l-Cysteine electrooxidation in alkaline and acidic media: a combined spectroelectrochemical and computational study

Abstract

The wide possibilities of l-cysteine and l-cystine adsorbed and/or oxidized species onto Pt were explored by FTIR experiments and DFT calculations.

The long and successful journey of electrochemically active amino acids. From fundamental adsorption studies to potential surface engineering tools.

“…The oxidation by the chemical reaction with the surface oxides (Koryta and Pradac 1968a, Pradac and Koryta 1968) would also explain the oxidation, but the mechanism should generate more oxidized species, being necessary to reduce the oxide back to Pt. So, spectroscopic data also suggest that direct oxidation of the adsorbed species is possible as well as generation of a wide range of intermediates, depending on pH (Dourado et al 2017). Another aspect studied was the side reaction of the descarboxilation of the adsorbed species; this was a point not considered in any other work dealing with the electrooxidation of L-Cys but treated as being of high importance for L-Tyr (Ogura et al 1999, Zinola et al 2005) and L-His (Ogura et al 1999).…”

“…The first group performed spectroscopic measurements by XPS to investigate the oxidation state of the adsorbed sulfur atom; a complex mechanism is proposed whereby the adsorbed species is combined, generating L-Cys 2 and this species absorbing by losing one of the R chains ). This kind of mechanism was only proposed in this study, and the probability of breaking an S-S bond should be higher than for a C-S one; so, an adsorption step where the former bond is broken should be more suitable, as suggested by many other works (Dourado et al 2017, Hager and Brolo 2003, Watanabe and Maeda 1989. The literature about electrochemical dessorption of thiols (Widrig et al 1991 does not present much information on the mechanism since currently only the regeneration of the S-S bond is considered, and if the oxidation is analyzed, not many advances apart from a multielectronic mechanism were related.…”

“…As pointed out in an earlier work (Dourado et al 2017), the adsorption of L-Cys is a key point for understanding certain side reactions, such as the Kolbe descarboxylation (Vijh andConway 1967, Wang et al 2015). Also, the formation of a film at the surface is an example of a self-assembling monolayer (SAMs), which was reviewed recently for different amino acids on metallic surfaces with the expectation that different effects would take place on the oxides (Costa et al 2015).…”

“…Another aspect studied was the side reaction of the descarboxilation of the adsorbed species; this was a point not considered in any other work dealing with the electrooxidation of L-Cys but treated as being of high importance for L-Tyr (Ogura et al 1999, Zinola et al 2005) and L-His (Ogura et al 1999). This kind of reaction could be related to a Kolbe Mechanism (Vijh andConway 1967, Wang et al 2015), but this kind of behavior should be more expressed in alkaline media due to the higher reactivity of the deprotonated carboxyl; but interestingly, in acid media (pH 0), this reaction was more pronounced (Dourado et al 2017). This fact was explained by the structure of the first adsorbed species, which were different from those reported for Au (Bieri and Bürgi 2005, Di Felice et al 2003, Di Felice and Selloni 2004 and Ag (Brolo et al 2002, Santos et al 2007, meaning that the carboxyl was away from the surface for Au and Ag at lower pH values and closer in alkaline media.…”

“…Nevertheless, pulsed amperometric techniques (Johll et al 1997) present good reproducibility, sensitivity and detection limits and were selected by Johnson's group for developing an amino acid analyzer, in which an HPLC column is coupled with an electrochemical detector (Vandeberg and Johnson 1993); there has been a great deal of interest in this piece of equipment in biochemical structural studies. Recently, Dourado et al (2017) studied the electrochemical oxidation of L-Cys and L-Cys 2 by infrared reflection absorption spectroscopy (IRRAS) in parallel with DFT calculations to predict the possible structures of the intermediates and the theoretical IR spectra. In this work, IRRAS spectra obtained at different applied potentials made it possible to detect intermediates that were not very common in the literature and others that were never suggested, Figure 6.…”