ (MC”+‘)+ L/M”+ L) < 0.94 V. Reduction of a metal complex by superoxide or ascorbate and subsequent oxidation by hydrogen peroxide to form a hydroxyl radical requires 0.02 V < U,,’ (M(“+‘)+ L/M”+ L) < 0.32 V. Thus, a metal complex that is required to catalyze the dismutation reaction but not the reduction of hydrogen peroxide should have a reduction ) in excess of 0.32 V. However, correction for non-standard conditions potential Ua’ (M(“+‘)+/M”+ drives this value up to about 0.7 V, which is now very close to the upper limit which allows superoxide dismutase catalysis. The reaction of a reduced transition metal complex with hydrogen peroxide might not result in the hydroxyl radical, but in a higher oxidation state of the metal. An upper limit of approx. 1.3 V is derived for the couple M(“+*)+L/M”+L, in which M(“+2)fL represents a compound as indicated, a M = On+ L, or a M”+ H202L complex.
INTRODUCTION
Dioxygen was introduced into the atmosphere of the earth most probably by photosynthetic algae. A partial oxygen pressure of 2 X lop3 atm (the Pasteur point) was reached approx. 2.65 billion years before present time [l]. With it came the dangers of radical reactions, which are at the heart of oxygen toxicity. It can be shown that if life were based on reactions involving sulphate, carbonate or nitrate, as is the case for some organisms, one-electron transfer reactions are very unfavourable and that any radicals would be scavenged by the products of anaerobic
* Presented at the Bioelectrochemical Society Meeting on “Formation and Reactions of Peroxides in Biological Systems”, Obemai, 22-24 October 1986. l * Present address: Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
0302-4598/87/$03.50
0 1987 Elsevier Sequoia
S.A.
4
metabolism [2]. Thus, chain reactions that amplify damage are not possible under those conditions. It would seem that harmful reactions of oxyradicals are a trade-off with the large amount of energy that is available through the four-electron reduction of dioxygen [3]. Reactive radicals initiate lipid peroxidation and are formed during phagocytosis, the metabolism of toxic chemicals and microsomal oxidation [4-71. They are implicated in pathological conditions such as rheumatoid arthritis [8] and reperfusion injury [9]. Damage can often be prevented by superoxide dismutase, catalase, removal of iron by desferrioxamine, or by compounds which react with the hydroxyl radical and form stable products [4]. It is therefore widely accepted that a reduced metal ion reacts with hydrogen peroxide to form the reactive hydroxyl radical [lo]. The superoxide anion can serve as a reducing agent for the metal and thereby mobilize it, as is the case for iron [ll], or act as a precursor of hydrogen peroxide. Superoxide is formed in small amounts by mitochondria [12] and possibly from oxyhaemoglobin and oxymyoglobin [2]. Organisms exposed to dioxygen use the enzymes superoxide dismutase, catalase, glutathione peroxidase and cytochrome c peroxidase as a first-line defense against oxidative damage [13,14]. However, there is no enzyme to scavenge the hydroxyl radical since its reactions with proteins and many other molecules are diffusion controlled. Recently, it has been suggested that higher oxidation states of metals, rather than hydroxyl radicals, might be responsible for damage at neutral pH [15-181. Fe02+, or ferryl, as an alternative to the hydroxyl radical was first proposed by Bray and Gorin [19]. Evidence for this species in acetonitrile was obtained by Groves and coworkers [20]. Ferry1 porphyrins are formed during the catalytic cycles of catalase and peroxidases [14]. Recently, a higher oxidation state of iron has been identified in the reaction of hydrogen peroxide with a water soluble ferric porphyrin [21]. Hypervalent iron complexed by OH- has an appreciable lifetime in aqueous solution [22]. If such a compound is formed, it could be expected to be more selective in its reactions than the hydroxyl radical. It has been estimated that the reduction potential of a ferryl/ferric complex is about 1 V, 1.3 V lower than the couple OH’/H,O at pH 7 [23]. Standard Gibbs energy changes can be calculated for a variety of reactions now that reliable Gibbs energies of formation and reduction potentials are available. When a particular thermodynamic quantity is unknown useful upper or lower limits can often be derived. THERMODYNAMIC
PARAMETERS
Standard reduction potentials, U,, refer to 1 molal concentrations or a pressure of one atmosphere (101.3 kPa) if a gas is involved, at 25 o C. Values of all potentials are with respect to the Normal Hydrogen Electrode. Table 1 lists the thermodynamic data used for the calculation of reduction potentials of oxygen and hydrogen-containing species (see Ref. 2 and literature cited therein). Reduction potentials are shown in an oxidation state diagram for oxygen. Fig. 1 [24]. This
5
TABLE
1
Gibbs energies of formation and solution, 25’ C and 1 atm (101.3kPa) [2] Species
State
AGo.1 kcal/mol
0,
g ao g g
0 3.9 22.6 37 39.1 41.6 55.4 18.5 6.4 1.2 7.6 -25.3 - 33.1 - 17.1 8.2 6.2 22.4 - 54.7 - 56.7 - 59.1 - 37.6
(3xg-) (‘Ag) (‘Xg’)
03
g
0 0;. HO;
g aq g aq aq g aq aq
aq
0;. H,Oz HO; OH’
g aq
0-’ Hz0
aq g hq aq ’ aq
OH-
pK,‘s
and electron
AG;,, (PH 7) kcal/mol
affinities
AGO,, a kcal/mol
pH7 Pop= tam
species
at
Electron affinity V
3.9 1.42 2.04 2.14
41.6
2.5 1.46
18.5
-74
10.7 7.6
-5.2 -85
1.19
- 14.1 -7.6
- 7.80 b -98
4.7
11.7 1.83
15.7 19.4
-2.0 -102
- 37.6 -28.1
11.9
14.0 -4.4 - 106 [25]. of - 6.6 kcal reflects our more
23
.O‘ OH’
r = 25oc
PK,
oxygen
0.44 3.9
a AGO,, for ionic species are with respect to AGo,,(H+ ) - 102.5 kcal/mol b The difference with the accepted Gibbs energy of the solution of H,O, negative value of AGO,, of H,O, (as). ’ HsO,, refers to a hypothetical 1 M solution of water in water.
1
of various
.‘G OS.,
i
/
N +.0.6
