Article pubs.acs.org/JPCC
Investigating the Enantioselectivity of Alanine on a Chiral Cu{421}R Surface Lars Thomsen,†,‡ Anton Tadich,†,§ Daniel P. Riley,∥,⊥ Bruce C. C. Cowie,† and Michael J. Gladys*,‡ †
Australian Synchrotron, Clayton, VIC 3168, Australia School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW 2308, Australia § Department of Physics, La Trobe University, Bundoora, VIC 3083, Australia ∥ Institute of Materials Engineering, ANSTO, Lucas Heights, NSW 2234, Australia ⊥ Department of Mechanical Engineering, University of Melbourne, Melbourne, VIC 3010, Australia ‡
ABSTRACT: Saturated chemisorbed layers of D- and Lalanine amino acids were adsorbed onto a chiral Cu{421}R surface. Very little enantioselectivity was observed in the HRXPS thermal desorption; however, there was a large difference in the dissociation products left on the surface between the two enantiomers. NEXAFS spectroscopy also exhibited only minor differences in the molecular adsorption of the two enantiomer layers; however, low-energy electron diffraction (LEED) images revealed significant differences in the structure of the adsorbed layers. D-alanine forms a p(2 × 3) overlayer while L-alanine forms a combination of p(1 × 2) overlayer and faceted steps oriented in the [−110] direction. Intermolecular bonding and steric effects play a significant role in these stereoselective differences by maximizing the hydrogen bonding between the molecules. This study clearly shows that single chiral tests are not adequate to ascertain the true enantioselective properties of the given system.
1. INTRODUCTION It is well-known that a chiral surface can display enantioselectivity with regards to chiral adsorbates1−5 in a “lock and key” scenario where the chiral adsorbents (i.e., “the key”) will, for example, stick more easily to one chiral surface (“the lock”) over the opposite chiral surface and vice versa. As such, a “matching” chiral surface (designated either R or S6,7) will contain a preferred geometry for a particular molecule and be able to distinguish between the enantiomers of chiral molecular adsorbates. This stereoselectivity is similar to that observed for enzymes; however, the mechanism behind the matching surface/molecule pair interactions is still somewhat elusive. To gain a more comprehensible insight into enantiospecific interactions between chiral adsorbates and chiral substrates, it is often useful to simplify the system. As such, many studies have been conducted involving adsorption of chiral molecules on achiral substrates (ref 8 and references therein), chiral molecules on chiral substrates (refs 1, 9−16 and references therein), or achiral molecules on chiral substrates (ref 17 and references therein) in order to acquire additional knowledge into the phenomenon of chiral adsorption and stereoselectivity. Smerieri et al.8 performed a recent study into the selfassembly of chiral (S)-glutamine acid molecules on an achiral Ag{100} surface and found that H bonds between carboxylic groups form a square OCOH−OCOH−OCOH−OCOH structure via intermolecular bonding, which was concluded to be the main driving force behind the self-assembly of this © 2012 American Chemical Society
system. Studies of adsorption of small and medium sized chiral molecules on chiral surfaces1,9−16 showed differences in stereoselectivity depending on the system. Some of the systems that showed the largest chiral selectivity were those investigated by Greber et al.14 and Gladys et al.15 The former group demonstrated that D- and L-cysteine adsorbs onto an Au{17 11 9} surface in a “two point” footprint with two different enantiomeric conformations while the latter group examined the “three point” adsorption of alanine enantiomers on Cu{531}R and found that they attach to the first and second layer surface atoms in both linear and kinked conformations depending on the enantiomers used. Further studies of a Cu{531}S surface by Thomsen et al.16 showed the adsorption footprint of L-cysteine and L-methionine to be “triangular” and “quadrangular”, respectively. As observed with alanine adsorption on Cu{531}, L-cysteine and L-methionine also occupy all adsorption sites on the {110} and {311} microfacets of the Cu{531}S surface, and these facets do exhibit enantioselectivity.15,17 Through recent work1,15,16 it is also becoming clear that the adsorption sites alone do not define the enantioselectivity of adsorbing molecules. The key interactions that cause enantioselectivity on chiral interfaces are: (1) Chiral adsorption Received: August 15, 2011 Revised: March 28, 2012 Published: April 4, 2012 9472
dx.doi.org/10.1021/jp207847j | J. Phys. Chem. C 2012, 116, 9472−9480
The Journal of Physical Chemistry C
Article
sites (microfacets). (2) Adsorption kinetics at chiral sites: the initial adsorption of a chiral molecule can influence the adsorption geometry of subsequent molecules.a (3) Steric effects (is there enough space?) and intermolecular bonding between the adsorbates. Intermolecular bonding has the potential to either reduce or enhance substrate enantioselectivity depending on the geometry of the adsorbed molecule and the proximity of neighboring molecules. As an example, the enantioselective adsorption observed on chiral modified Pd{111} and Pt{111} surfaces18−21 was attributed to the hydrogen-bonding interaction between the propylene oxide probe molecule and the OH group in the 2-butanol chiral modifier. However, when this modifier−probe combination was used on low index copper surfaces, no enantioselectivity was observed.22 It was concluded that the critical hydrogen-bonding interactions occur between adjacent molecules at the surface and in effect inhibit the enantioselectivity. The aforementioned study on the intrinsically chiral copper surface Cu{531}R shows that alanine adsorbs onto {311}- and {110}-orientated microfacets on the surface (as alaninate) in such a way that the molecules on the {110} facet have the ability to pair up via a hydrogen-bonded link between the NH3 moiety and the deprotonated oxygen atom on an adjacent molecule. The rate at which the {110} and {311} microfacets are filled when L-alaninate is adsorbed onto a Cu{531}R substrate is unambiguous, but twice as many molecules are adsorbed onto the {110} microfacet than on the {311} microfacet when D-alaninate is adsorbed.1,15 A recent paper by Eralp et al.17 concerning the adsorption of achiral glycine onto Cu{531} revealed similar intermolecular hydrogen-bonded links between the NH3 and the oxygen moieties of neighboring molecules. The study presented here investigates the adsorption of Dand L-alanine on Cu{421}R. From previous work conducted with alanine on copper surfaces, it is recognized that alanine adsorb onto these surfaces as alaninate (CH3−CH(NH2)− COO).3,15,23−25 From studies of alanine on Cu{531}R it was determined that the amino acid is expected to form a triangular footprint on the surface via bonding through the nitrogen and oxygen atoms on {311} and {110} microfacets.15 The {311} and {110} microfacets on Cu{421} are identical to the microfacets on the {531} surface; however, the Cu{421} surface differs from the Cu{531} surface by having a unit cell that is one atom longer in the [−110] direction, and hence a lower surface kink-site density, leading to the chiral adsorption sites being further apart. This generates a slightly larger distance between adsorbing molecules and substantially decreases the intermolecular interactions on the surface. In this way we can target the contribution of intermolecular bonding and adsorption site interaction to enantioselectivity. The similarity between adsorption sites on the Cu{531} and Cu{421} surface is illustrated on Figure 1 where (among other) two different triangles show the {311} and the {110} microfacet adsorption sites on both surfaces (with the Cu{531} surface inserted in the top left-hand corner for comparison). The local chiral sites are identical between the two surfaces but are further apart on the Cu{421} surface. It should be noted, to avoid confusion, that while the atomic arrangements around the chiral sites (referring to the 6-coordinate surface atoms which defines the chirality) are identical for both Cu{531} and Cu{421}, the {311} and {110} microfacets around the chiral sites on the Cu{531} surface consist of atoms from the first and second surface layers.
Figure 1. Schematic model of the atoms on a bulk-terminated Cu{421}R. Added to the model at the bottom are the most relevant crystallographic directions along with the surface unit cell. At the top the {311} and {110} microfacets are specified. The insert is of Cu{531}R with its equivalent {311} and {110} microfacets.
However, for the Cu{421} surface this is only true for the {311} microfacet, whereas the {110} microfacet consists of atoms from the first and third atomic surface layers. The results in this publication indicate that the adsorption of alanine on Cu{421} show a considerable decrease in enantioselectivity compared to Cu{531}. This selectivity was probed via the CO π* transition in carbon K-edge near-edge X-ray absorption fine structure (NEXAFS) scans which elucidate the geometrical conformations of the molecules. Despite the low level of enantioselectivity of this surface, X-ray photoelectron spectroscopy (XPS) does show differences in the thermal stability of the D- and L-alanine on the Cu{421}R surface, with D-alanine decomposing differently than L-alanine. The elemental interactions observed in this study are part of an ongoing investigation that will provide a greater understanding of amino acid interaction with chiral interfaces.
2. EXPERIMENTAL DETAILS A Cu{421}R single crystal with a
