Self-Assembled Chiral Gold Supramolecules with Efficient Laser Absorption for Enantiospecific Recognition of Carnitine

Stereospecific recognition of chiral molecules is ubiquitous in chemical and biological systems, thus leading to strong demand for the development of enantiomeric drugs, enantioselective sensors, and asymmetric catalysts. In this study, we demonstrate the ratio of d-Cys and l-Cys playing an important role in determining the optical properties and the structures of self-assembled Cys-Au(I) supramolecules prepared through a simple reaction of tetrachloroaurate(III) with chiral cysteine (Cys). The irregularly shaped -[d-Cys-Au(I)]_n- or - [l-Cys-Au(I)]_n- supramolecules with a size larger than 500 nm possessing strong absorption in the near-UV region and chiroptical characteristics were only obtained from the reaction of Au(III) with d-Cys or l-Cys. On the other hand, spindle-shaped -[d/l-Cys-Au(I)]_n- supramolecules were formed when using Au(III) with mixtures of d/l-Cys. Our results have suggested that Au(I)···Au(I) aurophilic interactions, and stacked hydrogen bonding and zwitterionic interactions between d/l-Cys ligands are important in determining their structures. The NaBH₄-mediated reduction induces the formation of photoluminescent gold nanoclusters (Au NCs) embedded in the chiral -[d-Cys-Au(I)]_n- or -[l-Cys-Au(I)]_n- supramolecules with a quantum yield of ca. 10%. The as-formed Au NCs/-[d-Cys-Au(I)]_n- and Au NCs/-[l-Cys-Au(I)]_n- are an enantiospecific substrate that can trap l-carnitine and d-carnitine, respectively, and function as a nanomatrix for surface-assisted laser desorption/ionization mass spectrometry (LDI-MS). The high absorption efficiency of laser energy, analyte-binding capacity, and homogeneity of the Au NCs/-[Cys-Au(I)]_n- allow for quantitation of enantiomeric carnitine down to the micromolar regime with high reproducibility. The superior efficiency of the Au NCs/-[d-Cys-Au(I)]_n- substrate has been further validated by quantification of l-carnitine in dietary supplements with accuracy and precision. Our study has opened a new avenue for chiral quantitation of various analytes through LDI-MS using metal nanocomposites consisting of NCs and metal-ligand complexes.