Electronic Transport via Proteins (original) (raw)

option in mesoscopic devices in terms of functionality, fabrication and stability. In principle the properties of molecules can be tailored to produce functions comparable to those of traditional macroscopic circuit components. For instance, electrical spacers and wires can be formed from saturated and conjugated molecules, respectively. Simple molecular diodes, [ 1 ] switches [ 2 ] and transistors [ 3 ] have been built using functionalized conjugated molecules. However, going beyond simple molecular systems to ones exhibiting multiple functionalities, such as those with molecular selectivity and long-term stability, entails overcoming formidable obstacles. Proteins present a family of molecules that includes attractive candidates for integration in multi-functional molecular devices. [ 4 ] They are the result of evolutionary routes of diversity that culminated in fully functional entities that can be viewed as "nanometer-scale machines" or "nanoparticles" for biochemical processes. However, notwithstanding the fact that their redox-active properties are well known, they are not normally viewed as candidates for molecular electronics, in marked contrast to DNA. [ 5 ] Many experimental and theoretical studies have been conducted to understand biological electron transfer (ET). Typically, an intramolecular electron transfer rate constant (k ET) is measured between electronically localized donor (D) and acceptor (A) states, which could either be the naturally occurring sites or ones purposefully implanted by means of (bio-)chemical modifi cation of the protein. The majority of k ET data for proteins has been obtained using the spectroscopic method of fl ash-quench (cf., ref. [ 6 ]). By measuring ET rate constants between protein redox centers and various locations in the protein, Gray, Wink ler and co-workers could show experimentally the relation between the natural logarithm of k ET and the D-A distance, L , k ET ∝ exp(− βL). In this way they were able to calculate distance decay constants, β , from the slope of the ln(k ET) vs L plot. Typical β values for redox-active proteins range from 1.0 Å −1 to 1.4 Å −1 for ET along β-strands and α-helices, respectively. [ 6a , 7 ] Another approach to study ET processes within protein is pulseradiolysis. This is accomplished by monitoring the ET process between redox centers of the protein, e.g., between an intramolecular disulfi de radical and the Cu(II) center in azurin. [ 8 ] An additional methodology is electrochemistry, where the ET