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Review—Electrochemical Properties of 13 Vitamins: a Critical Review and Assessment Matthew D G18 Journal of The Electrochemical Society, 165 (2) G18-G49 (2018) Review—Electrochemical Properties of 13 Vitamins: A Critical Review and Assessment Matthew D. Lovander, ∗ Jacob D. Lyon, Daniel L. Parr IV, ∗ Junnan Wang, ∗ Brenna Parke, ∗ and Johna Leddy ∗∗,z Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, USA Review of the literature on the currently recognized, thirteen vitamins yields an overview of the electrochemical properties that include estimates of the formal potentials at physiological pH and identification of the general classes of redox mechanisms. All vitamins are electroactive and map a range of formal potentials E0 over a 3 V window. The vitamins are grouped as lipid soluble (vitamins A, D, E, and K) and water soluble (B vitamins and vitamin C). Mechanisms are grouped as single electron transfer agents (B3, B7, B2, C, and D), vitamins that can be both oxidized and reduced (B1, B5, B6, B9, and E), and vitamins that undergo two successive, distinct reductions (B12 and K). Vitamin A voltammetry is uniquely complex. Plot of the formal potentials on a potential axis allows assessment of mechanistic paths to vitamin recycling, antioxidant behavior, pH dependence, electrochemical stability in air, acid, and water, electrochemical instability of vitamin pairs, and cooperative interactions between vitamins in medicine. The potential axis is shown as an effective tool for mapping thermodynamically complex interactions. The voltammetry literature for each vitamin is critically assessed. © The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.1471714jes] Manuscript submitted August 7, 2017; revised manuscript received December 4, 2017. Published January 27, 2018. This paper is a Critical Review in Electrochemical and Solid State Science and Technology (CRES3T). This article was reviewed by Shelley D. Minteer ([email protected]) and David E. Cliffel ([email protected]). The National Institutes of Health, Office of Dietary Supplements The review contains two main sections. One section compiles the defines vitamin as A nutrient that the body needs in small amounts to data for the vitamins and provides perspective on the electrochemical function and maintain health.1 Merriam Webster Dictionary2 defines behavior and properties. The second section summarizes the available vitamin as: literature for each vitamin where papers judged the best available are included. In the first section, some compilation of the electrochemical any of various organic substances that are essential in minute quanti- and voltammetric properties of vitamins are summarized in Table I. ties to the nutrition of most animals and some plants, act especially as The order of the reduction potentials, as approximated from voltam- coenzymes and precursors of coenzymes in the regulation of metabolic metric literature data at pH 7, is mapped in Figure 1. The notations for processes but do not provide energy or serve as building units, and are kinetics and potentials embedded in Table I and Figure 1 as well as a present in natural foodstuffs or sometimes produced within the body. brief discussion of how redox potentials are estimated are provided.4 A list of abbreviations is provided at the end of the document in By modern standards, the US federal government identifies 13 Table AI. vitamins.3 The water soluble vitamins are vitamin C and the eight B vitamins (B1, B2, B3, B5, B6, B7, B9, B12). The lipid or fat soluble vitamins are A, D, E, and K. The fat soluble vitamins can accumulate Kinetic notation.—Much of the data evaluated in this review is in the body whereas the water soluble vitamins do not. There are no drawn from voltammetry. Voltammetrydrives electron transfer events, current vitamins designated beyond E except for K because materials generically represented as A + ne B, where formal potential E 0 historically assigned the interposed letter designations either no longer characterizes the energy of the process. Voltammetric morphology fall under the modern definition of vitamin or several related mate- can be impacted by a wide variety of experimental conditions that rials were reclassified. Several vitamins exist in different but related include solvent, solution components, and pH as well as chemical chemical structures. processes. Kinetics are captured in abbreviations of E and C,thatde- Questions considered in this perspective include whether all vi- note mechanistic steps of interfacial (heterogeneous) electron transfer tamins are electroactive; what is the redox potential of the vitamins; and homogeneous (bulk phase) chemical reactions. E characterizes what are the kinetics and mechanisms of vitamins on oxidation and an electron transfer process at the electrode. If the electron transfer reduction; when are vitamins antioxidants; are vitamins stable to wa- is fast compared to the rate of voltammetric perturbation (e.g., cyclic ter and oxygen; do vitamins interact cooperatively. Although papers voltammetric scan rate v), then the process is said to be reversible and are available on the electrochemistry and voltammetry of individual Er is assigned to the process. Absent chemical reactions, Er charac- vitamins, no single resource summarizes the electrochemical data for terizes a Nernstian process. If the electron transfer is slow compared all vitamins. This review compiles and critically assesses the avail- to the voltammetric perturbation, the irreversible electron transfer is able literature on vitamin voltammetry. The compiled data serve to designated E . When rates of electron transfer and perturbation are develop perspective on the electrochemical properties of vitamins i comparable, the reaction is quasireversible, Eq .Whenthereareno and the role of vitamins individually and collectively as electroactive E 3 associated chemical reactions, designation with only is appropriate species. This CRES T review assimilates fundamentals of thermo- and reversibility of the electron transfer is usually readily determined. dynamics and estimated formal potentials, kinetics, and mechanisms When chemical steps couple with the electron transfer, the designa- as assessed voltammetrically for individual vitamins to provide per- tion is C. The chemical step can precede the electron transfer (CE)or spective on the collective electrochemical properties of the thirteen follow the electron transfer (EC). For more than one electron transfer, vitamins. Perspective on vitamin electrochemical properties may con- designations include EE, ECE,andECEC. When there is more than tribute to assess yet more complex bioelectrochemical processes. one electron transfer, disproportionation may occur. Chemical steps may be further characterized as irreversible, reversible, catalytic, and dimerizations. To identify the nature of the chemical step commonly ∗Electrochemical Society Student Member. requires extensive chemical and electrochemical characterization. For ∗∗Electrochemical Society Fellow. this review, a simple label of C is typically used. When the chemical z E-mail: [email protected] process is chemically irreversible, Ci is denoted. 'RZQORDGHGRQWR,3DGGUHVV5HGLVWULEXWLRQVXEMHFWWR(&6WHUPVRIXVH VHHHFVGORUJVLWHWHUPVBXVH XQOHVV&&/LFHQVHLQSODFH VHHDEVWUDFW Journal of The Electrochemical Society, 165 (2) G18-G49 (2018) G19 Table I. Table of Electrochemical Properties of 13 Vitamins. a b Vitamin Estimated E1/2 (vs NHE) pH/aprotic E1/2 from pH dependent ≈ mechanism √ C 0.39 V C|Cox 7.4 Ep EC ascorbate B1 +0.6V B1|B1ox 7.4 Ep pH ind. 9 to 13 EC thiamine −0.38 V B1red|B1 3.5to6 Eave for pH 3.5 to 6: E Eaveind. ipdecreases B2 −0.54 V B2 |B2 DMSO E − EC ...E red p √ − . | 0 E riboflavin 0 22 V B2red B2 7 Eave √ E shifts neg. w/pH − . | p EC EC B3 1 4V B3red B3 7Ep E1/2 shifts neg. w/pH (likely i ) niacin for 2.5 < pH < 10 B5 √ +0.5V B5|B5 4.2 E E shifts pos. w/pH EC (likely EC ) ox p √ p i pantothenic + c −0.2V B5red|B5 6.08 Ep/ 2e/2H , PCET EC (likely ECi ) acid 2 − +1.1V B6|B6 MeCN Ep EC (likely EC ) B6 ox − i −0.8V B6red|B6 MeCN Eave E pyridoxine likely as pK and p a 5 −1.5V B6red|B6 6.5 E / E reported 1 2 pK predicted B7 DMF b −1.03 V B7red|B7 E − Ei biotin DMSO ave √ + EC or EEC B9 +1.0V B9|B9ox 7 Ep, Eave 2e/2H ,PCET adsorption, folate −0.3V B9red|B9 5.2 Ep likely as 2 pK a multiple peaks EE B12 +0.21 V B12red|B12 7.1 Eave likely, pK , pK pred. √ a b aquocobalamin; cobalamin −0.83 V B12red, |B12 7.1 Eave E / shifts neg. w/pH 2 red 1 2 cyano, methyl differ + . | − EC A 1 0V AA CH3OH; CH2Cl2 Ep i − . | − EC retinol (A) 1 95 V Ared A MeCN Ep − . | − EC retinal(A ) 2 1V Ared,2 Ared MeCN Ep + . | − EC β − carotene(Aβ) 1 3V A Aox CH2Cl2 Ep i − . | − E 1 1V Ared A MeCN Ep − | − EC 2V Ared,2 Ared MeCN Ep i +0.8V Aβ|Aβ,ox CH2Cl2 Eave − E −1.4V Aβ,red|Aβ CH2Cl2 Ep − ECi D D3 cholecalciferol +1.45 V D|Dox MeCN,CH2Cl2 Ep − ECi D2 ergocalciferol E +1.2V E|E MeCN − ox ECE 6 α − + . | − disp tocopherol 0 8V Eox Eox,2 MeCN − . | EE K 0 6V Kred K MeCN,50 mM H2O Eave √likely as 3 pKa − . | EE phylloquinone 1 2V Kred,2 Kred MeCN,50 mM H2O Eave CVs H2O dependent √ + −0.5V Kred,2|K MeCN,7.2 M H2O Eave 2e/2H , PCET EE ... aUnderscore indicates the native state vitamin in solution. Charge in the native state is noted in the Synopsis for the individual vitamin. bApproximate mechanism denoted in electron transfer steps (E) and chemical steps (C). c + PCET is proton coupled electron transfer (e.g., A + 2e + 2H H2A).
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