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

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-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.

Mechanism of Electrochemical L‐Cysteine Oxidation on Pt

“…L‐Cysteine (L‐Cys) is one of the five electrochemically active amino acids ,. Because it is the only amino acid presenting a thiol group on its side chain, it controls the redox activity of many enzymes and is crucial in the immobilization of proteins on metallic surfaces ,,. This property is also the reason for the use of L‐Cys as an electrode modifier .…”

“…It was, and still is, assumed as the only mechanism in many other works ,. A recent review presents all this controversy and recent efforts for a better understanding of this process. Several studies have found that some other intermediates formed in steps with transfer of fewer electrons.…”

“…Vibrational spectroscopy was used to study L‐Cys adsorption on different surfaces, such as Ag, Au and Pt,,,, and the structure of the adsorbed species was found to be dependent on the surface material …”

Electrochemical Sensing of Ovalbumin Based on the Interaction between Lysozyme Origin/Tyrosine‐rich Peptides Modified on Magnetic Beads and Oligothreonine/Ovalbumin‐origin Peptide

“…In that case, the amino acid sequence indicated a specific electrode response. Generally, oxidation responses are observed in tyrosine, tryptophan, and cysteine, because these are examples of electroactive amino acids . Our group found that (tyrosine) 4 cysteine was one of the electron‐transfer peptides.…”

Kinetics, Assembling, and Conformation Control of L‐Cysteine Adsorption on Pt Investigated by in situ FTIR Spectroscopy and QCM‐D

“…Most of these investigations consider metallic surfaces such as Au, Ag,, and Cu ,. These metals are known as “softer” acids than Pt, which explains the higher affinity they present by the S atom and why thiols are standard moieties to modify these surfaces ,. Since Pt is a “harder” acid, the interaction is not as favourable than for the other surfaces, and this can be a reason for the smaller amount of investigations with this material.…”