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A perspective of biological supramolecular electron transfer

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Title A perspective of biological supramolecular electron transfer
 
Creator Ramasarma, T
 
Description 379-397
Electron transfer is an essential activity in
biological systems. The migrating electron originates from water-oxygen in photosynthesis
and reverts to dioxygen in respiration. In this cycle two metal porphyrin
complexes possessing circular conjugated system and macrocyclic π-clouds, chlorophyll
and heme, play a decisive role in mobilising electrons for travel over
biological structures as extraneous electrons. Transport of electrons within
proteins (as in cytochromes) and within DNA (during oxidative damage and
repair) is known to occur. Initial evaluations did not favour formation of semiconducting
pathways of delocalized electrons of the peptide bonds in proteins and of the
bases in nucleic acids.Direct measurement of conductivity of bulk material and
quantum chemical calculations of their polymeric structures also did not support
electron transfer in both proteins and nucleic acids.


   New
experimental approaches have revived interest in the process of charge transfer
through DNA duplex. The fluorescence on photoexcitation of Ru-complex was found
to be  quenched by Rh-complex, when both
were tethered to DNA and intercalated in the base stack. Similar experiments
showed that damage to G-bases and repair of T-T dimers in DNA can occur by possible
long range electron transfer through the base stack. The novelty of this
phenomenon prompted the apt name, "chemistry at a distance".


   Based on
experiments with ruthenium modified proteins, intramolecular electron transfer
in proteins is now proposed to use pathways that include C-C σ-bonds and
surprisingly hydrogen bonds which remained out of favour for a long time. In support
of this, some experimental evidence is now available showing that hydrogen
bond-bridges facilitate transfer of electrons between metal-porphyrin
complexes. By molecular orbital calculations over 20 years ago we found that "delocalization
of an extraneous electron is pronounced when it enters low-lying virtual
orbitals of the electronic  structures of
peptide units linked by hydrogen bonds". This review focuses on
supramolecular electron transfer pathways that can emerge on interlinking by
hydrogen bonds and metal coordination of some unnoticed structures with π-clouds
in proteins and nucleic acids, potentially useful in catalysis and energy
missions.
 
Date 2012-12-31T20:08:14Z
2012-12-31T20:08:14Z
1999-12
 
Type Article
 
Identifier 0975-0959 (Online); 0301-1208 (Print)
http://hdl.handle.net/123456789/15475
 
Language en_US
 
Rights CC Attribution-Noncommercial-No Derivative Works 2.5 India
 
Publisher NISCAIR-CSIR, India
 
Source IJBB Vol.36(6) [December 1999]