Formation of Induced-Fit Chiral Templates by Amino Acid-Functionalized Pd(111) Surfaces

Chiral probe molecules, propylene oxide, and glycidol are used to measure the enantioselectivity of a range of amino acid-functionalized Pd(111) surfaces. Only those surfaces that contain tetrameric amino acid assemblies are found to be enantioselective, indicating that they act as chiral templates in which several modifiers operate in concert to form a chiral reaction pocket. It has previously been shown that the tetramers assemble from antiparallel anionic-zwitterionic dimers where three of the amino acids then undergo a concerted translational motion to form a more stable tetramer. However, density functional theory calculations reveal that the most stable tetramer has a pocket that is too small to accommodate the chiral probes, while the more open antiparallel anionic-zwitterionic dimer structure provides sufficient space for the epoxide to adsorb enantioselectively on the most stable atop palladium adsorption site. Amino acid destabilization is confirmed by its lower desorption temperature measured in temperature-programmed desorption.