The dipeptide, Arg–Cys, and the related molecule, Arg–cysteamine, are adsorbed to gold surfaces and the monolayers are characterized. Chemical binding and electronic structure of the monolayers are obtained by X-ray photoelectron spectroscopy (XPS). Strong molecular binding of the adsorbates to gold surface through the sulfur atom is attained. Orientation of the adsorbates on gold is studied using infrared reflection absorption spectroscopy (IRAS). Arg–Cys is interpreted to be adsorbed on gold in a compact configuration. The Arg–cysteamine molecule is adsorbed on gold with the main molecular axis perpendicular to the surface. Interaction of G-protein with the adsorbates was studied using the surface plasmon resonance (SPR) technique. It is believed that arginine has a major role in G-protein recognition since the G-protein-coupled receptor (GPCR) α₂A has an arginine-rich region in the G-protein-binding part of the third intracellular loop.

Interactions between the functional bovine brain G-protein and receptor-derived peptidea chemically adsorbed on gold surfaces are studied. The peptides are designed to mimic the third ic-loop (aa 361-373) of the Alpha 2a-adrenergic receptor (α ₂AR). These segments are linked to a surface using the thiol-gold chemistry, and the protein interaction studies are conducted to investigate the key function of recognition. The chemical composition and binding strength of the peptide monolayers onto a gold surface are characterized using X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. Strong molecular binding of the adsorbates to the gold surface is attained, and the presence of amide-related IR vibrations verified the composition of the peptides. Bovine brain G-protein adsorption studies on these molecular monolayers are performed using the surface plasmon resonance technique. The arginine-rich peptide, which is a direct mimicry of the receptor, has a higher affinity for G-protein than the lysine-rich and alanine-rich derived peptides, showing that arginine residue has special importance for the G-protein interaction with the receptor.

The affinity of α_2A-adrenergic receptor (α_2A-AR) derived peptide adsorbates for the functional bovine brain G-protein is studied in the search for the minimum sequence recognition. Three short peptides (GPR-i2c, GPR-i3n, and GPR-i3c) are designed to mimic the second and third intracellular loops of the receptor. X-ray photoelectron spectroscopy is used to study the chemical composition of the peptides and the binding strength to the surfaces. Chemisorption of the peptides to the gold substrates is observed. Infrared spectroscopy is used to study the characteristic absorption bands of the peptides. The presence of peptides on the surfaces is verified by prominent amide I and amide II bands. The interaction between the peptides and the G-protein is studied with surface plasmon resonance. It is shown that GPR-i3n has the highest affinity for the G-protein. Equilibrium analysis of the binding shows that the G-protein keeps its native conformation when interacting with GPR-i3c, but during the interaction with GPR-i2c and GPR-i3n the conformation of G-protein is changed, leading to the formation of aggregates and/or multilayers.

Tyrosine-terminated propanethiol (TPT), tyrosine linked to 3-mercaptopropionic acid through an amide bond, is adsorbed to gold surfaces. The adsorbates are characterized by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). XPS is used to investigate the chemical binding and electronic structure of the monolayer. Strong molecular binding of the tyrosine derivative on the gold surface through the sulfur atom is attained. Angle-dependent XPS results shows that TPT molecules are oriented with the sulfur atoms molecularly oriented close to the gold surface and that the phenol moiety is oriented away from the gold surface. Average orientation of the adsorbate on gold is deduced using the NEXAFS results. It shows that the main molecular axis is tilted approximately 38° relative to the Au surface normal. The ring plane of the phenol moiety exhibits a preferential orientation with an average tilt angle of the normal of the ring plane from the surface normal of about 60°.

3,4-Dihydroxyphenylalanine-terminated propanethiol (DOPA-PT), an amino acid DOPA linked to 3-mercaptopropionic acid through an amide bond, is adsorbed and self-assembled to polycrystalline gold surfaces. The structure of the adsorbates was characterized by means of X-ray photoelectron spectroscopy (XPS), infrared reflection−absorption spectroscopy (IRAS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Strong molecular binding of a DOPA derivative on gold surfaces through the sulfur atom was attained. Angle-dependent XPS results showed that the aromatic ring is oriented away from the gold surface. Both IRAS and NEXAFS results showed parallel orientation of the C=O bond of the amide moiety relative to the gold surface. Hydrogen bonding between amide moieties is achieved, and it seemed to provide additional orientation stabilization. Deduced orientation of the amide moiety on the short alkyl chain (or the peptide plane) is assumed to give the average orientation of the main molecular axis. The main molecular axis is estimated to have an average tilt angle of approximately 37° relative to the gold surface normal based on NEXAFS results. The aromatic ring exhibits a preferred orientation with an average tilt angle of about 64°. The experimental results showed that DOPA-thiol with amide linkage is able to self-assemble and form a layered structured film consisting of a layer of alkane chains with a gauche conformation beneath an oriented layer of DOPA.

Monolayers of tert-butyl carbamate-terminated thiol were formed by adsorption of the molecules onto polycrystalline gold substrate. The adsorbates were studied using techniques as X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and infrared reflection−absorption spectroscopy (IRAS). The results provide the electronic structure, composition, characteristic fingerprint, and orientation of the molecular adsorbate. XPS verified that the thiolate group is chemically bonded to the gold surface and that a complete chemisorption of the molecule occurs. Elemental depth profiling by varying the excitation energy in XPS supports the angle-dependent XPS results. Both techniques showed that the tert-butyl group is oriented away from the gold surface. A nearly parallel orientation of the carbonyl group relative to the gold surface is deduced from the IRAS results. The main molecular axis is estimated to have an average tilt angle of about 38° relative to the gold surface normal on the basis of the NEXAFS results. Cyclic voltammetry indicates a less blocking capability of the adsorbates. Overall, the molecules are oriented in an upright manner with indications of presence of pinholes and/or defects possibly due to steric hindrance of the bulky tert-butyl group. This molecular system is envisioned to be of use for surface-based organic synthesis on gold substrates.

Phosphorylated tyrosine analogue molecules (pTyr-PT) were assembled onto gold substrates, and the resulting monolayers were used for metal ion interaction studies. The monolayers were characterized by X-ray photoelectron spectroscopy (XPS), infrared reflection−absorption spectroscopy (IRAS), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), both prior to and after exposure to metal ions. XPS verified the elemental composition of the molecular adsorbate and the presence of metal ions coordinated to the phosphate groups. Both the angle-dependent XPS and IRAS results were consistent with the change in the structural orientation of the pTyr-PT monolayer upon exposure to metal ions. The differential capacitance of the monolayers upon coordination of the metal ions was evaluated using EIS. These metal ions were found to significantly change the capacitance of the pTyr-PT monolayers in contrast to the nonphosphorylated tyrosine analogue (TPT). CV results showed reduced electrochemical blocking capabilities of the phosphorylated analogue monolayer when exposed to metal ions, supporting the change in the structure of the monolayer observed by XPS and IRAS. The largest change in the structure and interfacial capacitance was observed for aluminum ions, compared to calcium, magnesium, and chromium ions. This type of monolayer shows an excellent capability to coordinate metal ions and has a high potential for use as sensing layers in biochip applications to monitor the presence of metal ions.