An Insight into the Reactivity of the Electrogenerated Radical Cation of Caffeine

Abstract

Controlled potential electrolyses of caffeine (CAF) were carried out at a Pt electrode in undried acetonitrile (ACN) and ACN-H2O and the products of the anodic oxidation were analyzed by HPLC-PDA-ESI-MS/MS. A higher current efficiency occurred in ACN-H2O, but an analogous chromatographic outline was found in both media, evidencing a reactive pathway of the electrogenerated radical cation CAF•+ with water, added or in trace, as nucleophile. No dimeric forms were evidenced, excluding any coupling reactions. Neither was 1,3,7-trimethyluric acid found, reported in the literature as the main oxidative route for CAF in water. Four main chromatographic peaks were evidenced, assigned to four proposed structures on the base of chromatographic and spectral data: a 4,5-diol derivative and an oxazolidin-2-one derivative were assigned as principal oxidation products, supporting a mechanism proposed in a previous work for the primary anodic oxidation of the methylxanthines olefinic C4 = C5 bond. Two highly polar degradation products were also tentatively assigned, that seemed generating along two different pathways, one opening the imidazolic moiety and another one opening the purinic one.

Electrochemical Transformations of Methylxanthines in Non‐Aqueous Medium

“…A similar systematic study of the oxidation products of 1 in organic solvent has been recently reported [15] …”

“…The electrochemical oxidation of methylxanthines in an aprotic medium has been studied, too [3,10,13–15] …”

“… Proposed oxidation mechanisms for caffeine 1 : R. 1: in water; [8] R. 2: in strongly dried acetonitrile; [10] R. 3: in bottle acetonitrile [15] …”

“…Conversely, the electrochemistry of methylxanthines in an aprotic medium has been sparingly studied, [3,10–15] which is unexpected taking into account the suitability of the aprotic medium to elucidate redox mechanisms (often involving radical species more stable in organic solvent), as well as to better control electrochemical synthetic paths, avoiding chemical side‐reactions often due to the presence of water. Furthermore, the aprotic medium can simulate the lipophilic environment of biological membranes, in which methylxanthines are bioavailable and exert their bioactivity.…”

Recent Advances in Electrochemical Chitosan-Based Chemosensors and Biosensors: Applications in Food Safety

“…Caffeine (CAF, chemical structure in Figure 5 ) is a natural alkaloid, distributed in seeds, nuts, or leaves of a number of plants, mainly coffee, cocoa, tea, and it is the most widely consumed psychoactive substance in human dietary. Recently, an antioxidant activity has been suggested for CAF, showing protective effects against oxidative stress [66][67][68][69]. The presence of CAF in many beverages and drug formulations of worldwide economic importance [69] makes it an analyte of great interest.…”