Effect of Conformational Constraints on Gated Electron-Transfer Kinetics. 3. Copper(II/I) Complexes with cis- and trans-Cyclopentanediyl-1,4,8,11-tetrathiacyclotetradecane¹

Abstract

Previous kinetic and electrochemical studies of copper complexes with macrocyclic tetrathiaethers-such as 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4)-have indicated that electron transfer and the accompanying conformational change occur sequentially to give rise to a dual-pathway mechanism. Under appropriate conditions, the conformational change itself may become rate-limiting, a condition known as "gated" electron transfer. We have recently hypothesized that the controlling conformational change involves inversion of two donor atoms, which suggests that "gated" behavior should be affected by appropriate steric constraints. In the current work, two derivatives of [14]aneS4 have been synthesized in which one of the ethylene bridges has been replaced by either cis- or trans-1,2-cyclopentane. The resulting copper systems have been characterized in terms of their Cu(II/I)L potentials, the stabilities of their oxidized and reduced complexes, and their crystal structures. The electron self-exchange rate constants have been determined both by NMR line-broadening and by kinetic measurements of their rates of reduction and oxidation with six or seven counter reagents. All studies have been carried out at 25 degrees C, mu = 0.10 M (NaClO4 and/or Cu(ClO4)2), in aqueous solution. Both Cu(II/I) systems show evidence of a dual-pathway mechanism, and the electron self-exchange rate constants representative of both mechanistic pathways have been determined. The first-order rate constant for gated behavior has also been resolved for the Cu(I)(trans-cyclopentane-[14]aneS4) complex, but only a limiting value can be established for the corresponding cis-cyclopentane system. The rate constants for both systems investigated in this work are compared to values previously determined for the Cu(II/I) systems with the parent [14]aneS4 macrocycle and its derivatives involving phenylene and cis- or trans-cyclohexane substituents. The results are discussed in terms of the influence of the fused rings on the probable conformational changes accompanying the electron-transfer process.

A cytochrome C electron transfer switch modulated by heme ligation and isomerization of a peptidyl‐prolyl bond

“…Under steady‐state conditions, the dependence of k obs,2 on a 6 Ru 2+ concentration is given by Eq. (1),15, 16, 41, 42 where the rate constants are defined in Figure 2. For data at pH 5.5 to 9.5, the slopes of most plots of k obs,2 as a function of a 6 Ru 2+ concentration are near zero (Figure S3 in the Supporting Information).…”

The response of Ω-loop D dynamics to truncation of trimethyllysine 72 of yeast iso-1-cytochrome c depends on the nature of loop deformation

“…Thus, it is possible to fit the dependence of k gET,2 on a 6 Ru 2+ concentration to the full form of Eq. 6, which is derived assuming a steady-state approximation [59-61], so that both rate constants, k b,His and k f,His , associated with the $$k_{\mathrm{gET},2}=\frac{\left(k_{\mathrm{b},\mathrm{His}}{k}_{\mathrm{ET}}\left[a_{6}R{u}^{2+}\right]\right)}{(k_{\mathrm{f},\mathrm{His}}+k_{\mathrm{ET}}\left[a_{6}R{u}^{2+}\right])}$$ conformational transition can be obtained (Fig. S7).…”

Electron Transfer by Copper Centers

“…134 A prime example of such multifold behavior is illustrated in Figure 15 for the oxidation of Cu I (trans-cypt- [14]aneS 4 ) + by Ni III ( [14]aneN 4 ) 3+ for which the three limiting conditions overlap significantly as the oxidant concentration is varied. 135 The initial and final linear portions of this plot represent the limiting k 21 values for pathways A and B, respectively, while the intermediate curved segment represents the onset of first-order "gated" electron-transfer mixing in with the second-order contribution. It is pertinent to note that both linear segments extrapolate through the origin.…”