The Development of Unnatural Amino Acid-Based Probes and Methods for Biological Studies

Proteins form a diverse ensemble of dynamic structures to carry out all life-sustaining functions. Therefore, many efforts have gone into studying the structure-dynamics-function relationship of proteins using a wide range of techniques, including fluorescence and infrared (IR) spectroscopies. While very useful, intrinsic fluorescence and IR signals arising from the natural amino acid side chains within the protein are often insufficient or unable to provide the information needed to understand the biological question of interest. To this end, various extrinsic spectroscopic probes, such as fluorescent dyes, have been used to increase the information content in specific measurements and applications. However, incorporation of a foreign moiety into any protein unavoidably affects its native structure and dynamics; hence effort must be made to reduce such perturbation. In this regard, the overarching aim of this thesis is to develop novel spectroscopic probes based on scaffolds of natural amino acids (NAAs). Because of their small size and similarity to NAAs, such unnatural amino acid-based (UAA-based) probes are expected to be minimally perturbing. Specifically, we show that (1) 4-cyanotryptophan (4CN-Trp) is a blue fluorescent amino acid useful for fluorescence microscopy applications; (2) 4CN-Trp and DiO (a common dye used to stain membranes) are a useful FRET pair to study peptide-membrane interactions; (3) 4CN-Trp, and tryptophan constitutes a dual FRET-PET pair which was used to study peptide end-to-end termini interactions and protein ligand-binding; and (4) the functional group of 4CN-Trp, 4-cyanoindole can be used in the form of a nucleoside as a dual fluorescence-IR reporter for DNA-protein studies. Furthermore, we extended applications of previously known UAAs and showed (5) p-cyanophenylalanine is useful as a fluorescence-based pH sensor which we used to determine peptide pKa’s and peptide membrane penetration kinetics and (6) we use a simple synthetic method for post-translationally installing an ester moiety on to proteins via cysteine alkylation as an UAA-based vibrational probe in proteins to study fibril formation and protein-ligand interaction