Adlayer structures of dl-homocysteine and l-homocysteine thiolactone on Au(1 1 1) surface: an in situ STM study

Abstract

The adsorption of dl-homocysteine (Hcy) and l-homocysteine thiolactone (HTL) on Au(1 1 1) electrode was investigated in 0.1 M HClO 4 by cyclic voltammetry and in situ scanning tunneling microscopy (STM). Hcy and HTL molecules formed highly ordered adlayers on Au (1 1 1) surface. High-resolution STM images revealed the orientation and packing arrangement in the ordered adlayers. Hcy molecules formed (2 √ 3 × 3 √ 3)R30 • adlayer structure and H-bonds between carboxyl groups were assumed to be responsible for the origin of tail-to-tail or head-to-head molecular arrangement, while HTL molecules formed (4 × 6) adlayer structure, and two different orientations and appearances in the ordered adlayer were found. Structural models were proposed for the two adlayers.

Enantioselectivity of Redox Reaction of DOPA at the Gold Electrode Modified with a Self-Assembled Monolayer of Homocysteine

“…The amount of Hcy molecules self-assembled on gold was estimated from the electric charge for the reductive desorption of the SAM in 0.5 M KOH, and it was found to be approximately 1.8 × 10 14 molecules cm -2 . This value is close to the number of molecules expected from the (2√3 × 3√3) R 30° structure reported for the adlayer of dl -Hcy on the gold(111) surface in 0.1 M HClO 4 , which corresponds to a fractional coverage of one-ninth of the number density of surface gold atoms. If it is assumed that the nanosized open spaces or apertures between Hcy molecules in the SAM, through which bare gold atoms are exposed, allow further access of small molecules to the gold surface, the enantioselective blocking of the redox reaction of DOPA can be attributed to the steric hindrance between DOPA and the chiral space provided by arranged Hcy molecules.…”

“…23 Notably, the enantioselective blocking of the redox reaction of DOPA was not observed in the buffered 0.25 M K 2 SO 4 solution at pH 5.5, which is higher than the pK a of the carboxyl group in both Hcy and DOPA. As the intermolecular hydrogen bond between carboxyl groups is responsible for the formation of the ordered adlayer structure of Hcy in the acidic condition, 21 the orientation and conformation of Hcy molecules self-assembled on gold are considered to depend on pH, providing the different environment for nanosized spaces. 24 In summary, the Hcy-modified gold electrode with (111)-oriented surface was shown to exhibit enantioselectivity in the redox reaction of DOPA in acidic solution.…”

Effect of pH on the Enantiospecificity of Homocysteine Monolayer on Au(111) for the Redox Reaction of 3,4‐Dihydroxyphenylalanine

“…It appears quite reasonable to consider that the carboxy groups (COOH) in a Hcy molecule form a dimer through a hydrogen-bond, because the distance between the sulfur and the oxygen of carboxy group in Hcy is in good agreement with that between the bright spot and the less bright spot in Figure 3b. Based on the above analysis, a plausible structural model of l-Hcy SAM in acidic solution is proposed in Figure 3c, which is similar to the model [30] proposed by H-M Zhang et al for the adlayer of dl-Hcy containing the racemic mixture of d-Hcy and l-Hcy, although their STM images are considerably different from ours. These results suggest that l-Hcy molecules form a highly ordered (2 p 3 Â 3 p 3)R308 structure, and two homocysteine molecules exist in the unit cell.…”

“…The lengths of the sides, 1.0 nm and 1.5 nm, correspond to 2 p 3 and 3 p 3 times the distance between the nearest gold atoms, respectively. From the contrast of these spots, they are considered to represent sulfur atoms [19,30,31]. Spots with a less bright contrast are also seen, some of which are encircled by white dotted lines.…”

STM Characterization of Self-Assembled Monolayers of Cysteine Betaine on Au(111) Electrode in Perchloric and Sulfuric Acids

“…The formation of multiple adlattices of Cys-b on Au(111) in perchloric acid is unique for thiol molecules adsorbed at the electrified interface of Au(111). For example, the spatial arrangement of cysteine and homocysteine adsorbed on Au(111), before they are desorbed and oxidized, is unchanged with respect to the electrode potential. , This contrast is also manifested in the CV profiles, where multiple peaks are seen with Cys-b, and essentially featureless CV profiles are reported for cysteine and homocysteine. This comparison reflects the complicated intermolecular interactions and charged species at electrified interfaces, which both contribute to guide the spatial structures.…”