Kationima pospješeni spregnuti prijelaz elektrona i protona

Abstract

Uĉinak iona Na+, K+ i (CH3)4N+ na protonom spregnuti prijelaz elektrona (PCET) u reakciji askorbata s heksacijanoferat(III) ionom u vodi ukazuje na utjecaj kationa na dvostruko elektronsko/protonsko tuneliranje u PCET procesu, što kod većih koncentracija kationa moţe dovesti do “ekstenzivnog” tuneliranja. Dobiveni uvidi obuhvaćaju, uz ostalo: a) opaţanje primarnog kinetiĉkog solnog uĉinka kod niskih koncentracija iona, te linearnu ovisnost konstante brzine o koncentraciji kationa u prisutnosti viših koncentracija kationa; b) kinetiĉki izotopni uĉinak i termokemijsku analizu, koja ukljuĉuje usporedbu ΔG° odgovarajućih konsekutivnih ET/PT ili PT/ET procesa s opaţenom ΔG‡ u reakciji, što jasno ukazuje na PCET reakciju; c) promjenu kinetiĉkog izotopnog uĉinka (KIE), kH/kD, od 4,6 u vodi bez dodatka iona, na 3,3 u prisutnosti 1,0 mol dm-3 Na+, gdje se KIE linearno smanjuje s porastom koncentracije kationa; d) izotopnu razliku Arrheniusovih predeksponencijalnih faktora AH/AD = 2,3 kod 0,5 mol dm-3 Na+, u odnosu na AH/AD = 0,97 u vodi bez dodatka iona; e) izotopnu razliku aktivacijskih entalpija H‡(D2O/H2O) = 1,3 kJ/mol u prisutnosti 0,5 mol dm-3 Na+ iona, koja se znaĉajno razlikuje od semiklasiĉno predviĊenog iznosa, kao i od H‡(D2O/H2O) = 3,9 kJ/mol u vodi bez dodatka iona. Rezultati su raspravljeni s obzirom na pretpostavljeni poĉetni trimolekularni asocijacijski kompleks u toj reakciji, koji bi se sastojao od redoks para i kationa, unutar kojega bi se kation mogao naći u neposrednoj blizini aktiviranog kompleksa. Opaţanje “ekstenzivnog” tuneliranja protona je objašnjeno primjenom modela za tuneliranje koji polazi od Marcusove teorije (Marcusianski model tuneliranja), uz pretpostavku da je dinamika solvatnog kaveza sliĉna onoj u enzimskim reakcijama, u skladu s postojećim teorijskim predviĊanjima.The effects of Na+, K+ and (CH3)4 N+ observed in the proton coupled electron transfer (PCET) reaction of ascorbate monoanion with hexacyanoferrate(III) ion in water suggest an influence of the cations on the proton/electron double tunnelling event in the PCET process, which can lead to the extensive tunnelling in the presence of higher concentrations of cations. The evidence comprised, among others: a) observation of the primary kinetic salt effect at low ion concentrations and the linear dependence of the rate constant on the cation concentration in the presence of higher salt concentrations; b) kinetic isotope effect and thermochemical analysis involving the comparison of driving forces for the sequential ET/PT or PT/ET pathways with the observed activation free energy in the reaction, which strongly suggests a PCET reaction; c) change of kinetic isotope effect (KIE) from kH/kD = 4.6 in the absence of ions to kH/kD = 3.3 in the presence of 1.0 mol dm-3 Na+, where KIE decreases linearly with the increase of cation concentration; d) the isotope effects on Arrhenius pre-factor AH/AD = 2.3 at 0.5 mol dm-3 Na+, which differs from the AH/AD = 0.97 in the absence of cations; e) isotope difference in the enthalpies of activation H‡(D2O/H2O) = 1.3 kJ/mol in the presence of 0.5 mol dm-3 Na+, which differs markedly from the expected semiclassical value, and also from the H‡(D2O/H2O) = 3.9 kJ/mol in the absence of added cations. The results are discussed 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. The observation of “extensive” tunnelling in the reaction has been explained applying a „Marcus-like tunneling“ model for hydrogen tunnelling, where a solvent-cage dynamics similar to that in an enzyme active site is assumed, in accordance with existing theoretical predictions