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The Chemistry of Concrete Biodeterioration
Published in Thomas Dyer, Biodeterioration of Concrete, 2017
For many acids, this reaction will reach equilibrium with only a proportion of molecules deprotonated. The extent to which deprotonation occurs for a given acid is expressed in terms of the acid dissociation constant (Ka). This constant is defined by the equation: Ka=[H+][A−][HA]
Charge Storage Mechanism in Proteotronic Capacitors
Published in Angsuman Sarkar, Arpan Deyasi, Low-Dimensional Nanoelectronic Devices, 2023
Shrabani Guhathakurata, Nabin Baran Manik, Sheetikanta Mohanty, Dipun Jena, Subhasis Satapathy, Debasish Panda, Ajit Dash, Rohini Mahato, Gufran Ahmad, Prashant Kumar Singh, Sandipan Mallik
In the biological world, the most abundant and novel macromolecules present in all parts of cell are proteins, whose structural units are amino acids. There are 20 different amino acids commonly found in proteins.91 The main elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in the side chains of some amino acids. Amino acid consists of a central carbon atom (the α carbon), which is attached to an amine (-NN2) group, a carboxyl (–COOH) group, and a side chain (R group).92,93 These four different groups amid α-carbon atom can engage two unique spatial configurations due to tetrahedral arrangement of bonding orbitals and hence giving rise to two possible stereoisomers of the amino acid (L & D) and the α- carbon, which is called as chiral center.94 These arrangements are contrasting in their side chains (R group), which vary with certain parameters such as size, structure, and electrical charge due to which it influences the solubility of amino acids in water. Depending on the side chains, amino acids are categorized into hydrophobic, aromatic, polar uncharged, basic, and acidic. When an amino acid is dissolved in water, it exists in solution as the dipolar ion, that is, the carboxyl group (COO) and the amino group (NH3+). Because the resulting amino acid contains one positive and one negative charge, it is a neutral molecule called a zwitterion.94Figure 4.4a and b shows both the non-ionic and zwitter-ionic forms of amino acids, respectively. Therefore, in aqueous solution, there is an existence of equilibrium between the molecular form and the zwitterion form of amino acids. Amino acids are also referred as amphiprotic (both donate and accept protons) in nature, as it contains both amine and carboxylic acid functional groups. A conjugate acid–base pair is formed up of proton donor and its corresponding proton acceptor.95 The tendency of any acid (HA) to lose a proton and form its conjugate base (A) is defined by the equilibrium constant (Keq). For ionization reactions, equilibrium constants are called as ionization or dissociation constants (Ka). Its negative logarithm (pKa) can be defined as pKa = log (1/Ka) = -log Ka,96 which is a tool to measure the tendency of groups to liberate a proton and it is evident from the pKa expression that for tenfold decreasing tendency,pKa value increases by one unit.97 When pH < pKa, protonation occurs and similarly when pH > pKa, deprotonation occurs.
Theoretical investigation on the structure and antioxidant activity of (+) catechin and (−) epicatechin – a comparative study
Published in Molecular Physics, 2020
S. Anitha, S. Krishnan, K. Senthilkumar, V. Sasirekha
According to the SPLET mechanism, PA and ETE of CT and ECT are calculated and summarised in Table 4 at gas and solvent environment. In the first step, PA is the measure of binding of the proton; lower PA corresponds to easy deprotonation. The lowest PA of 333.58, 97.39, 47.75 and 34.33 kcal/mol in gas, benzene, methanol and water medium, respectively, is observed at 4’-OH site for CT and for ECT PA is 336.66, 99.97, 47.21 and 33.94 kcal/mol in gas, benzene, methanol and water medium, respectively. The PA decreases from gas to the solvent medium as the polarity of the solvent increases, which is similar to PDE [64]. That is, polar solvents favour the deprotonation process. ETE is the second step of the SPLET mechanism; here the electron transfer occurs from deprotonated radical. From Table 4 it is observed that ETE of both the compounds is higher in the solvent medium than the gas medium. ETE of protonated radical is lower than the IP of the neutral molecule. This indicates that a single-electron transfer from the deprotonated form is easier than from the neutral form. The sum of PA and ETE (SPLET) of each O–H site shows that 4’-OH site of both the compounds has the high dehydrogenation ability, and the water medium supports SET-PT mechanism as well. Also, in terms of SET-PT, CT is having better antioxidant property than ECT.