engleski

Observation of a cation promoted proton-coupled electron transfer (PCET) reaction

The direct influence of cations on the rates and on the kinetic isotope effects (KIEs) kH/kD in a proton-coupled electron transfer (PCET)1-7 reaction, in this case in the interaction of ascorbate with hexacyanoferrate(III) ion in water and 50/50 v/v water-dioxane mixed solvent, has been observed experimentally for the first time (to the best of our knowledge). The facts that support the above statement comprise: i) the interaction of ascorbate with hexacyanoferrate(III) ion in water and 50/50 v/v water-dioxane must be a PCET reaction, as the comparison of the experimentally obtained activation parameters and the driving force (formal reduction potentials) for the reaction (the analysis of thermochemistry of reaction using the well known “square scheme4), in combination with the observed KIEs and spectroscopic and kinetic evidence clearly show that the process is neither a sequential electron transfer/proton transfer (ET/PT) nor a sequential proton transfer/electron transfer (PT/ET) ; ii) the observation of change of the kinetic isotope effect kH/kD in the reaction in water with change of a cation concentration: thus, the KIE decreases from 4.34(0.12) at zero cation concentration to 3.82(0.09) at 0.10 moldm-3 of Na+, and 2.82(0.08) at 1.00 moldm-3 of Na+ ; iii) the obtained kinetic data clearly indicates the primary kinetic salt effect in the range 0.00 to 0.01moldm-3 of NaCl in water and in heavy water ; the slope of log (k/ko) vs.the square root of ionic strength in D2O is 3.1, (R=0.999) ; iv) the data fitted with the Brønsted-Bjerrum equation in the range 0.0 to 1.0 moldm-3 of NaCl very well, suggesting a role of interionic attractions in the process ; v) in 50/50 dioxane-water solvent the observed KIE is 6.23(0.32) at 0.002 moldm-3 of Na+ while at 0.200 moldm-3 of Na+ the observed value is 5.03(0.25). The result probably suggests a role of ion pairing phenomena in the reaction ; vi) taking into regard the KIE of 4.34 at zero cation concentration in water, the observation of the KIE of 6.23 in 50/50 v/v dioxane-water at low cation concentration should be interpreted as a possible example of a change of KIE with change of the solvent polarity, in line with theoretical predictions6-9 vii) a role of ion pairing phenomena is corroborated by the experiments with added calcium ions in the water system ; at 0.000053 moldm-3 of Ca2+ the observed KIE is 4.28 and at 0.00053 moldm-3 of Ca2+ the KIE is 3.76. The corresponding rate enhancements are, expectedly, greater for Ca2+ than for Na+ ion. The obtained results should be of importance for a better understanding of fundamental features of PCET reactions in systems involving charged reactants and other ions in less polar solvent environment (especially in enzyme reactions) and could present a challenge for theoreticians as well. REFERENCES 1 Huynh, H. V. ; Meyer, T. J. Chem. Rev. 2007, 107, 5004. 2 Reece, S.Y. ; Hodgkiss, J. M. ; Stubbe, J. ; Nocera, D.G. Phil. Trans. R. Soc. B 2006, 361, 1351. 3 Hammarström, L ; Styring, S.Phil. Trans. R. Soc. B 2008, 363, 1283. 4 Mayer, J. M. Annu. Rev. Phys. Chem. 2004, 55, 363. 5 Itoh, S. ; Fukuzumi, S. Acc. Chem. Res. 2007, 40, 592. 6 Hammes-Schiffer, S. ; Soudackov, A. V. J. Phys. Chem. B 2008, 112, 14108. 7 Hammes-Schiffer, S. ; Hatcher, E. ; Ishikita, H. ; S. Skone, J. H. Soudackov, Coord. Chem. Rev. 2008, 252, 384. 8 Decornez, H. ; Hammes-Schiffer, S. J. Phys. Chem. A 2000, 104, 9370.8 9 Ludlow, M. K. ; Soudackov, A. V. ; Hammes-Schiffer, S. J. Am. Chem. Soc. 2009, 131, 7094.

PCET; Cation; Kinetic isotope effect; Ascorbate; Hexacyanoferrate; Tunnelling

nije evidentirano