Febs Letters Volume 83, Number 1-2./Volume 84, Number 1-2. [antikvár]

Volume 83, number 1 FEBS LETTERS November 1977 Discussion Letter MECHANISM OF REACTIONS INVOLVING SINGLET OXYGEN AND THE SUPEROXIDE ANION W. H. KOPPENOL and J. BUTLER* Department of Molecular Biophysics, Physics Laboratory, University of Utrecht, Sorbonnelaan 4, Utrecht, The Netherlands and Christie Hospital and Hoh Radium Institute, Paterson Laboratories, Manchester M20 9BX, England Received 25 August 1977 1. Introduction Recently it was shown [1] on thermodynamic grounds that singlet oxygen could be a product of the following redox reactions: Oî + Oi + 2ir Oi + H2O2 + H* 0," + OH' + H* H2O2 + H2O2 'Ag02 + H2O2 'A,02 + HjO + OH- 'EtOj+HjO '2^02+2H20 Since superoxide anions and hydrogen peroxide are formed in living systems and since singlet oxygen as well as the hydroxyl radical are considered to be harmful species, it is important to know in which reactions the formation or involvement of these species is kinetically feasible. For instance, does the dismutation of Oi yield 'A^ or '2g02? In two recent papers by Khan [2,3] it was calculated that 'Zg02 would be formed if Oi were surrounded by six or more water molecules. It is obvious that the calculations of Khan require the strict control of many parameters; therefore we maintain that the formation of '2*02 as a product of the dismutation reaction is most improbable [1]. To decide whether oxygen will be formed in its 'A^ or ^Sg state we must know the relative rates of the respective reactions. •Present address: Hahn-Meitner-Institut für Kernforschung Berlin GmbH, Sektor Strahlenchemie, 1 Berlin 39, Germany 2. Discussion According to the well-known Arrhenius equation the rate constant, k, of a reaction is determined by (i)^, a frequency factor, which is influenced by effects such as diffusion and electrostatic interaction, as well as the entropy of the activated complex with respect to the reactants, and (ii) activation energy, which can be related to the height of the energy barrier. For an electron transfer reaction it is necessary that during the transition state overlap should occur between the electron donating and the electron accepting orbital [4]. This is possible if they have the correct topology and symmetry. These effects are not considered in the Marcus theory [5]. If such an overlap does occur, making mixing in of higher states unnecessary, we expect the reaction to have a small activation energy and a high rate constant. Oi has the electron configuration: (Og 1 s)^« 1 s)^(ag2s)^ «2s)Hag2p)^ (7r„2p)''(7r|2p)^(a*2p)° In Oi one of the two 7r| orbitals is filled and the other one contains one electron. It is the latter orbital which can easily overlap with a half-filled n* orbital of HO2 (figs. la-c). After electron transfer, an oxygen molecule will be formed which has an empty and a filled ?ri orbital; 'A O2 (figs. 2a-c). A mechanism North-Holland Publishing Company - Amsterdam