Electron Transfer Proteins as Electronic Conductors: Significance of the Metal and Its Binding Site in the Blue Cu Protein, Azurin

Abstract

Electron transfer (ET) proteins are biomolecules with specific functions, selected by evolution. As such they are attractive candidates for use in potential bioelectronic devices. The blue copper protein azurin (Az) is one of the most‐studied ET proteins. Traditional spectroscopic, electrochemical, and kinetic methods employed for studying ET to/from the protein's Cu ion have been complemented more recently by studies of electrical conduction through a monolayer of Az in the solid‐state, sandwiched between electrodes. As the latter type of measurement does not require involvement of a redox process, it also allows monitoring electronic transport (ETp) via redox‐inactive Az‐derivatives. Here, results of macroscopic ETp via redox‐active and ‐inactive Az derivatives, i.e., Cu(II) and Cu(I)‐Az, apo‐Az, Co(II)‐Az, Ni(II)‐Az, and Zn(II)‐Az are reported and compared. It is found that earlier reported temperature independence of ETp via Cu(II)‐Az (from 20 K until denaturation) is unique, as ETp via all other derivatives is thermally activated at temperatures >≈200 K. Conduction via Cu(I)‐Az shows unexpected temperature dependence >≈200 K, with currents decreasing at positive and increasing at negative bias. Taking all the data together we find a clear compensation effect of Az conduction around the Az denaturation temperature. This compensation can be understood by viewing the Az binding site as an electron trap, unless occupied by Cu(II), as in the native protein, with conduction of the native protein setting the upper transport efficiency limit.

Protein Electronic Conductors: Hemin–Substrate Bonding Dictates Transport Mechanism and Efficiency across Myoglobin

“…[4,31,32] This type of ETp suggests (coherent) tunneling up to room temperature.Azisoriented in relation to the surface via the covalent bonds between its cysteine thiolate residues and the surface.W eh ypothesize that its temperature-independent ETp is due to the existence,i na ddition to the covalent bond between the protein and one electrode,o f strong electronic coupling between the other end of azurin and the second electrode.That latter end contains the Cu ion with its first coordination shell being less than 5away from its respective electrode.Insuch aconfiguration, the electrode contact may well be more efficient than otherwise. [4,31,32] This type of ETp suggests (coherent) tunneling up to room temperature.Azisoriented in relation to the surface via the covalent bonds between its cysteine thiolate residues and the surface.W eh ypothesize that its temperature-independent ETp is due to the existence,i na ddition to the covalent bond between the protein and one electrode,o f strong electronic coupling between the other end of azurin and the second electrode.That latter end contains the Cu ion with its first coordination shell being less than 5away from its respective electrode.Insuch aconfiguration, the electrode contact may well be more efficient than otherwise.…”

“…Proteins capable of electron transfer are prime candidates for the study of ETp in solid-state electronic junctions.W e have studied the ETp properties of various proteins,including azurin (Az) [3,4] and cytochrome C(Cyt C). [5] We also explored ETp in human serum albumin (HSA) by doping it with hemin, which made HSA redox-active.…”

“…[7] Its O 2 binding and electron transfer involve its prosthetic group, heme,aconjugated macrocycle with ar edox-active Fe ion. Mb,therefore,offers ameans to evaluate whether the unique behavior, temperature-independent ETp,o bserved via holo-Az, [4] is due to the covalent binding of the protein to one of the electrodes,t oi ts defined orientation to the electrodes or both. Mb,therefore,offers ameans to evaluate whether the unique behavior, temperature-independent ETp,o bserved via holo-Az, [4] is due to the covalent binding of the protein to one of the electrodes,t oi ts defined orientation to the electrodes or both.…”

Protein Electronic Conductors: Hemin–Substrate Bonding Dictates Transport Mechanism and Efficiency across Myoglobin

“…Proteins capable of electron transfer are prime candidates for the study of ETp in solid‐state electronic junctions. We have studied the ETp properties of various proteins, including azurin (Az)3, 4 and cytochrome C (Cyt C) 5. We also explored ETp in human serum albumin (HSA) by doping it with hemin, which made HSA redox‐active 6.…”

“…In this respect, the hemin/heme group resembles the exposed disulfide bond of Az, which can serve as a binding anchor. Mb, therefore, offers a means to evaluate whether the unique behavior, temperature‐independent ETp, observed via holo‐Az,4 is due to the covalent binding of the protein to one of the electrodes, to its defined orientation to the electrodes or both. Moreover, Mb may also be used, at a later stage, to study the effects of chemical reactions on protein ETp by way of ligand binding to Mb.…”