Naslov
New Insights into Vitamin C Activity

Autori
Karković Marković, Ana ; Fabijanić, Ivana ; Jakobušić Brala, Cvijeta ; Pilepić, Viktor ; Barbarić, Monika ; Uršić, Stanko

Vrsta, podvrsta i kategorija rada
Radovi u zbornicima skupova, cjeloviti rad (in extenso), znanstveni

Izvornik
Conference Proceedings of the 2nd Zagreb International Conference on Pharmaceutical Sciences 'Sharing a vision - Towards better and safe medicines' (2nd ZICPS 2013) ; ISBN 978-953-7897-02-4 ; CIP 865515 / Jadrijević-Mladar Takač, Milena ; Klebovich, Imre ; Shah, Vinod P. - Zagreb : Hrvatsko farmaceutsko društvo, 2013, 358-359

ISBN
978-953-7897-02-4

Skup
2nd Zagreb International Conference on Pharmaceutical Sciences 'Sharing a vision - Towards better and safe medicines'

Mjesto i datum
Zagreb, Hrvatska, 25.10.2013. - 26.10.2013

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Ascorbate; PCET; tunneling

Sažetak
Vitamin C, more precisly ascorbate monoanion, is well known antioxidant and cofactor in large number of enzyme reactions in human body. Some ascorbate redox reactions, for example reactions with N-containing toxins, nitrosobenzene and TEMPO radical ; and metal complexes, hexacyannoferrate(III) ion and ferricinium ion, are PCET reactions, reactions based on concerted electron and proton transfer, that avoided highly reactive intermediates, which are present in case of stepwise electron/proton transfer reactions. Some other important antioxidants, as tocopherol, ubiqinon, uses PCET mechanism, as well. Generally, PCET reactions are of essential importance in many biological systems, and enzyme reactions [1, 2]. Figure 1. A graphical presentation of a PCET (Proton Coupled Electron Transfer) reaction of ascorbate and a reactant R (reactants R are listed in a Table 1). The evidence for PCET reaction mechanism usually comes from thermochemical analysis and kinetic isotope effects, determined to be 3.10, 24.2, 4.60, 1.91 for the reaction between ascorbate and nitrosobenzene, TEMPO radical, hexacyannoferrate(III) ion and ferricinium ion, respectively [3]. In these ascorbate reactions, an important role has quantum-mechanical phenomenon of hydrogen tunnelling, as seen from KIE and isotopic activation parameters, H/D ratio of Arrhenius preexponential factors AH/AD and ΔEa(D-H). AH/AD determined in the reaction between ascorbate and hexacyannoferrate(III) ion are 0.07 and 2.30 in 0.25:0.75 dioxane-water solvent mixture and in 0.5 M aqueous NaCl solution, respectively ; and in the reaction between ascorbate and TEMPO radical, in MeCN-water 0.50:0.50 solvent mixture, in presence of 0.5 M tetraethylammonium chloride AH/AD 0.22. The observed values are typical evidence indicating H tunnelling. The results are explained within quantum-mechanical model for H tunnelling (Marcus like tunnelling model) [4], which concerns dynamical role of activated complex and it`s environment in the process. The obtained results are of importance for better understanding the reactivity of ascorbate, and the role of ascorbate in biological and biochemical systems. Table 1. Experimental data for the reactions of ascorbate with reactant R [3, 5-7] ; a - in water ; b - in 50:50 v/v MeCN-water solvent mixture, in presence of 0.5 M tetraethylammonium chloride ; c - 25:75 v/v dioxane-water solvent mixture ; d - 0.5 M aqueous NaCl solution References: 1. Weinberg, D. R. ; Gagliardi, C. J. ; Hull, J. F. ; Fecenko Murphy, C. ; Kent, C. A. ; Westlake, B. C. ; Paul, A. ; Ess, D. H. ; Granville McCafferty D. ; Meyer, T. J. Proton-Coupled Electron Transfer, Chem. Rev., 2012, 112, 4016−4093. 2. Reece, S. Y. ; Nocera, D. G. Proton-coupled electron transfer in biology: results from synergistic studies in natural and model systems, Annu. Rev. Biochem., 2009, 78, 673-99. 3. Sajenko, I. ; Pilepić, V. ; Jakobušić Brala, C. ; Uršić, S. Solvent Dependence of the Kinetic Isotope Effect in the Reaction of Ascorbate with the 2, 2, 6, 6-Tetramethylpiperidine-1-oxyl Radical: Tunnelling in a Small Molecule Reaction. J. Phys. Chem. A, 2010, 114, 3423-3430. 4. Knapp, M. J. ; Rickert, K. ; Klinman J.P. Temperature-dependent isotope effects in soybean lipoxygenase-1: Correlating hydrogen tunneling with protein dynamics, J. Am. Chem. Soc., 2002, 124, 3865-3874. 5. Vuina, D. ; Pilepić, V. ; Ljubas, D. ; Sanković, K. ; Sajenko, I. ; Uršić, S. PCET in the oxidation of ascorbate. Dramatic change of the kinetic isotope effect on the change in solvent polarity, Tetrahedron Lett., 2007, 48, 3633–3637. 6. Jakobušić Brala, C. ; Pilepić, V. ; Sajenko, I. ; Karković, A. ; Uršić, S. Ions Can Move a Proton-Coupled Electron-Transfer Reaction into Tunnelling Regime, Helv. Chim. Acta, 2011, 94, 1718 – 1731. 7. Jakobušić Brala, C. ; Karković, A. ; Klepac, K. ; . Vučinović, A. M. ; Pilepić, V. ; Uršić, S. Small Molecule Tunnelling Systems. Variation of Isotope Effects, Z. Phys. Chem., 2011, 225, 821-841.

Izvorni jezik
Engleski

Znanstvena područja
Kemija, Farmacija

Napomena
Extended poster presentation abstract

POVEZANOST RADA