Adsorption kinetics of l-glutathione on gold and structural changes during self-assembly: an in situATR-IR and QCM study

Noble Metal Nanoparticles with Nanogel Coatings: Coinage Metal Thiolate-Stabilized Glutathione Hydrogel Shells

Developing biocompatible nanocoatings is crucial for biomedical applications. Noble metal colloidal nanoparticles with biomolecular shells are thought to combine diverse chemical and optothermal functionalities with biocompatibility. Herein, we present nanoparticles with peptide hydrogel shells that feature an unusual combination of properties: the metal core possesses localized plasmon resonance, whereas a few-nanometer-thick shells open opportunities to employ their soft framework for loading and scaffolding. We demonstrate this concept with gold and silver nanoparticles capped by glutathione peptides stacked into parallel β-sheets as they aggregate on the surface. A key role in the formation of the ordered structure is played by coinage metal(I) thiolates, i.e., Ag(I), Cu(I), and Au(I). The shell thickness can be controlled via the concentration of either metal ions or peptides. Theoretical modeling of the shell's molecular structure suggests that the thiolates have a similar conformation for all the metals and that the parallel β-sheet-like structure is a kinetic product of the peptide aggregation. Using third-order nonlinear two-dimensional infrared spectroscopy, we revealed that the ordered secondary structure is similar to the bulk hydrogels of the coinage metal thiolates of glutathione, which also consist of aggregated stacked parallel β-sheets. We expect that nanoparticles with hydrogel shells will be useful additions to the nanomaterial toolbox. The present method of nanogel coating can be applied to arbitrary surfaces where the initial deposition of the seed glutathione monolayer is possible.

Phenol as a Tethering Group to Gold Surfaces: Stark Response and Comparison to Benzenethiol

Understanding the adsorption of organic molecules on metals is important in numerous areas of surface science, including electrocatalysis, electrosynthesis, and biosensing. While thiols are commonly used to tether organic molecules on metals, it is desirable to broaden the range of anchoring groups. In this study, we use a combined spectroelectrochemical and computational approach to demonstrate the adsorption of 4-cyanophenols (CPs) on polycrystalline gold. Using the nitrile stretching vibration as a marker, we confirm the adsorption of CP on the gold electrode and compare our results with those obtained for the thiol counterpart, 4-mercaptobenzonitirle (MBN). Our results reveal that CP adsorbs on the gold electrode via the OH linker, as evidenced by the similarity in the direction and magnitude of the nitrite Stark shifts for CP and MBN. This finding paves the way for exploring new approaches to modify electrode surfaces for controlled reactivity. Furthermore, it highlights adsorption on metals as an important step in the electroreactivity of phenols.

Self-assembling Bioadhesive Inspired by the Fourth Repetitive Sequence of Balanus albicostatus Cement Protein 20 kDa (Balcp-20 k)

Barnacle cement proteins are multi-protein complexes composed of a series of functionally related synergistic proteins that enable barnacles to adhere strongly and consistently to various underwater substrates. There is no post-translational modification of barnacle cement proteins, which provides a possibility for the synthesis of similar adhesive materials. Balcp-20 k has four repetitive sequences with multiple conserved cysteine groups. Whether these repeats are separate functional units and the role of cysteine in adhesion is not clear. In order to investigate the adhesion properties of Balcp-20 k, we amplified and expressed R4 (DHLACNAKHPCWHKHCDCFC)₄, which is a quadruple repeat of Balcp-20 k's fourth repetitive sequence, and S0R4 (DHLASNAKHPSWHKHSDSFS)₄, all cysteine of R4 replaced by serine. Analysis showed that R4 had a similar structure to Balcp-20 k, and the amyloid fibrils structure formed by self-assembly of R4 played an important role in improving the adhesion strength. The absence of disulfide bonds in S0R4 prevents self-assembly, and the failure of self-assembly after the reduction of disulfide bonds of R4 by DTT indicates that disulfide bonds play an important role in self-assembly. With adhesion and coating analysis, it was found that R4 has good adhesion on different materials surfaces, which is better than Balcp-20 k, while S0R4 has weak adhesion, which is only better than BSA.

Self-Assembly and Rearrangement of a Polyproline II Helix Peptide on Gold

Polyproline peptide sequences have gained popularity as anchors for peptide-based self-assembled monolayers (SAMs) due to their attractive properties. In this work, peptides containing the polyproline II helix (PPII) conformation were designed and assembled on gold (Au). A quartz crystal microbalance with dissipation was used to characterize SAM formation kinetics and related properties. Peptides were designed with the sequence (GPPPPPG)₂C. It was discovered that a biexponential adsorption and rearrangement model describes the binding kinetics of the PPII-containing peptide on Au. In this model, an initial reversible binding step is followed by an irreversible rearrangement step, given by parameter k_t. This study found k_t to be approximately 0.00064 s^-1 for the PPII-containing peptides. Similarly, we found that the adsorption of the PPII-containing peptide on Au, given by ΔG_ads, was thermodynamically favorable (-7.8 kcal mol^-1) and comparable to other common thiol terminated SAMs on Au. Furthermore, we characterized SAM properties via QCM-D, Fourier-transform infrared (FTIR) spectroscopy, and electrochemical techniques to reveal high molecular density SAMs consisting of PPII helices. In addition, these SAMs were found to have high antifouling properties. Overall, this study characterizes the fundamental assembly mechanisms, particularly, rearrangement of PPII-containing peptides for the first time, which will be useful in the designing of future peptide-based SAMs with high surface coverage and antifouling properties.

Label-free monitoring of immuno-specific interactions of adsorbed multilayer of proteins

Protein-protein interactions in adsorbed multilayer of an immuno-specific system of proteins that include staphylococcal protein A (SpA), bovine serum albumin (BSA), anti-chicken immunoglobulin Y (ac-IgG), chicken serum IgG (cs-IgG), and rabbit serum IgG (rs-IgG) on polystyrene (PS) were studied using attenuated total reflection-Fourier transform infrared spectroscopy. A systematic analysis allowed a direct qualitative and quantitative determination of protein interactions at each step of specific and nonspecific binding conditions at the molecular level. The study also provided information about (1) the adsorption behavior of the proteins, (2) the role of SpA in enabling correct orientation of the adsorbed IgG and maintaining the stability of the adsorbed SpA/ac-IgG system on the PS surface, (3) the function of BSA as both blocking reagent and promoter of specific and selective binding, and (4) the bioactivity conserved accommodation of SpA molecules on the PS surface. Furthermore, the unique characteristics of cs-IgG such as passive toward SpA adsorption and exposure of the multivalence state at nonspecific binding conditions was revealed spectroscopically. The present investigation provides a platform for further extension of the adopted methodology to a more complex system of immuno-detection for highly sensitive and rapid diagnostics.

Glutathione Self-Assembles into a Shell of Hydrogen-Bonded Intermolecular Aggregates on "Naked" Silver Nanoparticles

A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the β-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using ¹³C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native β-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons' anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.

Surface-Directed Disparity in Self-Assembled Structures of Small-Peptide l-Glutathione on Gold and Silver Nanoparticles

Despite the key roles of l-glutathiones (GSHs) inbiology and nano-biotechnology, understanding their labile structures and hydrogen bond interactions with nanoparticles has posed a critical challenge to the scientific community. The structural conformation of GSH as a capping layer on gold nanoparticle (AuNP) and silver nanoparticle (AgNP) surfaces is investigated. In this report, we attempt to explore the material-dependent interaction of GSH with different spherical nanoparticle surfaces by employing Fourier transform infrared (FTIR) spectroscopy. The infrared signal of amide I of GSH is studied as a function of different materials' spherical nanoparticles with comparable size. We revealed the β-sheet secondary structure of GSH on AgNPs and the random structure on AuNPs even though both the nanoparticles have comparable shapes and sizes and belong to the same group of the periodic table. The GSH is firmly anchored on the gold and silver surface via the thiol of the cys part. However, our experimental data designate a further interaction with the AgNP surface via the carboxylic acid group of the gly- and glu- end of the molecule. It is observed that enhancement of IR absorption of amide I of GSH is pronounced by a factor of 10 on AuNP but, in contrast, on the same-sized AgNP, the suppression is perceived by a factor of 2, even though both are plasmonic materials with respect to free GSH. This study can be used as a point of reference for understanding the structural conformation of the capping layer on nanoparticle surfaces as well as surface enhancement of the IR absorption of amide I. We would like to emphasize that molecular self-assembly on the nanoparticle surfaces is definitely of very broad interest for chemists working in nearly any subdiscipline, spanning from the nanoparticle-based medicine to surface-enhanced spectroscopy to heterogeneous catalysis, etc.

Two-Dimensional Infrared Spectroscopy Reveals Molecular Self-Assembly on the Surface of Silver Nanoparticles

The conformation of molecules, peptides, and proteins, self-assembled into structured monolayers on the surface of metal nanoparticles (NPs), can strongly affect their properties and use in chemical or nanobiomedical applications. Elucidating molecular conformations on the NP surface is highly challenging, and the microscopic details mostly remain elusive. Using polarization-selective third-order two-dimensional ultrafast infrared spectroscopy, we revealed the highly ordered intermolecular structure of γ-tripeptide glutathione on the surface of silver NPs in aqueous solution. Glutathione is an antioxidant thiol abundant in living cells; it is extensively used in NP chemistry and related research. We identified conditions where the interaction of glutathione with the NP surface facilitates formation of a β-sheet-like structure enclosing the NPs. A spectroscopic signature associated with the assembly of β-sheets into an amyloid fibril-like structure was also observed. Remarkably, the interaction with the metal surface promotes formation of a fibril-like structure by a small peptide involving only two amino acids.

Amino-acid- and peptide-directed synthesis of chiral plasmonic gold nanoparticles

Understanding chirality, or handedness, in molecules is important because of the enantioselectivity that is observed in many biochemical reactions ¹ , and because of the recent development of chiral metamaterials with exceptional light-manipulating capabilities, such as polarization control^2-4, a negative refractive index ⁵ and chiral sensing ⁶ . Chiral nanostructures have been produced using nanofabrication techniques such as lithography ⁷ and molecular self-assembly^8-11, but large-scale and simple fabrication methods for three-dimensional chiral structures remain a challenge. In this regard, chirality transfer represents a simpler and more efficient method for controlling chiral morphology^12-18. Although a few studies^18,19 have described the transfer of molecular chirality into micrometre-sized helical ceramic crystals, this technique has yet to be implemented for metal nanoparticles with sizes of hundreds of nanometres. Here we develop a strategy for synthesizing chiral gold nanoparticles that involves using amino acids and peptides to control the optical activity, handedness and chiral plasmonic resonance of the nanoparticles. The key requirement for achieving such chiral structures is the formation of high-Miller-index surfaces ({hkl}, h ≠ k ≠ l ≠ 0) that are intrinsically chiral, owing to the presence of 'kink' sites^20-22 in the nanoparticles during growth. The presence of chiral components at the inorganic surface of the nanoparticles and in the amino acids and peptides results in enantioselective interactions at the interface between these elements; these interactions lead to asymmetric evolution of the nanoparticles and the formation of helicoid morphologies that consist of highly twisted chiral elements. The gold nanoparticles that we grow display strong chiral plasmonic optical activity (a dis-symmetry factor of 0.2), even when dispersed randomly in solution; this observation is supported by theoretical calculations and direct visualizations of macroscopic colour transformations. We anticipate that our strategy will aid in the rational design and fabrication of three-dimensional chiral nanostructures for use in plasmonic metamaterial applications.

Nanoliter Sensing for Infrared Bioanalytics

Nondestructive label-free bioanalytics of microliter to nanoliter sample volumes with low analyte concentrations requires novel analytic approaches. For this purpose, we present an optofluidic platform that combines surface-enhanced in situ infrared spectroscopy with microfluidics for sensing of surface-immobilized ultrathin biomolecular films in liquid analytes. Submonolayer sensitivity down to surface densities of few ng/cm² is demonstrated for the adsorption of the thiolate tripeptide glutathione and for the recognition of streptavidin on a biotinylated enhancement substrate. Nonfunctionalized and functionalized metal island films on planar oxidized silicon substrates are used for signal enhancement with quantifiable enhancement properties. A single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is used with ordinary planar, IR-transparent windows. The geometry circumvents the strong IR absorption of common polymer materials and of aqueous environments in the IR fingerprint region. This practice enables straightforward quantitative analyses of, e.g., adsorption kinetics as well as chemical and structural properties in dependence of external stimuli.

Multimodality of Structural, Electrical, and Gravimetric Responses of Intercalated MXenes to Water

Understanding of structural, electrical, and gravimetric peculiarities of water vapor interaction with ion-intercalated MXenes led to design of a multimodal humidity sensor. Neutron scattering coupled to molecular dynamics and ab initio calculations showed that a small amount of hydration results in a significant increase in the spacing between MXene layers in the presence of K and Mg intercalants between the layers. Films of K- and Mg-intercalated MXenes exhibited relative humidity (RH) detection thresholds of ∼0.8% RH and showed monotonic RH response in the 0-85% RH range. We found that MXene gravimetric response to water is 10 times faster than their electrical response, suggesting that H₂O-induced swelling/contraction of channels between MXene sheets results in trapping of H₂O molecules that act as charge-depleting dopants. The results demonstrate the use of MXenes as humidity sensors and infer potential impact of water on structural and electrical performance of MXene-based devices.

Theoretical spectroscopic insights of tautomers and enantiomers of penicillamine

B3LYP and MP2 calculations have been carried out to investigate tautomers and enantiomers of penicillamine (Pen). Their infrared (IR), ultraviolet (UV), circular dichroism (CD) and nuclear magnetic resonance (NMR) spectra were obtained at linear-response, time-dependent DFT (TD-DFT). IR, UV and NMR spectra cannot be used to identify Pen enantiomers, showing nearly equal spectral profiles. CD spectra, however, give rise to completely symmetric signals, forming a perfect specular image to each other. Distinct CD profiles were also obtained for Pen tautomers. Important IR differences were found in positions and intensities of the vibrational stretching bands involving acid and amine groups of Pen tautomers. The highest electron transitions involving HOMO-LUMO orbitals show to be of major importance in the computed UV spectra, showing a large red-shift around 30nm as the zwitterionic and neutral Pen spectra are compared. NMR results show to be quite useful for identification of Pen tautomers since clear differences are found by means of the computed shielding tensors as well as spin-spin coupling constants ¹J(N,H) data.

In situ studies of the adsorption kinetics of 4-nitrobenzenediazonium salt on gold

Self-assembled organic layers are an important tool for modifying surfaces in a range of applications in materials science. Covalent modification of metal surfaces with aryldiazonium cations has attracted much attention primarily because this reaction offers a route for spontaneously grafting a variety of aromatic moieties from solution with high yield. We have investigated the kinetics of this process by performing real-time, in situ nanogravimetric measurements. The spontaneous grafting of 4-nitrobenzene diazonium salts onto gold electrodes was studied via quartz crystal microbalance (QCM) from aqueous solutions of the salt at varying concentrations. The concentration dependence of the grafting rate within the first 10 min is best modeled by assuming a reversible adsorption process with free energy comparable to that reported for arylthiols self-assembled on gold. Multilayer formation was observed after extended grafting times and was found to be favored by increasing bulk concentrations of the diazonium salt. Modified gold surfaces were characterized ex situ with cyclic voltammetry, infrared reflection absorbance spectroscopy, and X-ray photoemission spectroscopy. Based on the experimentally determined free energy of adsorption and on the observed grafting rates, we discuss a proposed mechanism for aryldiazonium chemisorption.

Formation process and solvent-dependent structure of a polyproline self-assembled monolayer on a gold surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.

Peptide interactions with metal and oxide surfaces

Increasing interest in bio-interfaces for medical, diagnostic, or biotechnology applications has highlighted the critical scientific challenge behind both the understanding and control of protein-solid surface interactions. In this context, this Account focuses on the molecular-level characterization of the interactions of peptides, ranging in size from a few amino acids to long sequences, with metal and oxide surfaces. In this Account, we attempt to fill the gap between the well-known basic studies of the interaction of a single amino acid with well-defined metal surfaces and the investigations aimed at controlling biocompatibility or biofilm growth processes. We gather studies performed with surface science tools and macroscopic characterization techniques along with those that use modeling methods, and note the trends that emerge. Sulfur drives the interaction of cysteine-containing peptides with metal surfaces, particularly gold. Moreover, intermolecular interactions, such as hydrogen bonds may induce surface self assembly and chiral arrangements of the peptide layer. Depending on the solvent pH and composition, carboxylates or amino groups may also interact with the surface, which could involve conformational changes in the adsorbed peptide. On oxide surfaces such as titania or silica, researchers have identified carboxylate groups as the preferential peptide binding groups because of their strong electrostatic interactions with the charged surface. In high molecular weight peptides, systematic studies of their interaction with various oxide surfaces point to the preferential interaction of certain peptide sequences: basic residues such as arginine assume a special role. Researchers have successfully used these observations to synthesize adhesive sequences and initiate biomineralization. Studies of the interaction of peptides with nanoparticles have revealed similar binding trends. Sulfur-containing peptides adhere preferentially to gold nanoparticles. Peptides containing aromatic nitrogen also display a high affinity for various inorganic nanoparticles. Finally, we describe a novel class of peptides, genetically engineered peptides for inorganics (GEPIs), which are selected from a phage display protocol for their high binding affinity for inorganic surfaces. Extended investigations have focused on the mechanisms of the molecular binding of these peptides to solid surfaces, in particular the high binding affinity of some sulfur-free sequences of GEPIs to gold or platinum surfaces. We expect that this clearer view of the possible preferential interactions between peptides and inorganic surfaces will facilitate the development of new, more focused research in various fields of biotechnology, such as biocompatibility, biomimetics, or tissue engineering.

Chiral gold nanoparticles

Monolayer-protected gold nanoparticles have many appealing physical and chemical properties such as quantum size effects, surface plasmon resonance, and catalytic activity. These hybrid organic-inorganic nanomaterials have promising potential applications as building blocks for nanotechnology, as catalysts, and as sensors. Recently, the chirality of these materials has attracted attention, and application to chiral technologies is an interesting perspective. This minireview deals with the preparation of chiral gold nanoparticles and their chiroptical properties. On the basis of the latter, together with predictions from quantum chemical calculations, we discuss different models that were put forward in the past to rationalize the observed optical activity in metal-based electronic transitions. We furthermore critically discuss these models in view of recent results on the structure determination of some gold clusters as well as ligand-exchange experiments examined by circular dichroism spectroscopy. It is also demonstrated that vibrational circular dichroism can be used to determine the structure of a chiral adsorbate and the way it interacts with the metal. Finally, possible applications of these new chiral materials are discussed.

Interparticle interactions in glutathione mediated assembly of gold nanoparticles

The understanding of the detailed molecular interactions between (GSH) glutathione molecules in the assembly of metal nanoparticles is important for the exploitation of the biological reactivity. We report herein results of an investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The interparticle interactions and reactivities were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method and the hydrodynamic sizes of the nanoparticle assemblies using the dynamic light scattering technique. The interparticle reactivity of glutathiones adsorbed on gold nanoparticles depends on the particle sizes and the ionic strength of the solution. Larger-sized particles were found to exhibit a higher degree of interparticle assembly than smaller-sized particles. The assembly-disassembly reversibility is shown to be highly dependent on pH and additives in the solution. The interactions of the negatively charged citrates surrounding the GSH monolayer on the particle surface were believed to produce more effective interparticle spatial and electrostatic isolation than the case of OH (-) groups surrounding the GSH monolayer. The results have provided new insights into the hydrogen-bonding character of the interparticle molecular interaction of glutathiones bound on gold nanoparticles. The fact that the interparticle hydrogen-bonding interactions in the assembly and disassembly processes can be finely tuned by pH and chemical means has implications to the exploitation of the glutathione-nanoparticle system in biological detection and biosensors.

Probing chiral interfaces by infrared spectroscopic methods

Biological homochirality on earth and its tremendous consequences for pharmaceutical science and technology has led to an ever increasing interest in the selective production, the resolution and the detection of enantiomers of a chiral compound. Chiral surfaces and interfaces that can distinguish between enantiomers play a key role in this respect as enantioselective catalysts as well as for separation purposes. Despite the impressive progress in these areas in the last decade, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination are lagging behind due to the lack of powerful experimental techniques to spot these interactions selectively with high sensitivity. In this article, techniques based on infrared spectroscopy are highlighted that are able to selectively target the chiral properties of interfaces. In particular, these methods are the combination of Attenuated Total Reflection InfraRed (ATR-IR) with Modulation Excitation Spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid-liquid interfaces and Vibrational Circular Dichroism (VCD), which is used to probe the structure of chirally-modified metal nanoparticles. The former technique aims at suppressing signals arising from non-selective interactions, which may completely hide the signals of interest due to enantiodiscriminating interactions. Recently, this method was successfully applied to investigate enantiodiscrimination at self-assembled monolayers of chiral thiols on gold surfaces. The nanometer size analogues of the latter--gold nanoparticles protected by a monolayer of a chiral thiol--are amenable to VCD spectroscopy. It is shown that this technique yields detailed structural information on the adsorption mode and the conformation of the adsorbed thiol. This may also turn out to be useful to clarify how chirality can be bestowed onto the metal core itself and the nature of the chirality of the latter, which is manifested in the metal-based circular dichroism activity of these nanoparticles.

Enantioselective hydrogenation of ethyl pyruvate over diop modified Pt nanoclusters. Determination of geometry of the ligand adsorption mode via DRIFTS

Immobilization of chiral ligands on the surface of metal nanoparticles is one concept for heterogenation of enantioselective catalysts. Diop modified Pt nanoclusters were found to be able to induce enantioselectivity and enhancement of the reaction rate in hydrogenation of ethyl pyruvate. The model of geometry of the diop adsorption has been proposed based on DRIFTS and molecular modelling investigations. The ability of diop to induce enantioselectivity and enhanced reaction rate was explained through the adsorption geometry and comparison with known models of enantioselectivity established for the cinchonidine on Pt system.

D-penicillamine adsorption on gold: an in situ ATR-IR spectroscopic and QCM study

The adsorption of penicillamine from ethanol on gold was studied in situ by attenuated total reflection infrared (ATR-IR) and quartz crystal microbalance (QCM) experiments. Both ATR-IR and QCM reveal a fast mass uptake. In ethanol, the molecule adopts a zwitterionic form. Upon adsorption, part of the molecules deprotonate at the amine group, which is a relatively slow process that goes along with a strong shift of the nu(as)(COO(-)) mode. Both ATR-IR and QCM confirm a physisorbed layer. ATR-IR furthermore shows that the latter consists of zwitterionic molecules only, whereas both zwitterionic and anionic species are found in the chemisorbed layer. The infrared spectra of the physisorbed and chemisorbed layers are rather different, and the molecules within both layers seem to be oriented with respect to the surface. The ATR-IR spectra furthermore indicate that all three functional groups of penicillamine (i.e., thiol, carboxylate, and amine) interact with the surface, and density functional theory calculations support this finding. QCM also shows that the molecule uses considerably more space on the surface than molecules of similar size, which supports a three-point interaction. The latter leads to a strong anchoring of the molecule to the metal, which may explain the exceptional capability of penicillamine to bind metals.

Chiral N-Isobutyryl-cysteine Protected Gold Nanoparticles: Preparation, Size Selection, and Optical Activity in the UV−vis and Infrared

We have prepared gold nanoparticles covered with N-isobutyryl-l-cysteine and N-isobutyryl-d-cysteine, respectively. These particles with a mean particle size smaller than 2 nm are highly soluble in water and are amenable to chiroptical techniques such as vibrational circular dichroism (VCD) and circular dichroism (CD) spectroscopy. Density functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the experimental VCD spectrum and the calculated VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addition to the sulfur. The particles could furthermore be separated according to their charge and size into well-defined compounds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compounds showed basically identical absorption spectra as analogous gold particles protected with l-glutathione. This shows that these particles have identical core sizes (10-12, 15 and 18 gold atoms, respectively) and indicates that the number and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some separated compounds show strong optical activity with opposite sign when covered with the d- and l-enantiomer, respectively, of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.