Structures of Glycine, Enantiopure Alanine, and Racemic Alanine Adlayers on Cu(110) and Cu(100) Surfaces

Abstract

We have determined the structures of dense adlayers of glycine and alanine on the Cu(110) and Cu(100) surfaces using plane wave density functional theory. These calculations resolve several experimental controversies regarding these structures. Glycine exists on Cu(110) as a single adlayer structure, while on Cu(100) two distinct glycine adlayers coexist. The glycine structures serve as useful starting points for constructing alanine adlayer structures. We considered separately the adsorption of enantiopure alanine and racemic alanine on each surface. Adlayers of enantiopure alanine are found to be closely related to the adlayers observed for glycine. Racemic alanine adlayers on Cu(110) are structurally analogous to those observed for glycine on this surface and adopt a pseudo-racemate ordering. On Cu(100), in contrast to glycine, racemic alanine is found to adopt a single adlayer structure that is an ordered racemate. Spontaneous segregation of molecular enantiomers does not occur in racemic adsorbed mixtures on either surface. Consideration of the orientationally distinct domains that may exist for each adlayer on these surfaces provides important information for the interpretation of the adlayer domain boundaries that are commonly observed in scanning tunneling microscopy images of amino acid adlayers. Examining this set of amino acid adlayers provides useful insight into the range of subtle behaviors that can arise in these and related systems where chiral molecules form ordered adlayers on flat metal surfaces.

Molecular chirality at surfaces

“…As observed for the glycine (2 Â 3) structure, calculations for the pure enantiomers favor a structure including opposite footprints, as if the methyl group would have no influence. For the racemate the two enantiomers coexist in a single domain and remain true enantiomers by occupying the opposite footprint [141][142][143]. Basically, the same conclusion was made for racemic alanine on Cu(100) [142].…”

“…One of the four diastereomers is the enantiomer of one of the others. Such diastereoisomerism has been discussed controversially for the (2 Â 3) structure of alanine on Cu(110) [139][140][141][142][143][144]. As observed for the glycine (2 Â 3) structure, calculations for the pure enantiomers favor a structure including opposite footprints, as if the methyl group would have no influence.…”

Adsorption and self-assembly of bio-organic molecules at model surfaces: A route towards increased complexity

“…At step sites on the surface, intrinsically chiral Cu (3,1,17) and Cu(3,1,17) facets form by adsorbateinduced reconstruction [139]. Dense adlayers of glycine and alanine structures on the Cu(110) and Cu(100) surfaces have been determined using plane wave density functional theory [142]. These calculations resolve several experimental controversies regarding these structures.…”

Chiral segregation driven by a dynamical response of the adsorption footprint to the local adsorption environment: bitartrate on Cu(110)

“…1,[9][10][11][12][13][14][15] Recent work has shown that the adsorption footprints adopted by individual molecules also play a critical role in determining the local and global organization of the adsorbed molecules. 16,17 For example, in racemic systems that contain an equal population of right-and left-handed molecules, adsorption footprints are key in determining whether the system organizes into an ordered heterochiral array, spontaneously segregates to form distinct homochiral domains, or assembles as a solid solution with a random arrangement of enantiomers, 18,19 with each arrangement expected to elicit very different physical, chemical and biological responses. The assembly of enantiopure bitartrate on Cu(110) represents the first global homochiral surface reported in the literature and has become a model system, serving as a benchmark for understanding chiral phenomena in 2D systems.…”