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A molecular tip about chirality
YOSHIO UMEZAWA
The investigators then generated four Chiral surfacesOmonolayers consisting of monolayers to assign chirality to molecules surfaces for analysis with the chiral molecular within the racemic monolayer. Within the chiral moleculesOare becoming increasingly important in the catalysis and separa- tips: D-cysteine, L-cysteine, racemic cysteine, racemic cysteine surface, the R molecular tion of pure chiral molecules, which are and achiral 3-mercaptopropionic acid. These tip found regions with more and less tunuseful as pharmaceuticals and in other neling, indicating interactions with Dapplications. However, the study of and L-cysteine molecules, respecchiral surfaces has been limited because tively. The S molecular tip performed of a lack of analytical techniques that similarly but with opposite chiral can detect chirality on a single-molpreferences. ecule basis. The achiral 3-mercaptopropionic In a new AC paper (DOI 10.1021/ acid surface elicited similar tunneling ac800818f), Yoshio Umezawa and responses from the R and S molecular Tomoaki Nishino at the University of tips, verifying that chirality itself is Tokyo demonstrate the ability of a responsible for the observed tunnelchiral molecular tip to distinguish being properties between the tips and tween enantiomers on a chiral surface the chiral surfaces. one molecule at a time. This “molecuFinally, to better understand the lar tip is the first to discriminate singleatomic-level surface structure, molecule chirality and is able to visualUmezawa and Nishino used theoretiize chirality of each molecule within a cal calculations to generate models of monolayer without any restriction on the orientation of the cysteine molthe sample,” says Umezawa. ecules and then used their observaChiral molecular tips are built on tions with the chiral tips to model standard scanning tunneling microsmolecular arrangements for the copy (STM) metal tips, which can be monolayers. The models show that Lused to explore the morphological and D-cysteine monolayers are mirror Schematic diagram of a chiral molecular tip interacting properties of surfaces but not to disimages. The researchers propose that with a monolayer of two sample isomers on a gold surcriminate among types of atoms or these arrangements are the result of face. Differences in the interactions between tip and isomolecules. STM detects when elecinteractions among the cysteine molmer are determined by chirality and are detected by trons jump between two conductors in variations in electron tunneling efficiency between the ecules themselves and between the close proximity; the idea behind using chiral tip and the surface molecule. cysteine molecules and the underlychiral molecular tips is that this elecing structure, in this case gold. tron jumping can be facilitated or reduced The models indicate that hydrogen monolayers were created by immersing a by favorable or unfavorable interactions bonding between cysteine molecules is the gold, single-crystal bead in an aqueous solubetween a chiral molecule on a tip and central determinant of the enantiopure tion of the desired surface molecules, which another chiral molecule on a surface of monolayers’ structures. In contrast, the racethen, says Umezawa, formed a covalent interest. The concept is similar to a chiral bond with the gold atoms via the molecules’ mic monolayer is mostly disordered because stationary phase, which has more favorable cysteinyl thiol groups. the hydrogen bond network is disrupted by interactions with one enantiomer than the the presence of both enantiomers. However, Using the R molecular tip, the researchother, facilitating separation. Umezawa says that within the racemic ers observed greater tunneling with D-cysIn this study, the researchers first admonolayer were pockets of enantiopure doteine than with L-cysteine. The S molecular sorbed a monolayer of (S)- or (R)-Nmains, which were observed with the molectip had the opposite tunneling properties: (3,5-dinitrobenzoyl)cysteine n-propyl- greater tunneling with L-cysteine than D-cys- ular tips. amide molecules on a gold STM needle. “The molecular tip thus provides a first teine. The standard, underivatized STM Umezawa says the needle is very thin, way to investigate chirality of a variety of metal tip yielded less tunneling overall and and thus only a single chiral molecule identical tunneling efficiency between L- and functional chiral surfaces,” says Umezawa. protrudes from the tip. Only one S or R “With the molecular tip we have tried to D-cysteine, confirming the chiral blindness molecule interacts with the chiral surface of standard STM. visualize what remains unseen by convenbeing probed; multiple molecular intertional STM but [is] vital for a scientific Next, the authors used the tunneling actions would result in tunneling profile advance.” efficiencies observed between the chiral —Erika Gebel distortions. tips and enantiopure L- and D-cysteine 7182
ANALYTICAL CHEMISTRY /
OCTOBER 1, 2008
10.1021/AC801692X 2008 AMERICAN CHEMICAL SOCIETY
Published on Web 09/30/2008
