Unravelling single metalloprotein electron transfer by scanning probe techniques

Abstract

This review is intended to account for the experimental and theoretical achievements obtained in a period of about 15 years on the investigation of the electron transport through single redox metalloproteins by scanning probe techniques. A highly focussed research effort has been deployed by the scientists active in this particular field towards measuring and interpreting electronic current signals flowing via blue copper, redox metalloproteins (e.g. azurin). The field has taken a remarkable advantage of the use of electrochemically assisted scanning tunnelling microscope (EC-STM) which has allowed to probe single molecule signals under full control of all the potential values involved in the experiments. This experimental activity has both triggered more comprehensive theoretical interpretations and has been, in its turn, stimulated by theoreticians to test always new predictions. The authors hope to have succeeded in providing the reader with a valuable appraisal of this fascinating field.

Electric double layer effect on observable characteristics of the tunnel current through a bridged electrochemical contact

“…Figures 8 and 9 show that the experimental data of Ref. 33 are reproduced rather well with respect to the width of the current/overpotential maximum. It was assumed that the Debye screening is strong so that ␥ Ϸ 0 and Ϸ 1.…”

“…32 The tunneling current through a single azurin molecule attached to a gold electrode also showed an approximately linear dependence of the maximum position on the bias voltage with the slope approximately equal to −0.5. 33 This would correspond to a redox center located symmetrically within the tunneling gap ͑S =1/2 in the terms of the present report͒ or to another location in the case of strong screening ͑S Ϸ 1/2͒. In spite of the scatter of the data one may make some observations concerning the width and the height of the maximum.…”

“…These results were fitted in Ref. 33 the use of the approximate equations ͑20͒ ͑with a =0͒ within the sequential stepwise mechanism. The electronic coupling matrix element was also used as a fitting parameter.…”

“…20 This expectation has been supported by a number of recent experimental studies. [31][32][33][34][35][36][37][38] Not only the maximum itself has been clearly observed but the dependence of maximum current and the position of the maximum on the bias voltage has also been studied 32,33 and analyzed. 32 Closer examination of Tao's data shows that the current maximum for this system is also close to the equilibrium potential.…”

Insulator-protected mechanically controlled break junctions for measuring single-molecule conductance in aqueous environments

“…The field of molecular electronics provides a versatile testbed for studying the electronic properties of single molecules, including quantum interference effects, 1-3 tuning of charge transport by electrochemistry, [4][5][6][7][8] interactions with environmental variables such as pH, 9,10 solvent induced effects, [11][12][13][14] and charge transport in biological moieties. [15][16][17][18][19] Over the past decade, such charge transport measurements through single molecules have been conducted with metallic break junctions formed by means of scanning probe or nanolithography techniques; with scanning tunnelling microscopy being an example for the former and mechanically controlled break junctions (MCBJ) for the latter. Both break junction techniques can be operated at room temperature, although research on the role of environmental variables such as solvent interactions and electrochemical gating has not received the same level of attention for the MCBJ approach [20][21][22][23][24] compared to the STM method.…”