Most of the biological molecules such as enzymes, proteins, and carbohydrates have different interactions with compounds with different chirality. In fact, chirality plays an important role in biochemical systems functioning in the living organisms through enantioselective interactions [1, 2]. Nowadays, strict regulations have been established by health authorities on the commercialization of chiral drugs [3]. Therefore, the existence of sensitive and accurate methods for the determination enantiomeric excess is necessary to obey the international regulations and quality control purposes for pharmaceutical industries. Methods available for enantiomeric compounds determination usually relay on different interactions of enantiomers of interest with other enantiomers most often chiral acid-base interactions and can be divided into two main categories including separation based methods and chiroptical methods. This work reports on the development of a chiroptical method for the determination of enantiomeric excess of tryptophan amino acid. The developed method uses tryptophan as both the reducing and complexation regents in the presence of Cu 2+ ions. Preliminary studies showed that tryptophan could be utilized for the synthesis of gold nanoparticles under ambient conditions. Also, it was observed that gold nanoparticles synthesized using sodium citrate reducing reagent agglomerate in the presence of only D-tryptophan-Cu 2+ complex (confirmed using DLS and TEM analyses). Therefore, a method was developed based on these facts to determine the enantiomeric excess of tryptophan amino acid by recording different mole fractions of D-tryptophan solution absorbance at 570 nm. The experimental conditions including Au 3+ and Cu 2+ concentrations, solution pH, temperature, and time were optimized. The results showed that it is possible to determine the enantiomeric excess of tryptophan amino acid in the concentration of 104 µM to 3.33 mM under the optimized conditions with acceptable accuracies and precisions.
