Determination of the Molar Fraction and Enantiomeric Excess of Electrosprayed Amino Acid Anions Employing Photoelectron Circular Dichroism

The quantification of molar fractions and enantiomeric excess has been demonstrated in mixtures of d- and l-tryptophan and d- and l-phenylalanine, respectively, avoiding derivatization of the analyte with additional reagents or separation steps. The technique is based on electrospray ionization (ESI), which allows the generation of anions of nonvolatile compounds such as amino acids or large biomolecules. Electrons are photodetached from these anions. The distribution of forward and backward scattered photoelectrons is analyzed, leading to photoelectron circular dichroism (PECD), the observable of interest. The quantification of the concept is proven by blind measurements analyzing mixtures of unknown composition. The quantification of enantiomeric excess (ee) values is not only possible for signals originating from the molecular anion but also for the molecular dimer anion. The ESI-PECD technique is known to be applicable to large chemical entities of several thousand Daltons.