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

Marta Feroci,Fabiana Pandolfi,Rita Petrucci,Leonardo Mattiello,Isabella Chiarotto,Martina Bortolami,Daniele Rocco,Paola Di Matteo

doi:10.3390/electrochem1010005

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.

Highlights

The widespread natural caffeine (1,3,7-trimethylxanthine, CAF, Scheme 1), taken daily with the diet as an important component of highly popular beverages as coffee and tea, has been widely studied for decades
A higher current efficiency was found when the electrolysis was carried out in the presence of 1 equivalent of added water, as evidenced by the strong decrease of the anodic peak of CAF after 2 F electrolysis (Figure 1 right, blue curve): the ratio between the peak current density I measured after 2 F electrolysis (I2F) and that one measured before starting the electrolysis (I0) evidenced a 14% of the starting material left, versus a 42% evidenced in the electrolysis carried out without added water (Figure 1 left, blue curve)
A higher current efficiency was found in the presence of water: only 14% of the starting material was found in the final 2 F electrolyzed solution versus a 42% in ACN, as calculated by both HPLC-photodiode array (PDA)-electrospray ionization (ESI)-MS/MS and voltammetric data

Summary

Introduction

The widespread natural caffeine (1,3,7-trimethylxanthine, CAF, Scheme 1), taken daily with the diet as an important component of highly popular beverages as coffee and tea, has been widely studied for decades. Many systemic and physiological effects of CAF in humans are well known, and some therapeutic properties have been used in medicine for different pathologic contexts, such as respiratory disease and cardiovascular disease [1,2,3,4,5,6]. Beneficial health effects have been more recently proposed on obesity and diabetes, fertility, and neurodegenerative disease as Alzheimer’s and Parkinson’s disease; in these last cases the proper mechanism of caffeine-induced protection has not been fully clarified yet. The xanthine structural core of CAF represents a useful natural renewable starting material in the synthesis of known and/or new drugs, especially addressed to diseases related to Electrochem 2020, 1, 44–55; doi:10.3390/electrochem1010005 www.mdpi.com/journal/electrochem

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