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

Anais da Academia Brasileira de Ciências (2018) 90(1 Suppl. 1): 607-630 (Annals of the Brazilian Academy of Sciences) Printed version ISSN 0001-3765 / Online version ISSN 1678-2690 http://dx.doi.org/10.1590/0001-3765201720170434 www.scielo.br/aabc | www.fb.com/aabcjournal The long and successful journey of electrochemically active amino acids. From fundamental adsorption studies to potential surface engineering tools. ANDRÉ H.B. DOURADO, FABIÁN C. PASTRIÁN and SUSANA I. CÓRDOBA DE TORRESI Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000 São Paulo, SP, Brazil Manuscript received June 6, 2017; accepted for publication on July 26, 2017 ABSTRACT Proteins have been the subject of electrochemical studies. It is possible to apply electrochemical techniques to obtain information about their structure due to the presence of five electroactive amino acids that can be oriented to the outside of the peptidic chain. These amino acids are L-Tryptophan (L-Trp), L-Tyrosine (L-Tyr), L-Histidine (L-His), L-Methionine (L-Met) and L-Cysteine (L-Cys); their electrochemical behavior being subject of extensive research, but it is still controversial. No spectroscopic investigations have been reported on L-Trp, and due to the short life time of the intermediates, ex situ techniques cannot be employed, leading to a never-ending discussion about possible intermediates. In the L-Tyr and L-His cases, spectroelectrochemical studies were performed and different intermediates were observed, suggesting that some intermediates may be observed under specific conditions, as proposed for L-Cys. This amino acid is the most interesting among the electroactive ones because of the presence of a thiol moiety at its side chain, leading to a wide range of oxidation states. It can adsorb onto surfaces of different crystallographic orientation in stereoselective conformation, modifying the surface for different applications.as a surface engineering tool since it plays the role of as an anchor for the growing of nanocrystals inside proteic templates. Key words: protein electro-oxidation, electrochemical active amino acids, L-Cysteine, SAM, Cu2O, low index facets nanoparticles. INTRODUCTION It is well known that proteins are widely used in electrochemical devices such as glucose sensors that use glucose oxidase enzyme to react with sugar, making indirect measurements by the electrochemical quantification of hydrogen peroxide (Cunningham et al. 2010, Gerlach et Correspondence to: Susana Ines Córdoba de Torresi E-mail: * Contribution to the centenary of the Brazilian Academy of Sciences. al. 2010). But, it must be pointed out that not just enzymes but proteins in general have been the subject of electrochemical studies since the discovery of polarography, which dates from the 30s (Brdička 1933, Heyrovský and Babička 1930, Jurka 1939); the first published paper on electrochemistry behavior of proteins was by Heyrovsky (Heyrovský and Babička 1930). At the time, a reduction wave was observed when a proteic media (Heyrovský and Babička 1930) such as urine, blood or protein solutions was added to an An Acad Bras Cienc (2018) 90 (1 Suppl. 1) 608 ANDRÉ H.B. DOURADO, FABIÁN C. PASTRIÁN and SUSANA I. CÓRDOBA DE TORRESI ammonium chloride solution. The explanation for this fact was the complexation of the ammonium cations and the protein, which was attributed to the electroactive species responsible for the “protein waves”. Three years later, the same group ( Brdička 1933) published a detailed paper showing that the addition of a metallic cation (Co2+) in the solution produced the splitting of the “protein wave” into two regions, and this was observed for several proteic media even at very low concentration. Many amino acids were tested, and just in the case of L-cystine (L-Cys2), a dimer of L-cysteine (L-Cys), the “wave” was observed, leading to the assumption of the existence of a catalytic effect of this amino acid on hydrogen reduction; L-Cys2 is very common in protein structures and is responsible for the “protein wave”. In 1939, Jurka suggested that not just the dissulfate was catalyzing the reaction but it was also due to the adsorption of these groups onto Hg (Jurka 1939). This method became an electroanalytical tool for cancer diagnosis, and it is still used as the base for studies in this area (Minevich and Tur’yan 2013). Previous examples show that protein electrochemistry is a subject that has merited attention since the beginning, and Jurka has noted that it can be used as a diagnostic tool (Jurka 1939), even if the protein does not present a redox catalytic site as an enzyme, a subject that became of interest during the 1970s (Paleček et al. 2015). At present, electrochemistry is still a useful a tool for protein studies, and it was recently reviewed by Paleček et al. (2015). Electrochemistry can be used for biochemical applications, not only as a source of diagnosis but also for genomics, gliconomics and proteomics studies (Paleček et al. 2015). Some of these studies have used the option of adding an iron center or a biomolecule that already has one (as cytochrome C) to simplify the system in a way that would make it possible to analyze the influence of the environment on this electroactive An Acad Bras Cienc (2018) 90 (1 Suppl. 1) group based on non-destructive analysis (Paleček et al. 2015). In the same review by Paleček and coworkers (2015), as in other works (Enache and Oliveira-Brett 2013, 2017, Xu et al 2005), it is highlighted that there are only five electroactive amino acids, L-histidine (L-His), L-methionine (L-Met), L-tyrosine (L-Tyr), L-tryptophan (L-Trp) and L-Cys. Although there is no difference in electrochemical activity in the case of L or D-stereoisomer, in the present work, all amino acids are referred to as L-isomer since this is the one that is naturally observed. The fact that only these five amino acids of the twenty-three natural ones are electroactive made possible the use of electrochemistry in proteomics; the oxidation of these molecules is observed in the same potential region and present the same voltammetric profile when tested free or linked in (poli)peptides (Enache et al. 2016, Enache and Oliveira-Brett 2013, 2017, Paleček et al. 2015) as shown in Figure 1. The current intensity can provide information about the way in which these amino acids are available to the electrode surface, in other words, if they are exposed on the external face of the protein structure or buried in the internal part of the biomolecule, also indicating if the protein is in its natural state or desaturated. (Enache et al. 2016, Enache and Oliveira-Brett 2017). Enache and co-workers published a series of papers on the application of voltammetric techniques for the investigation of the structure, aggregation and denaturation of amyloid-β peptide studied for Alzheimer disease (Enache et al. 2016, Enache and Oliveira-Brett 2017). The authors first compared the (...truncated)