Artificial cysteine

Fig. 19. Lipopeptides with the aminoacid sequence of the Ras C-terminus and the natural or artificial lipid-modifications can be coupled with C-terminally truncated Ras via a maleimi-docaproyl linker. This electrophile reacts with free thiol groups (here, a C-terminal cysteine at the Ras moiety)...

Sulfur (S) Sulfur is a yellow solid. Iron-sulfur clusters are found in cytochrome enzymes. The sulfur-containing amino acid cysteine is common in hair. Cysteine residues can connect to each other via disulfide bridges, giving hair a natural curl. Permanent waves are achieved by artificially removing and then re-forming these disulfide bridges. 

Covalent attachment has also been exploited for protein incorporation of non-native redox active cofactors. A photosensitive rhodium complex has been covalently attached to a cysteine near the heme of cytochrome c (67). The heme of these cytochrome c bioconjugates was photoreducible, which makes it possible for these artificial proteins to be potentially useful in electronic devices. The covalent anchoring, via a disulfide bond, of a redox active ferrocene cofactor has been demonstrated in the protein azurin (68). Not only did conjugation to the protein provide the cofactor with increased water stability and solubility, but it also provided, by means of mutagenesis, a means of tuning the reduction potential of the cofactor. The protein-aided transition of organometallic species into aqueous solution via increased solubility, stability and tuning are important benefits to the construction of artificial metalloproteins. 

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