Biocatalysis: Electrochemical Mechanisms of Respiration and Photosynthesis

Vectorial charge transfer and a molecular recognition at the interface between two dielectric media are important stages in many bioelectrochemical processes such as those mediated by energy-transducing membranes [1–4]. Many biochemical redox reactions take place at aqueous medium/membrane interfaces and some of them are multielectron processes. About 90% of the oxygen consumed on Earth is reduced in a four-electron reaction catalyzed by cytochrome c oxidase. Multielectron reactions take place in photosynthesis, which is the most important process on earth [5–10]. Life on Earth began as photosynthesis. Synchronous multielectron reactions may proceed without formation of intermediate radicals, which are highly reactive and can readily enter a side reaction of hydroxylation and destruction of the catalytic complex. Since multielectron reactions do not poison the environment with toxic intermediates and they are ecologically safe, they are used by Nature for biochemical energy conversion in respiration and photosynthesis [11,12]. In the multielectron reaction that takes place in a series of consecutive one-electron stages, the Gibbs energy necessary per single electron transfer obviously cannot be completely uniformly distributed over the stages [7]. The energy needs for various stages will be different and the excess energy in the easier stages will be converted into heat. In a synchronous multielectron reaction the energy will be used very economically [11]. An important parameter in the quantum theory of charge transfer in polar media is the medium reorganization energy Es that determines activation energy [1,12–17]. The energy of medium reorganization in systems with complicated charge distribution was calculated by Kharkats [18]. Reagents and products can be represented by a set of N spherical centers arbitrarily distributed in a polar medium. The charges of each of the reaction centers in the initial and final state are z and z, respectively. Taking Rk to represent co-ordinates of the centers and "i for dielectric constants of the reagents it follows that