Self-assembly of monodispersed, chiral nanoclusters of cysteine on the Au(110)-(1 x 2) surface

Kühnle A, Linderoth TR, Besenbacher F. Self-assembly of monodispersed, chiral nanoclusters of cysteine on the Au(110)-(1 x 2) surface. Journal of the American Chemical Society. 2003;125(48):14680-14681

Chiral Symmetry Breaking Observed for Cysteine on the Au(110)-(1x2) Surface

Kühnle A, Linderoth TR, Besenbacher F. Chiral Symmetry Breaking Observed for Cysteine on the Au(110)-(1x2) Surface. Topics in Catalysis. 2011;54(19-20):1384-1391.A pronounced enantiomeric excess of LL-cysteine dimers is observed by scanning tunneling microscopy (STM) on the Au(110)-(1x2) surface after partial thermal desorption/decomposition of racemic cysteine. We systematically examine several possible origins for this intriguing observation of chiral symmetry breaking, including a chiral bias of the substrate, but remain unable to identify the source

Enantiospecific adsorption of cysteine at chiral kink sites on Au(110)-(1x2)

Kühnle A, Linderoth TR, Besenbacher F. Enantiospecific adsorption of cysteine at chiral kink sites on Au(110)-(1x2). Journal of the American Chemical Society. 2006;128(4):1076-1077

Global and local expression of chirality in serine on the Cu{110} surface

Establishing a molecular-level understanding of enantioselectivity and chiral resolution at the organic−inorganic interfaces is a key challenge in the field of heterogeneous catalysis. As a model system, we investigate the adsorption geometry of serine on Cu{110} using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The chirality of enantiopure chemisorbed layers, where serine is in its deprotonated (anionic) state, is expressed at three levels: (i) the molecules form dimers whose orientation with respect to the substrate depends on the molecular chirality, (ii) dimers of l- and d-enantiomers aggregate into superstructures with chiral (−1 2; 4 0) lattices, respectively, which are mirror images of each other, and (iii) small islands have elongated shapes with the dominant direction depending on the chirality of the molecules. Dimer and superlattice formation can be explained in terms of intra- and interdimer bonds involving carboxylate, amino, and β−OH groups. The stability of the layers increases with the size of ordered islands. In racemic mixtures, we observe chiral resolution into small ordered enantiopure islands, which appears to be driven by the formation of homochiral dimer subunits and the directionality of interdimer hydrogen bonds. These islands show the same enantiospecific elongated shapes those as in low-coverage enantiopure layers

L-cysteine adsorption structures on Au(111) investigated by scanning tunneling microscopy under ultrahigh vacuum conditions

Kühnle A, Linderoth TR, Schunack M, Besenbacher F. L-cysteine adsorption structures on Au(111) investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. Langmuir. 2006;22(5):2156-2160.Adsorption structures formed upon vapor deposition of the natural amino acid L-cysteine onto the (111) surface of gold have been investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. Following deposition at room temperature and at cysteine coverages well below saturation of the first monolayer, we found coexistence of unordered molecular islands and extended domains of a highly ordered molecular overlayer of quadratic symmetry. As the coverage was increased, a number of other structures with local hexagonal order emerged and became dominant. Neither of the room temperature, as-deposited, ordered structures showed any fixed rotational relationship to the underlying gold substrate, suggesting a comparatively weak and nonspecific molecule-substrate interaction. Annealing of the cysteine-covered substrate to 380 K lead to marked changes in the observed adsorption structures. At low coverages, the unordered islands developed internal order and their presence started to perturb the appearance of the surrounding Au(111) herringbone reconstruction. At coverages beyond saturation of the first monolayer, annealing led to development of a (root 3 x root 3)R30 degrees superstructure accompanied by the formation of characteristic monatomically deep etch pits, i.e., the behavior typically observed for alkanethiol self-assembled monolayers on Au(111). The data thus show that as-deposited and thermally annealed cysteine adsorption structures are quite different and suggest that thermal activation is required before vacuum deposited cysteine becomes covalently bound to single crystalline Au(111)

Growth of unidirectional molecular rows of cysteine on Au(110)-(1x2) driven by adsorbate-induced surface rearrangements

Kühnle A, Molina LM, Linderoth TR, Hammer B, Besenbacher F. Growth of unidirectional molecular rows of cysteine on Au(110)-(1x2) driven by adsorbate-induced surface rearrangements. Physical Review Letters. 2004;93(8):086101.Using scanning tunneling microscopy we have studied the nucleation and growth of unidirectional molecular rows upon adsorption of the amino acid cysteine onto the anisotropic Au(110)-(1x2) surface under ultrahigh vacuum conditions. By modeling a large variety of possible molecular adsorption geometries using density-functional theory calculations, we find that in the optimum, lowest energy configuration, no significant intermolecular interactions exist along the growth direction. Instead the driving force for formation of the unidirectional molecular rows is an adsorbate-induced surface rearrangement, providing favorable adsorption sites for the molecules

Exploring the transferability of large supramolecular assemblies to the vacuum-solid interface

We present an interplay of high-resolution scanning tunneling microscopy imaging and the corresponding theoretical calculations based on elastic scattering quantum chemistry techniques of the adsorption of a gold-functionalized rosette assembly and its building blocks on a Au(111) surface with the goal of exploring how to fabricate functional 3-D molecular nanostructures on surfaces. The supramolecular rosette assembly stabilized by multiple hydrogen bonds has been sublimed onto the Au(111) surface under ultra-high vacuum conditions; the resulting surface nanostructures are distinctly different from those formed by the individual molecular building blocks of the rosette assembly, suggesting that the assembly itself can be transferred intact to the surface by in situ thermal sublimation. This unanticipated result will open up new perspectives for growth of complex 3-D supramolecular nanostructures at the vacuum-solid interface

Adsorption of dodecanethiol on Cu(110): Structural ordering upon thiolate formation

Kühnle A, Vollmer S, Linderoth TR, Witte G, Wöll C, Besenbacher F. Adsorption of dodecanethiol on Cu(110): Structural ordering upon thiolate formation. Langmuir. 2002;18(14):5558-5565.The adsorption of dodecanethiol [CH3(CH2)(11)SH] films on Cu(110) by vapor deposition under ultrahigh vacuum conditions has been studied by means of thermal desorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction with a special emphasis on the structural changes accompanying the transition from a physisorbed monolayer to a chemisorbed saturation structure. Adsorption at 110 K leads to the formation of an ordered physisorbed layer with flat-lying thiol molecules. Upon room-temperature deposition, initially an ordered pinstripe phase is formed which may be a molecular double layer. This layer transforms with time into a stable saturation structure of upright-tilted thiolates in a local c(2 x 2) arrangement that exhibits a long-range c(12 x 16) modulation, attributed to a Moire pattern. The XPS measurements show that the room-temperature saturation structure contains a fraction of sulfide species formed by partial decomposition and desorption of alkyl chains. At 400 K, the thiolate monolayer desorbs dissociatively, eventually resulting in a p(5 x 2) sulfur structure

Chiral recognition in dimerization of adsorbed cysteine observed by scanning tunnelling microscopy

Kühnle A, Linderoth TR, Hammer B, Besenbacher F. Chiral recognition in dimerization of adsorbed cysteine observed by scanning tunnelling microscopy. Nature. 2002;415(6874):891-893.Stereochemistry plays a central role in controlling molecular recognition and interaction: the chemical and biological properties of molecules depend not only on the nature of their constituent atoms but also on how these atoms are positioned in space. Chiral specificity is consequently fundamental in chemical biology and pharmacology(1,2) and has accordingly been widely studied. Advances in scanning probe microscopies now make it possible to probe chiral phenomena at surfaces at the molecular level. These methods have been used to determine the chirality of adsorbed molecules(3-5), and to provide direct evidence for chiral discrimination in molecular interactions(6) and the spontaneous resolution of adsorbates into extended enantiomerically pure overlayers(3,7-9). Here we report scanning tunnelling microscopy studies of cysteine adsorbed to a (110) gold surface, which show that molecular pairs formed from a racemic mixture of this naturally occurring amino acid are exclusively homochiral, and that their binding to the gold surface is associated with local surface restructuring. Density-functional theory(10) calculations indicate that the chiral specificity of the dimer formation process is driven by the optimization of three bonds on each cysteine molecule. These findings thus provide a clear molecular-level illustration of the well known three-point contact model(11,12) for chiral recognition in a simple bimolecular system