Side-chain thioamides as fluorescence quenching probes.

Thioamides, single atom oxygen-to-sulfur substitutions of canonical amide bonds, can be valuable probes for protein folding and protease studies. Here, we investigate the fluorescence quenching properties of thioamides incorporated into the side-chains of amino acids. We synthesize and incorporate Fmoc-protected, solid-phase peptide synthesis building blocks for introducing Nε -thioacetyl-lysine and γ-thioasparagine. Using rigid model peptides, we demonstrate the distance-dependent fluorescence quenching of these thioamides. Furthermore, we describe attempts to incorporate of Nε -thioacetyl-lysine into proteins expressed in Escherichia coli using amber codon suppression.