The study the interaction of biomolecules on metal surfaces is of a great relevance due to the wide number of scientific and technological areas in which they are involved. Also, due to its simple structure, amino-acids can be used as a good model system to study biomolecule-surface interaction, which can indeed assist in the understanding of more complex systems. It has been found that a number of amino-acids self-organize to form well ordered-two-dimensional structures at metal surfaces. The local adsorption geometry and the two-dimensional self-assembly of S-cysteine are of particular interest because it is the only amino-acid having a thiol side group, and therefore can be used as a bonding group for self-assembled monolayers formation.
We present a characterization of cysteine adsorption on Au(111) at submonolayer coverage performed by a combination of experimental techniques (scanning tunnelling microscopy-STM, X-ray photoemission spectroscopy-XPS and ultraviolet photoemission spectroscopy-UPS) and theoretical ab-initio methods. XPS data show the adsorption of the zwitterionic form of S-cysteine molecule on Au(111) surface. Meaning the desprotonation of the carboxylic group (COO-) and the protonation of the amino group (NH3+). Furthermore, the interaction of the amino-acid on the surface goes through the COO- and the thiol group (S-Au), which is the anchoring point of the molecule. The STM characterization performed under UHV conditions at room temperature reveals an structural evolution with time of the Cys/Au(111) system. Just after cysteine dosing at room temperature, the diffusion of molecules on the surface is very high. These diffusing molecules coexist with both, molecular islands lacking of internal order, which appear mainly anchored to the step edges, and with long ordered molecular rows. After a period of time, the long molecular rows evolve to develop ordered molecular networks of S-cysteine. By a combination of STM images showing molecular structure and Density Functional Theory (DFT) calculations we present an atomic scale model for the self-organized molecular rows of S-cysteine on the Au(111) surface which consists of coupled molecules along the [112] surface direction.
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