With the addition of a weak acid, a new prepeak occurs at a positive potential when quinone is reduced in an unbuffered solution. This prepeak was initially attributed to the protonation of a quinone anion radical. However, two prepeaks of the reduced quinone are detected in a solution containing dilute acetic acid and sodium dihydrogen phosphate. Evidently, the two prepeaks could not be assigned to the protonation of the quinone anion radical. In the unbuffered solution, the pH near the electrode surface increases drastically during the electrochemical reduction of p-benzoquinone (BQ). Our previous work revealed that a steep change in pH near the electrode surface produces a new voltammetric wave in a dilute buffer solution. We suggest that the new prepeaks are generated by a drastic change in pH near the electrode surface. Weak acids can release protons and suppress the increase in pH on the electrode surface. Therefore, the height of the prepeak is proportional to the concentration of the weak acid but is independent of BQ. We then establish a differential pulse voltammetric method to measure the weak acid and acetic acid contents of two kinds of vinegars. Our results agree well with those measured through potentiometric titration.

Brønsted–Lowry acids and bases exert important effects on the electrochemistry of hydroquinone (QH2) and benzoquinone (Q) in acetonitrile. However, the “effective pH” in aprotic media has been neglected in the redox process of QH2 and Q. Our results show that neutral proline induces new voltammetric waves for QH2 and quinone in acetonitrile, that cannot be attributed to protonation or hydrogen bonding. We observed that only one cathodic wave and one anodic wave of quinone are produced in adequately buffered acetonitrile. Moreover, several amines generated a new anodic QH2 wave, whereas acetic acid produced a new cathodic wave of quinone. Proline acts as proton donor and proton acceptor during the redox of QH2 and Q, thereby indicating that the “effective pH” occurs in dilute buffered aprotic media. We thus concluded that the new waves in acetonitrile are caused by rapid pH changes at the electrode surface and not by new species.

The intramolecular cyclization step of oxidized catecholamines is crucial for neuromelanin (NM) and polydopamine (PDA). The product of the cyclization reaction of catecholamines has electrochemical activity, and its anodic and cathodic peaks occur at a more negative position than that of catecholamines. Moreover, in low pH, the amine group is protonated to a considerable extent; thus, the cyclization reaction is precluded. Our cyclic voltammetric results show that, in dilute buffer solution, the buffer concentration promotes the extent of the cyclization reaction of dopamine (DA), levodopa (L-D), and adrenaline (AD). A buffer component accepts the protons released by oxidized catecholamines, which suppresses the pH decrease near the electrode surface and protonation of amine group. Ascorbic acid (AA) more significantly inhibits the cyclization reaction of oxidized DA than those of L-D and AD. AA reduces catecholamine-quinone, and the cyclization rate constant of oxidized DA is smaller than those of L-D and AD. Consequently, buffer capacity and AA can be used to regulate the synthesis of NM and PDA.Parkinson's disease (PD), one of the most common neurological disorders in the elderly, is characterized by a progressive and massive loss of midbrain dopaminergic neurons. 1 Neuromelanin (NM) is found in deep brain regions, specifically in loci that degenerate in PD patients. 2 To date, the formation, structure, and biological significance of NM are not clearly defined. 3 However, evidence from several studies strongly indicates that the main component of the nigral pigment is formed through the oxidative copolymerization of dopamine (DA). [4][5][6][7] In recent years, the study on polydopamine (PDA) has been an important topic in the biology and chemistry fields. Stepien et al. 8 showed that DA-melanin can suppress the yield of hydroxyl radicals generated via Fenton reaction; however, after saturation with ferric ions, it promotes the formation of hydroxyl radicals via redox activation of the ions. Wang et al. 9 fabricated a novel core-shell structure based on upconversion fluorescent nanoparticles and developed DAmelanin for the evaluation of the antioxidant capacity of biological fluids. Klosterman et al. 10 analyzed the influence of surface composition on the growth of DA-melanin films formed through the polymerization of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid precursor solution in vitro. Wu et al. 11 developed a method of DA-melanin nanofilms for biomimetic structural coloration.The mechanism of DA in eumelanin-type polymer was fully investigated in vitro, 12-16 and the reaction pathway is shown in Scheme 1. DA can be easily oxidized in an aqueous solution through a two-electron and two-proton process transforming into quinone (1a→2a); 17 dopamine-quinone (DAQ) undergoes a cyclization reaction to form leucoaminochrome (LAC) (2a→4a); and LAC reacts immediately with DAQ to form aminochrome (AC) and DA (4a + 2a→5a + 1a). 16,18 AC has a short lifespan and ultimately reacts to form NM, a polyme...