A perspective of biological supramolecular electron transfer

Electron transfer is an essential activity in biological systems. The migra ting electron originates from water-oxygen in photosynthesis and reverts to dioxygen in respiration. In this cycle two metal porphyrin complexes posse ssing circular conjugated system and macrocyclic pi-clouds, chlorophyll and hems, play a decisive role in mobilising electrons for travel over biologi cal structures as extraneous electrons. Transport of electrons within prote ins (as in cytochromes) and within DNA (during oxidative damage and repair) is known to occur. Initial evaluations did not favour formation of semicon ducting pathways of delocalized electrons of the peptide bonds in proteins and of the bases in nucleic acids. Direct measurement of conductivity of bu lk 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-comp lex 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 p rompted the apt name, chemistry at a distance. Based on experiments with ruthenium modified proteins, intramolecular elect ron transfer in proteins is now proposed to use pathways that include C-C s igma-bonds and surprisingly hydrogen bonds which remained out of favour for a long time. In support of this, some experimental evidence is now availab le showing that hydrogen bond-bridges facilitate transfer of electrons betw een metal-porphyrin complexes. By molecular orbital calculations over 20 ye ars ago. we found that "delocalization of an extraneous electron is pronoun ced when it enters low-lying virtual orbitals of the electronic structures of peptide units linked by hydrogen bonds". This review focuses on supramol ecular electron transfer pathways that can emerge on interlinking by hydrog en bonds and metal coordination of some unnoticed structures with pi-clouds in proteins and nucleic acids, potentially useful in catalysis and energy missions.