IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Biomaterials Wednesday Sessions
       Session BI+AS-WeA

Paper BI+AS-WeA7
Chiral Recognition Observed at the Molecular Level by UHV-STM: Cysteine on Au(110)-(1x2)

Wednesday, October 31, 2001, 4:00 pm, Room 102

Session: Surface Characterization
Presenter: T.R. Linderoth, University of Aarhus, Denmark
Authors: A. Kühnle, University of Aarhus, Denmark
T.R. Linderoth, University of Aarhus, Denmark
B. Hammer, University of Aarhus, Denmark
F. Besenbacher, University of Aarhus, Denmark
Correspondent: Click to Email
Chirality is a frequently encountered property of organic molecules, leading to the existence of two mirror-image enantiomers. Interestingly, Nature is often homo-chiral in the sense that only one enantiomer participates in biological processes. Molecular recognition with chiral specificity is thus crucial within many fields of chemistry, biology and medicine. It is also essential for strategies to resolve racemic mixtures into enantiopure phases. Here we report on the adsorption of the chiral amino acid cysteine [HS-CH@sub 2@-CH(NH@sub 2@)-COOH] on the Au(110)-(1x2) surface under Ultra-High Vacuum conditions. Using Scanning Tunneling Microscopy (STM) we have discovered that the cysteine molecules can form isolated molecular pairs that break the mirror symmetry of the gold surface. This provides a model system for a molecular level study of chiral recognition: Deposition of the pure L and D- enantiomers, respectively, leads to identical, but mirror-reflected, cysteine pairs clearly distinguishable by STM. Most interestingly, deposition of the racemic D-L mixture only leads to the known homochiral pairs, showing that heterochiral D-L molecular interaction is avoided. To explore the origins of this novel chiral recognition mechanism, we have performed ab-initio DFT calculations. We find that the cysteine molecules are anchored to the surface via S-Au bonds and interact mutually through hydrogen bonds between the carboxylic groups. Importantly, a favorable interaction between the surface and the amino group is only geometrically feasible for homochiral pairs. The mechanism behind the observed chiral recognition is thus reminiscent of the generic so-called three-point contact model for chiral ligand-receptor interaction.