Ions Can Move a Proton‐Coupled Electron‐Transfer Reaction into Tunneling Regime

Abstract

The effects of charged species on proton-coupled electron-transfer (PCET) reaction should be of significance for understanding/application of important chemical and biological PCET systems. Such species can be found in proximity of activated complex in a PCET reaction, although they are not involved in the charge transfer process. Reported here is the first study of the above-mentioned effects. Here, the effects of Na , 5.5 (0.9) (at 0.001m Ca 2þ ), and 7.9 (2.8) (at 0.001m Mg 2þ ) kJ mol À1 ; e) nonlinear proton inventory in reaction. In the H 2 O/dioxane 1 : 1, the observed KIE is 7.8 and 4.4 in the absence and in the presence of 0.1m K þ , respectively, and A H /A D ¼ 0.14 (0.03). The changes when cations are present in the reaction are explained in terms of termolecular encounter complex consisting of redox partners, and the cation where the cation can be found in a near proximity of the reaction-activated complex thus influencing the proton/electron double tunneling event in the PCET process. A molecule of H 2 O is involved in the transition state. The resulting configuration is more rigid and more appropriate for efficient tunneling with Na þ or K þ (extensive tunneling observed), i.e., there is more precise organized H transfer coordinate than in the case of Ca 2þ and Mg 2þ (moderate tunneling observed) in the reaction.Introduction. -Proton-coupled electron-transfer (PCET) reactions are of crucial importance in a wide range of biological, biochemical, and chemical systems [1 -16], including those related to artificial photosynthesis and solar fuels [4 -8], and nanostructures and interfaces [14] [15]. It is recognized now that many processes in biology would not be possible without the coupling of proton and electron motion [3] [5]. Within a unified theoretical framework, both sequential electron transfer, followed by proton transfer (ET/PT) or vice versa (PT/ET), and the concerted transfer of these particles with conspicuous quantum-mechanical character could be viewed as PCET reactions [16]. However, the concerted transfer of a proton and an electron should be of central importance in the issue; here, the single chemical reaction step and

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

“…In the reaction (Scheme 2) performed in the "neat" water moderate kinetic isotope effect kh/kd of 4.6 and values Ah/Ad of 0.97 and ∆∆H ‡ (D,H) of 3.9 kJ/mol have been observed [19,20]. The KIE-s well beyond the semiclassical limits observed in the reaction strongly suggest hydrogen tunnelling in the reaction.…”

“…The kinetic isotope effects observed in almost all ascorbate PCET systems presented here are increased unexpectedly [17][18][19][21][22][23] with regard to the KIE-s observed in particular "neat" water reaction. The only exception is the case of hexacyanoferrate oxidant in more concentrated water solutions of inorganic salts [20] where the observed KIE-s are reduced in magnitude.…”

“…In addition, it has been widely used in the study of enzymatic reactions, many of which are PCET reactions [7,55,[64][65][66]69,70]. The temperature-dependences of KIE in PCET reactions vary from significantly temperature-dependent, to temperature-independent KIE-s [7,19,20,[69][70][71]. This wide span of KIE temperature-dependences, observed in a great number of enzymatic but also in solution PCET reactions are usually interpreted with Marcus like full-tunnelling model (see Section 2.3.)…”

“…The results of the study of KIE-s and tunnelling in the ascorbate reactions [11,[17][18][19][20][21][22][23]72] can contribute to the better understanding of PCET reactions; moreover, many enzymatic reactions involve PCET. The reactions of ascorbate presented are similar to the reactions catalysed by ascorbate peroxidase enzymes [83], and cytochrome b 561 proteins [84], being thus a good potential reference solution reactions which is of importance for the study of enzyme catalysis.…”

“…However, to the best of our knowledge, there are no systematic studies of solvent influences on KIE-s and hydrogen tunnelling in PCET reactions [16]. We have investigated KIE-s and hydrogen tunnelling in the following PCET oxidations of ascorbate: (a) reaction with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO radical) [17,18], (b) reaction with hexacyanoferrate(III) ions [19][20][21][22] and (c) reaction with ferricinium cation [22] in aqueous solutions containing substantial concentrations of organic solvents or salts. Furthermore, KIE-s and hydrogen tunnelling in reaction of ascorbate with hexacyanoferrate(III) ions have been investigated in a series of fairly diluted aqueous solutions containing small concentrations of the partially hydrophobic solutes [23].…”