First Aparesh Bhattacharya Memorial Lecture, 1996
The Journey Between Oxygen and Water and Other Chemical Adventures

Animesh Chakravorty

[ An alumnus of this college graduated as a student of Chemistry Honours in 1955. Presently INSA Golden Jubilee Research Professor in the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032. ]

Abstract

Two unusual substances

Water and oxygen are symbols of life. Virtually all that live are destined to depend directly or indirectly on these two unusual substances. Water is a highly structured liquid sustained by extensive hydrogen bonding. Oxygen is paramagnetic, a property that acts as a subtle control on its reactivity towards other molecules.

Interconversion

The interconversion of oxygen and water involves the transfer of four electrons and four protons. The forward reaction O2 – 2H2O is energy-releasing and is a primary energy source to life. On the other hand the energy-absorbing reversal of the process is an integral part of photosynthesis. The interconversion reaction forms an endless cycle encompassing the biosphere, atmosphere and hydrosphere. Free oxygen in the atmosphere initially arose via photosynthesis and the present equilibrium level was reached about this time that the great diversification of mammals took place.

Multiple transfer of electrons and protons

The transfer of electrons is usually a unitary process, i.e., it occurs one by one. Ordered transfer of multiple electrons is not easy and required considerable organization as well as mediation (catalysis). One approach is to use a variable-valence metal ion as a mediator and use several of them held in conjunction in an orderly matrix. This strategy has been very successfully utilized by Nature in the form of multimetal electron-transfer or redox enzymes. Two examples will be cited.

A tetra copper oxidase

We first consider ascorbate oxidase. The conversion of oxygen to water required the presence of a substance, often called a substrate that can provide the required electrons and protons (hydrogen atoms). The enzymes called oxidases mediate the electron transfer process. Ascorbate oxidase mediates such transfer between ascorbic acid (vitamin C) and oxygen affording the dehydro form of ascorbic acid and water. The structure of ascorbate oxidase incorporating 553 amino acid residues and four copper atoms has been fully elucidated (X-ray) by R. Huber (Nobel prize). The four copper atoms are bonded to histidine nitrogen and methionine or cysteins sulfur. The copper centres are of three types, 1 (mononuclear), 2 (mononuclear by wealy joined to 3), and 3 (dinuclear). Ascorbic acid is believed to be held by the type 1 centre and oxygen by type 3. Electrons are transferred from ascorbic acid to type 1 to type 2 to type 3 to oxygen. Copper utilizes the oxidation levels Cu(II) and Cu(I). One can marvel at the high level of organization. that is present.

Tetramanganese oxygen evolution

The second example is the oxygen-evolving center (OEC) in photosynthesis. The center is located in what is called photo-system II where water is photo chemically converted to oxygen. The liberated electrons and protons are then utilized to convert carbon dioxide into carbohydrates via photo-system I. The enzymes in OEC has four manganese atoms bonded to ligands with oxygen and nitrogen sites. The detailed structures are not known at present but there are Mn…Mn contacts of 2.7 and 3.3 Å. The manganese oxidation levels are believed to span the range +2 to +4.

Minicing

While attempts to understand the exact nature of OEC is in progress, chemists (including us) have tried to produce a variety of synthetic molecules incorporating four manganese atoms. The purpose is to have systems that would mimic the natural system in one or more properties closely or even remotely. Model studies of the type have been a powerful force in the development of important interdisciplinary areas such as bioinorganic and bioorganic chemistry.

Concluding quotation

There is no question that there is a primitive element in this approach. We quote Einstein, ”… our science measured against reality, is primitive and childlike – and yet it is the most precious thing we have.”