Which Amino Acids are Capable of Nucleating Fluorescent Silver Clusters in Proteins?

In this experimental and theoretical joint study, we used single amino acids as model systems for studying protein cluster interactions. We probed 12 natural amino acids with different functional groups as potential templates of fluorescent silver (Ag) nanoclusters obtained by sodium borohydride reduction of Ag ions. We also calculated the Gibbs free energies of the complexes formed between Ag+ ions, Ag atoms, and two-atom Ag clusters with the amino acids' various functional groups. Only cysteine and tyrosine could form fluorescent complexes with Ag clusters. This agrees with the calculated Gibbs free energies for the Ag cluster amino acid complexes. We also show that the tyrosine-based fluorescent Ag cluster could be obtained using a green synthetic method in which tyrosine, at alkali pH, acts as a reducing agent. The optimized structure of a complex of Ag-3(+) cluster with three semiquinone tyrosine rings is proposed. These results can be used in designing and synthesizing new peptide-templated biolabels.