Critical Review—Electrochemical Properties of 13 Vitamins: A Critical Review and Assessment

Abstract

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 E 0 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 National Institutes of Health, Office of Dietary Supplements defines vitamin as A nutrient that the body needs in small amounts to function and maintain health. 1 Merriam Webster Dictionary 2 defines vitamin as: any of various organic substances that are essential in minute quantities to the nutrition of most animals and some plants, act especially as coenzymes and precursors of coenzymes in the regulation of metabolic processes but do not provide energy or serve as building units, and are present in natural foodstuffs or sometimes produced within the body. By modern standards, the US federal government identifies 13 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 in the body whereas the water soluble vitamins do not. There are no current vitamins designated beyond E except for K because materials historically assigned the interposed letter designations either no longer fall under the modern definition of vitamin or several related materials were reclassified. Several vitamins exist in different but related chemical structures. Questions considered in this perspective include whether all vitamins are electroactive; what is the redox potential of the vitamins; what are the kinetics and mechanisms of vitamins on oxidation and reduction; when are vitamins antioxidants; are vitamins stable to water and oxygen; do vitamins interact cooperatively. Although papers are available on the electrochemistry and voltammetry of individual vitamins, no single resource summarizes the electrochemical data for all vitamins. This review compiles and critically assesses the available literature on vitamin voltammetry. The compiled data serve to develop perspective on the electrochemical properties of vitamins and the role of vitamins individually and collectively as electroactive species. This CRES 3 T review assimilates fundamentals of thermo-dynamics and estimated formal potentials, kinetics, and mechanisms as assessed voltammetrically for individual vitamins to provide perspective on the collective electrochemical properties of the thirteen vitamins. Perspective on vitamin electrochemical properties may contribute to assess yet more complex bioelectrochemical processes. The review contains two main sections. One section compiles the data for the vitamins and provides perspective on the electrochemical behavior and properties. The second section summarizes the available literature for each vitamin where papers judged the best available are included. In the first section, some compilation of the electrochemical and voltammetric properties of vitamins are summarized in Table I. The order of the reduction potentials, as approximated from voltam-metric literature data at pH 7, is mapped in Figure 1. The notations for kinetics and potentials embedded in Table I and Figure 1 as well as a brief discussion of how redox potentials are estimated are provided. 4 A list of abbreviations is provided at the end of the document in Table AI. Kinetic notation.-Much of the data evaluated in this review is drawn from voltammetry. Voltammetry drives electron transfer events, generically represented as A + ne B, where formal potential E 0 characterizes the energy of the process. Voltammetric morphology can be impacted by a wide variety of experimental conditions that include solvent, solution components, and pH as well as chemical processes. Kinetics are captured in abbreviations of E and C, that denote mechanistic steps of interfacial (heterogeneous) electron transfer and homogeneous (bulk phase) chemical reactions. E characterizes an electron transfer process at the electrode. If the electron transfer is fast compared to the rate of voltammetric perturbation (e.g., cyclic voltammetric scan rate v), then the process is said to be reversible and E r is assigned to the process. Absent chemical reactions, E r characterizes a Nernstian process. If the electron transfer is slow compared to the voltammetric perturbation, the irreversible electron transfer is designated E i. When rates of electron transfer and perturbation are comparable, the reaction is quasireversible, E q. When there are no associated chemical reactions, designation with only E is appropriate and reversibility of the electron transfer is usually readily determined. When chemical steps couple with the electron transfer, the designation is C. The chemical step can precede the electron transfer (CE) or follow the electron transfer (EC). For more than one electron transfer, designations include EE, ECE, and ECEC. When there is more than 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 requires extensive chemical and electrochemical characterization. For this review, a simple label of C is typically used. When the chemical process is chemically irreversible, C i is denoted.