Introduction: Cysteine is a versatile amino acid for selective chemical modification of proteins with
both chemical and biological innovations, which plays a key role in different organic reactions.
Materials and Methods: Chemical modification of proteins is a rapidly expanding area in chemical biology. Selective
installation of biochemical probes has led to a better understanding of natural protein modification and
macromolecular function. In other cases, such as chemical alterations, the protein function has entirely changed.
This review paper considers the organic reaction of cysteine, the reactivity of this α-amino acid containing sulfur,
and several methodologies are also discussed. Herein, we focused on the reaction of cysteine and its application
in organic synthesis, which includes addition, condensation, substitution, oxidation, and ring-opening reactions.
Results and Discussion: Hence monitoring of cysteine is pivotal through the preparation of some fluorescent
probes to detect cysteine in high sensitivity. Also, a bibliometric analysis was carried out using Web of Science
and Scopus databases that demonstrated significant contributions being observed in organic synthesis. Analysis
of keywords revealed that research hotspots were cysteine, sensor, unclassified drug, and amino acid.
Conclusion: Therefore, it seems that future research focuses on using cysteine amino acids in various fields as
natural products and organic reactions. This focused review highlights the enduring utility of cysteine in protein
modification and sensor preparation, with a special focus on recent innovations in chemistry and biology associated
with such modifications.