Effects of Varying Coverage on Chiral Recognition of PVBA on Pd(111)
Chirality, or the "handedness" of molecules, is a fundamental aspect of molecular biology and biochemistry, and is of central importance in pharmaceutical and its applications. Enantiomeric drugs are used to increase the chances of success in targeting enzymes, hormones, and receptors on cell surfaces, along with other compounds comprised of chiral amino acids, carbohydrates, and lipids. Using scanning tunneling microscopy (STM), a model molecule, 4-trans-2-(pyrid-4-yl-vynyl) benzoic acid (PVBA), was observed to have two chiral species, labeled δ and λ, on a highly symmetric Pd(111) under ultrahigh vacuum. At low coverage, the STM image showed just a few PVBA dimers and trimers formed on the substrate, exhibiting clear orientational preference of 20° and 40° depending on their chirality. No chiral recognition has been observed for the PVBA δ and λ molecules on Pd(111) at low coverage. Until now, the lack of chiral recognition on Pd(111) had not been investigated for higher coverage of PVBA. For this reason, we studied the effects of increasing coverage on molecular chiral recognition and orientations. At medium coverage, the STM image shows network-like chain structures where an increased number of PVBA molecules still maintain the λ and δ orientation preferences they exhibited in the lesser-coverage image. At high coverage, the image shows an ultrahigh density of PVBA on the Pd(111) surface where linear chain-like structures align parallel, forming small domains of tens of molecules. These domains suggest that, in addition to the hydrogen-bonding interactions, the close proximity of molecules causes lateral intermolecular interactions to start acting as the coverage increases. The orientation analysis of the high coverage PVBA with fast Fourier transform (FFT) confirms that overall directional preference was not disturbed and the 20° and 40° chiral resolutions remain. In all three varying coverage situations, PVBA λ and δ configurations persist with these specific orientations. The result gives credence to our assertion that the adsorbate-substrate interaction is the dominant force present in this molecular system and controls the rotational and translational motion of the molecules, regardless of the quantity of molecules present on the substrate. The knowledge acquired through this study of chiral molecules on Pd(111) will contribute to the understanding of chiral recognition processes and to future applications, such as the development of chirality based pharmaceutical drugs.
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