Abstract
The redox capacity, as well as the aurophilicity of the terminal thiol side groups, in poly(Cysteine) lend a unique characteristic to this poly(amino acid) or polypeptide. There are two major application fields for this polymer: (i) biomedical applications in drug delivery and surface modification of biomedical devices and (ii) as coating for electrodes to enhance their electrochemical sensitivity. The intended application determines the synthetic route for p(Cysteine). Polymers to be used in biomedical applications are typically polymerized from the cysteine N-carboxyanhydride by a ring-opening polymerization, where the thiol group needs to be protected during the polymerization. Advances in this methodology have led to conditions under which the polymerization progresses as living polymerization, which allows for a strict control of the molecular architecture, molecular weight and polydispersity and the formation of block copolymers, which eventually could display polyphilic properties. Poly(Cysteine) used as electrode coating is typically polymerized onto the electrode by cyclic voltammetry, which actually produces a continuous, pinhole-free film on the electrode via the formation of covalent bonds between the amino group of Cysteine and the carbon of the electrode. This resulting coating is chemically very different from the well-defined poly(Cysteine) obtained by ring-opening polymerizations. Based on the structure of cysteine a significant degree of cross-linking within the coating deposited by cyclic voltammetry can be assumed. This manuscript provides a detailed discussion of the ring-opening polymerization of cysteine, a brief consideration of the role of glutathione, a key cysteine-containing tripeptide, and examples for the utilization of poly(Cysteine) and poly(Cysteine)-containing copolymers, in both, the biomedical as well as electrochemical realm.
Highlights
Poly(L-Cysteine), p(L-Cys), does not exist as a homopolymer and there are no known extended p(L-Cys) sequences neither in normal nor pathologic proteins, as there are e.g., extended sequences of p(L-Glutamine) associated with Huntington’s disease [1,2] and poly(L-Alanine) sequences indicated in oculopharyngeal muscular dystrophy [3]
The main focus of the article will be on the synthesis of p(Cys) and p(Cys)-containing copolymers and will highlight the two main—but very different—applications for the polymers: (i) biomedical polymers for drug delivery and surface modification of biomedical devices exploiting p(Cys)’s redox sensitivity and aurophilicity and (ii) electrochemical-based detection of chemical moieties
This is in contrast to the inorganic definition of aurophilicity that refers to the formation of weak gold-gold bonds in the aggregation of gold complexes [4]
Summary
Poly(L-Cysteine), p(L-Cys), does not exist as a homopolymer and there are no known extended p(L-Cys) sequences neither in normal nor pathologic proteins, as there are e.g., extended sequences of p(L-Glutamine) associated with Huntington’s disease [1,2] and poly(L-Alanine) sequences indicated in oculopharyngeal muscular dystrophy [3]. L-Cys building blocks have been utilized in combination with other amino acid building blocks primarily for biomedical applications and a chemically different form of p(Cys) has been exploited in electrochemical settings for the detection of chemical substances. It should be noted that aurophilicity of polymers should be understood as the reactivity of a polymer towards gold moieties, typically the formation of a bond between a thiol group and a gold surface or particle. This is in contrast to the inorganic definition of aurophilicity that refers to the formation of weak gold-gold bonds in the aggregation of gold complexes [4].
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