Multiple Mechanisms for Electron Transfer at Metal/Self‐Assembled Monolayer/Room‐Temperature Ionic Liquid Junctions: Dynamical Arrest versus Frictional Control and Non‐Adiabaticity

Abstract

Electrochemical devices consisting of gold electrodes coated by electronically well-behaved self-assembled alkanethiol monolayers of variable thickness, a ferrocene/ferrocenium redox probe and a typical room-temperature ionic liquid (RTIL) [bmim][NTf(2)] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) as a unique reaction medium with an exceptionally broad spectrum of relaxational modes (probed under variable temperature and pressure conditions), have been used to vary the intrinsic electron-transfer (ET) rate constant over eight orders of magnitude (from 0.1 to 3x10(7) s(-1)) by further tuning of the overvoltage. A remarkable interplay of ET mechanisms was observed, which was accompanied by the stepwise drop in the reorganisation free energy of the medium from 1.0 to 0.1 eV. The first mechanistic changeover to the dynamically arrested regime, with a locking ultra-slow relaxation time of approximately 50 micros, occurred at donor-acceptor separations below 20 A. Another mechanistic changeover to the full solvent friction regime, controlled by a medium relaxation process of approximately 100 ns, emerged for ET distances smaller than 8 A.

Electron transfer with myoglobin in free and strongly confined regimes: disclosing diverse mechanistic role of the Fe-coordinated water by temperature- and pressure-assisted voltammetric studies

“…The unusually large positive value DV Exp a for Mb, even under the stabilizing impact of TMAO, compared to analogous processes involving, e.g., CytC [18,19] (table 1) or Azurin (the non-heme redox-active protein) [20,21], may indicate much more large-scale conformational fluctuations that accompany and are closely coupled to the respective ET event in Mb (see below). The value of DV Exp a = +24.5 cm 3 mol −1 found in this work for dynamically controlled ET with Mb resembles surprisingly well with those found for analogous dynamically controlled processes of ET involving model redox couple, ferrocene/ferrocenium ion, occurring within the Au/SAM/RTIL junction (RTIL = room temperature ionic liquid), DV Exp a = +22.6/25.3 cm 3 mol −1 [53,54,71]. The reasons for this resemblance may lie in a similarity of the free volume values in these two kinds of environments (protein versus RTIL, see Ref.…”

“…It was well documented that high-pressure kinetic studies of electron exchange at bare and SAM-modified electrodes involving model metallocomplexes (dissolved in non-aqueous solutions) and redox-active proteins (dissolved in aqueous solutions) provide the most reliable information regarding the intrinsic physical mechanism (non-adiabatic versus frictional) of ET. Specifically, the long-range ET processes associated with non-adiabatic mechanistic regime, equation (12), exhibited negative volume of activation, whereas those occurring in the frictional (dynamically controlled) regime, equation (13), displayed positive values [17][18][19][20][21][22]70] (for similar manifestations in nonbiological electrode processes, see [5,53,54,71,77]). In general, this interplay rigorously correlates with the biphasic character for the logarithmic dependence of the standard rate constant of electron exchange on the number of methylene units, n, within the alkanethiol chain, HS-(CH 2 ) n -ω, constituting SAM, where ω is the SAM's terminal group (here: -OH and/or -COOH), see figure 4.…”

“…Alternatively, for the same purpose, the total CV (background subtracted) curve simulation procedure can be applied [57]. We refer to our previous work [19,54] for numerous details regarding the T.D. Dolidze et al application of these procedures.…”

Fundamental signatures of short- and long-range electron transfer for the blue copper protein azurin at Au/SAM junctions

“…3, 4), the relaxation component ν eff is a thermally activated process as well. Hence, in this case, the value of ΔG aðEXPÞ (or ΔH aðEXPÞ , as its constituent part) should contain an additional component, namely, ΔG aðηÞ (or, respectively ΔH aðηÞ ), related to the protein friction (24,27,28,34,41,53). For short-range ET (n ¼ 4), the value of ΔH aðEXPÞ is twice that of the long-range (n ¼ 15) ET; see Table 1.…”

Temperature-Driven Changeover in the Electron-Transfer Mechanism of a Thermophilic Plastocyanin

“…The first assumes that, as a consequence of the increased electronic coupling at short protein-electrode distances, the ET rate is controlled by a frictional mechanism that involves the protein and its surrounding medium and takes the reactant to the top of the activation barrier. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] The second introduces a preceding barrier-crossing event (gating step), which may involve a conformational fluctuation 32 or a protein reorientation, 33 to optimize the rate of electron transfer. [32][33][34][35][36][37][38][39] Irrespective of the precise nature of the controlling event in the distance-independent kinetic regime, the transition between the two kinetic regimes has been demonstrated by varying systematically the strength of the electronic coupling between electrode and protein by using molecular spacers of variable composition and length.…”