Direct electron transfer of protein cytochrome

Biosensors based on direct electron transfer of proteins cytochrome c... 

Carbon nanotubes (CNTs), as a new class of nanomaterial, were discovered in 1991 by Iijima [55] and have also been employed in biosensors. Such an application is attributed to their unique electrical properties, which make a redox active close to the surface of proteins, and enable direct electron transfer between proteins and electrode [56]. CNTs are highly conductive (rapid electron transfer) nanomaterials with great promise for applications in biochemical sensing [57-59], Several successful sensors based on CNTs have been reported for the detection of substances, including for NADH [58], glucose [59], cytochrome c [60] and thymine [61]. 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

Although electron transfer as such is not considered as catalysis, most enzymatic redox reactions require the presence of electron-transfer proteins for fast and efficiently directed electron transfer to the active sites. The ferredoxins, azurins, and cytochromes are most well known in this respect. Variations of over 15 A in distance may occur, and as a consequence, the electron-transfer rate may vary over 10 orders of magnitude [35], Exciting developments are ongoing in this field, and are highly relevant for the bioinorganic catalytic subject. 

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