Adsorption Of Cysteine Research Articles

Inhibition of copper corrosion by modifying cysteine self‐assembled film with alkylamine/alkylacid compounds

AbstractComplex self‐assembled monolayers (SAMs) were prepared by modifying the adsorption of cysteine with dodecylacid (DAC) and with dodecylamine (DAM) on copper surfaces. Their protective effects against copper corrosion were investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in 0.5 M HCl aqueous solution. Results show that SAMs suppress cathodic current densities and shift the corrosion potential toward more negative values. Two types of complex SAMs enhance the anticorrosion effect of cysteine SAMs. PM3 semi‐empirical quantum calculations were used to obtain the quantum chemical parameters. The complex SAMs formed from cysteine and DAM have the higher EHOMO level and the better protection effect.

Cysteine, thiourea and thiocyanate interactions with clays: FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry studies

The present study examined the adsorption of cysteine, thiourea and thiocyanate on bentonite and montmorillonite at two different pHs (3.00, 8.00). The conditions used here are closer to those of prebiotic earth. As shown by FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry, the most important finding of this work is that cysteine and thiourea penetrate into the interlayer of the clays and reduce Fe(3+) to Fe(2+), and as consequence, cystine and c,c'-dithiodiformamidinium ion are formed. This mechanism resembles that which occurs with aconitase. This is a very important result for prebiotic chemistry; we should think about clays not just sink of molecules, but as primitive vessels of production of biomolecules. At pH 8.00, an increasing expansion was observed in the following order for both minerals: thiourea > thiocyanate > cysteine. At pH 3.00, the same order was not observed and thiourea had an opposite behavior, being the compound producing the lowest expansion. Mössbauer spectroscopy showed that at pH 8.00, the proportion of Fe(2+) ions in bentonite increased, doubling for thiourea, or more than doubling for cysteine, in both clays. However, at pH 3.00, cysteine and thiourea did not change significantly the relative amount of Fe(2+) and Fe(3+) ions, when compared to clays without adsorption. For thiocyanate, the amount of Fe(2+) produced was independent of the pH or clay used, probably because the interlayers of clays are very acidic and HSCN formed does not reduce Fe(3+) to Fe(2+). For the interaction of thiocyanate with the clays, it was not possible to identify any potential compound formed. For the samples of bentonite and montmorillonite at pH 8.00 with cysteine, EPR spectroscopy showed that intensity of the lines due to Fe(3+) decreased because the reaction of Fe(3+)/cysteine. Intensity of EPR lines did not change when the samples of bentonite at pH 3.00 with and without cysteine were compared. These results are in accordance with those obtained using Mössbauer and FT-IR spectroscopy.

Enantiospecific Adsorption of Amino Acids on Hydroxylated Quartz (0001)

Density functional theory calculations have been used to study the adsorption of glycine, alanine, serine, and cysteine on the hydroxylated (0001) surface of alpha-quartz. We found negligible differences in adsorption energies for the most stable minima of enantiomers of alanine on this surface. There are, however, measurable energy differences between the two enantiomers of both serine and cysteine in their most stable states. The source of this enantiospecificity is mainly the difference in the strength of hydrogen bonds between the surface and the two enantiomers. Our results provide initial information on how amino acids can exhibit enantiospecific adsorption on hydroxylated quartz surfaces.

Density functional study of the cysteine adsorption on Au nanoclusters

The adsorption of the cysteine amino acid (H-SCβH2-CαH-NH2-COOH) on the Au55 cluster is investigated through density functional theory calculations. Two isomers, with icosahedral (Ih )a nd chiral (C1) geometries, of the Au55 cluster are used to calculate the adsorption energy of the cysteine on different facets of these isomers. Results, only involving the S(thiolate)-Au bonding show that the higher adsorption energies are obtained when the sulfur atom is bonded to an asymmetrical bridge site at the facet containing Au atoms with the lowest coordination of the C1 cluster isomer. PACS. 36.40.-c Atomic and molecular clusters - 36.40.Jn Reactivity of clusters - 68.43.Bc Ab initio calculations of adsorbate structure and reactions

Submolecular Electronic Mapping of Single Cysteine Molecules by in Situ Scanning Tunneling Imaging

We have used L-cysteine (Cys) as a model system to study the surface electronic structures of single molecules at the submolecular level in aqueous buffer solution by a combination of electrochemical scanning tunneling microscopy (in situ STM), electrochemistry including voltammetry and chronocoulometry, and density functional theory (DFT) computations. Cys molecules were assembled on single-crystal Au(110) surfaces to form a highly ordered monolayer with a periodic lattice structure of c(2x2) in which each unit contains two molecules; this conclusion is confirmed by the results of calculations based on a slab model for the metal surface. The ordered monolayer offers a platform for submolecular scale electronic mapping that is an issue of fundamental interest but remains a challenge in STM imaging science and surface chemistry. Single Cys molecules were mapped as three electronic subunits contributed mainly from three chemical moieties: thiol (-SH), carboxylic (-COOH), and amine (-NH2) groups. The contrasts of the three subunits depend on the environment (e.g., pH), which affects the electronic structure of adsorbed species. From the DFT computations focused on single molecules, rational analysis of the electronic structures is achieved to delineate the main factors that determine electronic contrasts in the STM images. These factors include the molecular orientation, the chemical nature of the elements or groups in the molecule, and the interaction of the elements with the substrate and tip. The computational images recast as constant-current-height profiles show that the most favorable molecular orientation is the adsorption of cysteine as a radical in zwitterionic form located on the bridge between the Au(110) atomic rows and with the amine and carboxyl group toward the solution bulk. The correlation between physical location and electronic contrast of the adsorbed molecules was also revealed by the computational data. The present study shows that cysteine packing in the adlayer on Au(110) from the liquid environment is in contrast to that from the ultrahigh-vacuum environment, suggesting solvent plays a role during molecular assembly.

Protonation and deprotonation of cysteine and cystine monolayers probed by impedance spectroscopy

The desorption mechanism for electrochemically-formed self-assembled monolayers from cysteine and cystine on a Au(1 1 1) electrode was investigated. The monolayer desorption in 0.1 M KClO 4 is found to occur in two steps with differing potentials. At potentials more positive than the desorption potential (circa −0.65 V vs. SCE), a quasi-reversible process, assigned to protonation/deprotonation, is observed. This process is found to occur at very discreet potentials when cysteine monolayers are probed by impedance spectroscopy. Monolayers derived from the disulfide (cystine) are found to protonate/deprotonate over a wider potential range. This suggests that electrochemically-formed monolayers obtained from disulfides do not afford the same level of organization as those derived from thiol species. The value of the double layer capacitance increases from 15 to 30 μF cm −2 after deprotonation. This suggests that the adsorption of cysteine weakens following the deprotonation. As expected, the protonation/deprotonation step is pH dependent and it is not observed in strong basic medium (0.1 M NaOH).

Formation and Decomposition of Water Soluble Colloidal Manganese Dioxide During the Reduction of by Cysteine. A Kinetic Study

Upon addition of cysteine to a solution of , a transient species appears within the time of mixing, which is stable for several months. In order to conform the nature of the transient species, spectroscopic, kinetic and coagulation experiments were carried out. For the formation of water-soluble colloidal MnO2, the stiochiometry was found to be 1:1 (cysteine:). On adding of HClO4, there is a decrease in the absorbance of colloidal MnO2. At higher [cysteine] (≥2.0 × 10−4 mol dm−3), as the reaction became so fast, the formation and decomposition of colloidal MnO2 are not observed. Thus in presence of [HClO4] and excess of [cysteine], the intermediate colloidal MnO2 is unstable. Kinetic studies of the decomposition of water soluble colloidal MnO2 have been investigated by visible spectrophotometry at 25°C. The observed kinetics was explained by assuming fast adsorption of cysteine on the surfaces of colloidal MnO2. The adsorption formation constants between MnO2-cysteine and MnO2-F− have been determined (Kad = 8.5 × 103 dm3 mol−1 and Kf = 4.9 × 104 dm3 mol−1 for cysteine and F−, respectively). The deposition of solid MnO2 was observed in presence of externally added Mn(II) and Ag+ ions. On the basis of the various observations, a most plausible mechanism has been envisaged for the decomposition of water soluble colloidal MnO2.

Interfacial Behavior of Cysteine between Mild Steel and Sulfuric Acid as Corrosion Inhibitor

Interfacial behavior of cysteine (Cys) between mild steel and sulfuric acid solution as a corrosion inhibitor has been studied with electrochemical AC (alternating current) and DC (direct current) techniques at (25.0±0.1) °C. The AC impedance results were evaluated using equivalent circuits in which a constant phase element (CPE) has been replaced with double layer capacitance ( C dl) to represent the frequency distribution of experimental data. Changes in impedance parameters (charge transfer resistance and double layer capacitance) indicated that cysteine molecules acted by accumulating at the metal/solution interface. The fractional coverage of the metal surface (ϑ) was determined using AC impedance results and it was found that the adsorption of cysteine on the mild steel surface followed a Langmuir isotherm model with a standard free energy of adsorption (ΔG 0 ads) of −35.1 kJ·mol −1. To clarify the type of interaction between mild steel surface and cysteine molecules with a molecular orbital approach, electronic properties, such as, the highest occupied molecular orbital (HOMO) energy, the lowest unoccupied molecular orbital (LUMO) energy, and the frontier molecular orbital coefficients have been calculated. Energy gaps for the interaction of mild steel surface and cysteine molecules ( E LUMO Fe− E HOMO Cys and E LUMO Cys− E HOMO Fe) were used to determine whether cysteine molecules acted as electron donors or electron acceptors when they interacted with the mild steel surface. The local reactivity was evaluated through the condensed Fukui indices. Theoretical calculations were carried out using the density functional theory (DFT) at B3LYP level with the 6-311++G(d,p) basis set for all atoms by Gaussian 03W program.

Adsorption of Cysteine on Cu(110) Studied by Core-Level Photoelectron Spectroscopy

Adsorption and ordering of cysteine molecules on Cu(110) surface have been studied by photoelectron spectroscopy using synchrotron radiation and low-energy electron diffraction (LEED). Analysis of S 2p, N 1s, O 1s, and C 1s core levels as a function of the cysteine coverage on Cu(110) reveals quite different behavior from the previously studied case on a Au(110) surface. At low coverages there evolve sequentially two types of thiolates with the S 2p binding energy difference of 0.85 eV. The relative population of the two thiolates induces a structural change in the ordering from 2 × 1 to c(2 × 2). Only above half the saturation coverage the LEED pattern becomes diffuse and a configurational change of the molecule by protonation of the functional groups occurs. Such an unusual evolution of the cysteine adsorption on the Cu(110) surface is discussed in relation to chemical reactions and possible molecular rearrangement on the surface.

Supramolecular Assembly Facilitating Adsorbate-Induced Chiral Electronic States in a Metal Surface

Through the application of optically active second-harmonic generation measurements (OA-SHG) we have demonstrated that the adsorption of amino acids cysteine (HSCH(2)CHNH(2)COOH) and penicillamine (HSC(CH3)(2)CHNH(2)COOH) from solution can induce chiral electronic states in an initially achiral polycrystalline Au film. The chiral induction is strongly dependent upon the pH of the deposition solution; adsorption of penicillamine and cysteine under acidic conditions (pH = 3) induces the same level of optical activity, whereas at pH = 11, the optical activity induced by cysteine is reduced by ca. 50% and penicillamine does not induce optical activity at all. The pH dependence indicates that the presence of interadsorbate hydrogen bonds, and consequently the supramolecular assembly of the adsorbates, facilitates the induction of chiral electronic states in the Au surface. This observation demonstrates that the symmetry properties of the extended structure of the self-assembled layer, and not the local adsorption geometry of the isolated adsorbed moiety, play the lead role in the induction of chiral metallic electronic states. The dependence of the chiral induction on COOH groups is identical to that observed in studies of optical activity in chiral thiol-protected nanoparticles, suggesting a common mechanism for the chiral perturbation in extended films and nanoparticles.

Cysteine adsorption on the Au(111) surface and the electron transfer in configuration of a scanning tunneling microscope: A quantum-chemical approach

Adsorption of two forms, molecule and radical, of amino acid L-cysteine (Cys) on the Au12 cluster that simulates the (111) face of single-crystal gold is studied in the framework of the density functional theory. Effects of solvation of adsorbed Cys particles and lateral interaction in a monolayer are analyzed. The simulation predicts a commensurate adsorption energetics of the molecule and radical, with a difference between the “on-top,” “hollow,” and “bridge” positions. An analysis of lateral electrostatic interactions points to the stability of a cluster comprising six Cys particles, which conforms to the size of a fragment observed experimentally. Adsorption calculations are used to build three-dimensional isosurfaces (STM images), where the tungsten needle of the scanning tunneling microscope is simulated by a tungsten atom or by small clusters. The calculated images are sensitive to both the Cys shape and the orientation of adsorbed Cys particles. Calculation results are compared with fresh in situ submolecular-resolution STM data. Simulated images (with commensurate contributions made by sulfur atom and amino group) built for Cys radical adsorbed in the “on-top” position give best conformance to experiment.

Evaluation of cysteine as environmentally friendly corrosion inhibitor for copper in neutral and acidic chloride solutions

The efficiency of cysteine as a non-toxic corrosion inhibitor for copper metal in 0.6 M NaCl and 1.0 M HCl has been investigated by electrochemical studies. Potentiodynamic polarization measurements and electrochemical impedance spectroscopy “EIS” were used to study the effect of cysteine on the corrosion inhibition of copper. Inhibition efficiency of about 84% could be achieved in chloride solutions. The presence of Cu 2+ ions increases the inhibition efficiency to 90%. Potentiodynamic polarization measurements showed that the presence of cysteine in acidic and neutral chloride solutions affects mainly the cathodic process and decreases the corrosion current to a great extent and shifts the corrosion potential towards more negative values. The experimental impedance data were analyzed according to a proposed equivalent circuit model for the electrode/electrolyte interface. Results obtained from potentiodynamic polarization and impedance measurements are in good agreement. Adsorption of cysteine on the surface of Cu, in neutral and acidic chloride solutions, follows the Langmuir adsorption isotherm. The adsorption free energy of cysteine on Cu (∼−25 kJ mol −1) reveals a strong physical adsorption of the inhibitor on the metal surface.

L-Cysteine Adsorption Structures on Au(111) Investigated by Scanning Tunneling Microscopy under Ultrahigh Vacuum Conditions

Adsorption structures formed upon vapor deposition of the natural amino acid L-cysteine onto the (111) surface of gold have been investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. Following deposition at room temperature and at cysteine coverages well below saturation of the first monolayer, we found coexistence of unordered molecular islands and extended domains of a highly ordered molecular overlayer of quadratic symmetry. As the coverage was increased, a number of other structures with local hexagonal order emerged and became dominant. Neither of the room temperature, as-deposited, ordered structures showed any fixed rotational relationship to the underlying gold substrate, suggesting a comparatively weak and nonspecific molecule-substrate interaction. Annealing of the cysteine-covered substrate to 380 K lead to marked changes in the observed adsorption structures. At low coverages, the unordered islands developed internal order and their presence started to perturb the appearance of the surrounding Au(111) herringbone reconstruction. At coverages beyond saturation of the first monolayer, annealing led to development of a ( radical3 x radical3)R30 degrees superstructure accompanied by the formation of characteristic monatomically deep etch pits, i.e., the behavior typically observed for alkanethiol self-assembled monolayers on Au(111). The data thus show that as-deposited and thermally annealed cysteine adsorption structures are quite different and suggest that thermal activation is required before vacuum deposited cysteine becomes covalently bound to single crystalline Au(111).

Enantiospecific Adsorption of Cysteine at Chiral Kink Sites on Au(110)-(1×2)

Enantiospecific adsorption of cysteine molecules onto chiral kink sites on the Au(110)-(1x2) surface was observed by scanning tunneling microscopy. l- and d-cysteine dimers were found to adopt distinctly different adsorption geometries at S kinks, which can be understood from the need to reach specific, optimum molecule-substrate interaction points. Extended, homochiral domains of l/d-cysteine were furthermore observed to grow preferentially from R/S kinks. The results constitute the first direct, microscopic observation of enantiospecific molecular interaction with chiral sites on a metal single-crystal surface.

Amino Acid Adsorption Effects on Nanotube Electronics

We discuss effects of the adsorption of glycine, histidine, phenylalanine, and cysteine on the electronic structure of the metallic (3,3) carbon nanotube through calculations within density functional theory. Results show good nanotube electronic structure stability in the presence of the biomolecules. Molecule-nanotube band mixing and band degeneracy lifting are observed, though the limited extents to which these effects are manifested on the system electronics affirm the weak, noncovalent connection at their interface.

Tailoring the electronic properties of silicon with cysteine: A first-principles study

We discuss the electronic structure modifications induced on the dihydride-terminated Si(001) surface upon cysteine adsorption by means of ab initio calculations: several stable functionalization schemes are presented, providing different routes for biological recognition, surface nanostructuring, and biomolecular electronics applications. The resulting hybrid systems are discussed and compared in terms of stability, structural, and electronic properties. Based on our results, we propose STM and photoemission experiments to determine unambiguously the adsorption mechanism involved and the attached functional group.

The electro-oxidative activity of cysteine on the Au electrode as evidenced by surface enhanced Raman scattering

It is demonstrated from the surface-enhanced Raman scattering that cysteine is adsorbed on the Au electrode via its carboxylate moiety. The mechanism is mainly via the charge transfer involving the π electron-rich carboxylate moiety. The adsorption of cysteine is weak that its diffusion on the Au surface is possible. The collision of two cysteine molecules may lead to the electro-oxidative formation of cystine in an irreversible way due to that the disulfide bond is not in the close vicinity of the electrode and its rupture is hampered. The physical difference as compared to its adsorption on the Ag electrode is stressed.

Adsorption of Enantiomeric and Racemic Cysteine on a Silver Electrode − SERS Sensitivity to Chirality of Adsorbed Molecules

The adsorption of both (L- and D-) enantiomeric forms of cysteine on the silver electrode surface was studied by surface-enhanced Raman scattering spectroscopy (SERS) as a function of electrode potential and pH value of the solution. It was demonstrated that at potentials more positive than -0.7 V (for pH 3) or -0.8 V (for pH 2 or lower), in acidic environment L-cysteine molecules are adsorbed mainly as P(H) (gauche) conformer, in zwitterionic form with the COO- groups close to the surface. At more negative potentials, NH3+ groups deprotonate at the surface with simultaneous weakening of the interaction of the carboxylic groups with the surface. Spectroscopic evidence for at least partial protonation of the COO- groups at strongly acidic solutions was given by observing the C=O stretching band at frequency lowered by about 30 cm(-1) in comparison with that observed for crystalline cysteine hydrochloride. It points to the considerable enhancement of the strength of hydrogen bonds and may be ascribed to the formation of cyclic L-cysteine dimers at the electrode surface. In neutral and alkaline solutions, adsorbed L-cysteine molecules have deprotonated amino groups at wide potential range. Similar spectroelectrochemical experiments were performed for D-cysteine and for a racemic mixture of D,L-cysteine. As expected, results for D-cysteine were similar to those for L-cysteine. However, for racemic mixture at acidic pH, the spectral effects corresponding to potential-induced transition from adsorbed zwitterions to neutral molecule were considerably smaller. This effect was discussed in terms of stereoselective dimerization of cysteine molecule at the electrode surface.

A scanning photoemission microprobe study of the adsorption of cysteine on Pt{1 1 1}

High resolution photoemission spectroscopy using sub-micron beams of soft X-rays is used to probe in situ prepared cysteine/Pt{1 1 1} surfaces. Cysteine is found to adsorb intact in its zwitterionic form and only its thiol group is observed to bond with surface Pt. The bright beams of light used in these experiments eventually reduce the sulphur-bonded cysteine molecule to a sulphur-bonded C x H y type-molecule. In order to minimise local damage fast scanning is required when imaging using the scanning photoemission microprobe.

Adsorption modes of cysteine on Au(111): thiolate, amino-thiolate, disulfide.

The adsorption of cysteine on the (111) surface of gold has been studied by means of periodic supercell density-functional theory calculations. A number of different adsorption modes are examined, including adsorption through the thiol group in either thiolate or disulfide form, and adsorption through both the thiol and amino functional groups. We find that at intermediate coverage densities the latter mode of adsorption is favored, followed by thiolate adsorption at the bridge (slightly displace toward fcc) site. The N-Au and S-Au bond strengths in the amino-thiolate adsorption are estimated to be of the order of 6 and 47 kcal/mol, respectively. The electronic structure of the different systems is analyzed, with focus on the total and projected density of states, as well as on the detailed character of the electronic states at the interface. States near the Fermi energy are found to have a metal-molecule antibonding character, whereas metal-molecule bonding states mostly occur near the lower edge of the Au-d band.