biological functions with abiotic building blocks. It is therefore
very important to identify and compare different types of molec-
ular entities with an intrinsic functional relationship. Some
instructive examples of such a possibility, which may become
very useful for biomimetic photochemistry, are given below.
A.1. Two-electron redox relays
In many biocatalytic systems, the conversion of substrate
molecules is accompanied by the transfer of two electrons and
two protons. Several different types of organic ligands are able
to assist such a catalytic exchange of multiple redox equivalents.
They are either directly acting as electron and proton transfer
cofactors or may be coordinated to a redox-active metal site.
Typical examples in biological systems are quinone and
hydrochinone couples which shuttle redox equivalents between
different proteins or porphyrin and hydroporphyrin ligands
acting as catalytic subunits in oxidoreductases.
Frequently, such systems undergo reversible structural
distortions to better control the redox equilibria and to slow
down undesired back reactions. Sometimes, even an irreversible
reaction involving cofactor fragmentation is applied to
completely shift the redox equilibrium into the desired direction.
An important example in this context is the biocatalytic reactiv-
ity ofa-keto acid-dependent iron enzymes( 134 ), where the two-
electron oxidation of ferrous active sites to form iron(IV) oxo spe-
cies is assisted by a decarboxylation process of the coordinated
carbonic acid in the presence of dioxygen. Interestingly, quite
similar strategies to utilize the efficiency of irreversible pro-
cesses and thus to better couple two consecutive one-electron
radical reactions are well established in the field of inorganic
photochemistry. The classical ferrioxalate system for chemical
actinometry based on the irreversible LMCT-induced decomposi-
tion of iron-coordinated oxalato ligands ( 135 ) and the more
recent development of sacrificial two-electron sensitizers for
improving the sensitivity of silver halide photography should be
mentioned in this context ( 136 ).
The employment of robust aromatic ligand architectures able
to introduce the same kind of reactivity in a reversible manner
is of course much more desirable. For this purpose, biological
redox cofactors such as quinones, porphyrins, flavins, and their
functional analogues in biomimetic chemistry contain a built-in
butadiene-type moiety as part of theirp-electron system, which
can be considered as a minimum functional motif for assisting
the reversible uptake of two electrons and two protons. This
262 GÜNTHER KNÖR AND UWE MONKOWIUS