Combined experimental and computational study of the photoabsorption of the monodoped and nondoped nanoclusters Au24Pt(SR)18, Ag24Pt(SR)18, and Ag25(SR)18.

Assessing the accuracy of first-principles computational approaches is instrumental to predict electronic excitations in metal nanoclusters with quantitative confidence. Here we describe a validation study on the optical response of a set of monolayer-protected clusters (MPC). The photoabsorption spectra of Ag25(DMBT)18-, Ag24Pt(DMBT)182- and Au24Pt(SC4H9)18, where DMBT is 2,4-dimethylbenzenethiolate and SC4H9 is n-butylthiolate, have been obtained at low temperature and compared with accurate TDDFT calculations. An excellent match between theory and experiment, with typical deviations of less than 0.1 eV, was obtained, thereby validating the accuracy and reliability of the proposed computational framework. Moreover, an analysis of the TDDFT simulations allowed us to ascribe all relevant spectral features to specific transitions between occupied/virtual orbital pairs. The doping effect of Pt on the optical response of these ultrasmall MPC systems was identified and discussed.

Gas-Phase Anion Photoelectron Spectroscopy of Alkanethiolate-Protected PtAu12 Superatoms: Charging Energy in Vacuum vs Solution.

Charging behavior of molecular Au clusters protected by alkanethiolate (SCnH2n+1 = SCn) is, under electrochemical conditions, significantly affected by the penetration of solvent and electrolyte into the SCn layer. In this study, we estimated the charging energy EC(n) associated with [PtAu24(SCn)18]- + e → [PtAu24(SCn)18]2- (n = 4, 8, 12, and 16) in vacuum using mass-selected, gas-phase anion photoelectron spectroscopy of [PtAu24(SCn)18]z (z = -1 and -2). The EC(n) values of PtAu24(SCn)18 in vacuum are significantly larger than those in solution and decrease with n in contrast to the behavior reported for Au25(SCn)18 in solution. The effective relative permittivity (εm*) of the SCn layer in vacuum is estimated to be 2.3-2.0 based on the double-concentric-capacitor model. Much smaller εm* values in vacuum than those in solution are explained by the absence of solvent/electrolyte penetration into the monolayer. The gradual decrease of εm* with n is ascribed to the appearance of an exposed surface region due to the bundle formation of long alkyl chains.

Aggregation-Enhanced Photoluminescence and Photoacoustics of Atomically Precise Gold Nanoclusters in Lipid Nanodiscs (NANO2).

The authors designed a structurally stable nano-in-nano (NANO2) system highly capable of bioimaging via an aggregation-enhanced NIR excited emission and photoacoustic response achieved based on atomically precise gold nanoclusters protected by linear thiolated ligands [Au25(SC n H2n+1)18, n = 4-16] encapsulated in discoidal phospholipid bicelles through a one-pot synthesis. The detailed morphological characterization of NANO2 is conducted using cryogenic transmission electron microscopy, small/wide angle X-ray scattering with the support of molecular dynamics simulations, providing information on the location of Au nanoclusters in NANO2. The photoluminescence observed for NANO2 is 20-60 times more intense than that of the free Au nanoclusters, with both excitation and emission wavelengths in the near-infrared range, and the photoacoustic signal is more than tripled. The authors attribute this newly discovered aggregation-enhanced photoluminescence and photoacoustic signals to the restriction of intramolecular motion of the clusters' ligands. With the advantages of biocompatibility and high cellular uptake, NANO2 is potentially applicable for both in vitro and in vivo imaging, as the authors demonstrate with NIR excited emission from in vitro A549 human lung and the KB human cervical cancer cells.

Atomically Precise Metal Nanoclusters: Novel Building Blocks for Hierarchical Structures.

Atomically precise ligand-protected nanoclusters (NCs) constitute an important class of compounds that exhibit well-defined structures and, when sufficiently small, evident molecular properties. NCs provide versatile building blocks to fabricate hierarchical superstructures. The assembly of NCs indeed offers opportunities to devise new materials with given structures and able to carry out specific functions. In this Concept article, we highlight the possibilities offered by NCs in which the physicochemical properties are controlled by the introduction of foreign metal atoms and/or modification of the composition of the capping monolayer with functional ligands. Different approaches to assemble NCs into dimers and higher hierarchy structures and the corresponding changes in physicochemical properties are also described.

Predictive optical photoabsorption of Ag24Au(DMBT)18 - via efficient TDDFT simulations.

We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.

Nuclear and Electron Magnetic Resonance Spectroscopies of Atomically Precise Gold Nanoclusters.

Atomically precise gold nanoclusters display properties that are unseen in larger nanoparticles. When the number of gold atoms is sufficiently small, the clusters exhibit molecular properties. Their study requires extensive use of classic molecular physical chemistry and, thus, methods such as vibrational spectroscopies, electrochemistry, density functional theory and molecular dynamics calculations, and of course nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. NMR and EPR studies have been mostly carried out on the benchmark, stable molecules Au25(SR)18, Au38(SR)24, Au102(SR)44, and Au144(SR)60 (where SR = thiolate). In this Account, we showcase examples primarily taken from our previous and ongoing NMR and EPR studies, which we hope will trigger further interest in the use of these sensitive, though often underutilized, techniques. Indeed, 1D and 2D NMR spectra of pure, atomically precise clusters can be very detailed and informative. Molecular clusters are molecules and, thus, have discrete energy levels and undergo stepwise oxidation or reduction. The effect of the charge state on the chemical shifts and line shapes is a function of the ligand type (ligands differ due to specific bonds with different Au atom types) and the position of the chemical group along the ligand backbone: for groups near the Au core, they can be very dramatic. Ligand-protected gold clusters are hard-soft molecules where a hard metal core is surrounded by a dynamic molecular layer. The latter provides a nanoenvironment that interfaces the cluster core with the surrounding environment and can be permeated by molecules and ions. NMR spectroscopy is especially useful to assess its structure. For example, the data show that whereas long alkanethiolates form bundles, shorter chains exhibit more conformational freedom and are quite folded. NMR spectroscopy allows studying diastereotopic effects and provides information on possible hydrogen bonds of ligands with sulfur or surface gold atoms. EPR spectroscopy is a very precise technique to check and characterize the magnetic state of gold clusters or clusters doped with foreign-metal atoms. Electron nuclear double resonance (ENDOR) provides a powerful tool to assess the interaction of an unpaired electron with nuclei, as we showed for 197Au and 1H. It can be used as a sensitive probe of the spin-density distribution in nanoclusters: for example, it showed that the singly occupied molecular orbital may span outside the Au core by nearly 6 Å. Solid-state EPR spectroscopy has provided compelling evidence that the specific ligands and the crystallinity degree are very important factors in determining the interactions between clusters in the solid state. Depending on the condition, paramagnetic, superparamagnetic, ferromagnetic, or antiferromagnetic behavior can be observed. Time-resolved EPR was successfully tested to determine the efficiency of singlet-oxygen generation via sensitization of Au25 clusters. This Account thus demonstrates some of the remarkable insights that can be gained into the properties of atomically precise clusters through detailed NMR and EPR studies.

Preparative Mass Spectrometry Using a Rotating-Wall Mass Analyzer.

The design of functional interfaces is central to both fundamental and applied research in materials science and energy technology. We introduce a new, broadly applicable technique for the precisely controlled high-throughput preparation of well-defined interfaces containing polyatomic species ranging from small ions to nanocrystals and large protein complexes. The mass-dispersive deposition of ions onto surfaces is achieved using a rotating-wall mass analyzer, a compact device which enables the separation of ions using low voltages and has a theoretically unlimited mass range. We demonstrate an efficient deposition of singly charged Au144 (SC4 H9 )60 ions (33.7 kDa), which opens up exciting opportunities for the structural characterization of nanocrystals and their assemblies using transmission electron microscopy. Our approach also enables the high-throughput deposition of mass-selected ions from multicomponent mixtures, which is of interest to the controlled preparation of surface gradients and rapid screening of molecules in mixtures for a specific property.

Metal Doping of Au25(SR)18- Clusters: Insights and Hindsights.

The study of the structures and properties of atomically precise gold nanoclusters is the object of active research worldwide. Recently, research has been also focusing on the doping of metal nanoclusters through introduction of noble metals, such as platinum, and less noble metals, such as cadmium and mercury. Previous studies, which relied extensively on the use of mass spectrometry and single-crystal X-ray crystallography, led to the assignment of the location of each of these foreign-metal atoms. Our study provides new insights into this topic and, particularly, compelling evidence about the actual position of the selected metal atoms M = Pt, Pd, Hg, and Cd in the structure of Au24M(SR)180. To make sure that the results were not dependent on the thiolate, for SR we used both butanethiolate and phenylethanethiolate. The clusters were prepared according to different literature procedures that were supposed to lead to different doping positions. Use of NMR spectroscopy and isotope effects, with the support of mass spectrometry, electrochemistry, and single-crystal X-ray crystallography, led us to confirm that noble metals indeed dope the cluster at its central position, whereas no matter how the doping reaction is conducted and the nature of the ligand, the position of both Cd and Hg is always on the icosahedron shell, rather than at the central or staple position, as often reported. Our results not only provide a reassessment of previous conclusions, but also highlight the importance of NMR spectroscopy studies and cast doubts on drawing conclusions mostly based on single-crystal X-ray crystallography.

Electrocrystallization of Monolayer-Protected Gold Clusters: Opening the Door to Quality, Quantity, and New Structures.

Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal X-ray structures has been instrumental to enable advanced molecular understanding of their intriguing properties. So far, however, a general, reliable, chemically clean approach to prepare single crystals suitable for accurate crystallographic analysis was missing. Here we show that single crystals of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystallization. This method relies on the fact that charged clusters display a higher solubility in polar solvents than their neutral counterparts. Nucleation of the electrogenerated insoluble clusters directly onto the electrode surface eventually leads to the formation of a dense forest of millimeter-long single crystals. Electrocrystallization of three known Au25(SR)180 clusters is described. A new cluster, Au25(S-nC5H11)18, was also prepared and found to crystallize by forming bundles of millimeter-long Au25 polymers.

From Blue to Green: Fine-Tuning of Photoluminescence and Electrochemiluminescence in Bifunctional Organic Dyes.

We describe the synthesis, computational analysis, photophysics, electrochemistry and electrochemiluminescence (ECL) of a series of compounds formed of two triphenylamines linked by a fluorene or spirobifluorene bridge. The phenylamine moieties were modified at the para-position of the two external rings by electron-withdrawing or electron-donating substituents. These modifications allowed for fine-tuning of the photoluminescence (PL) and ECL emission from blue to green, with an overall wavelength span of 73 (PL) and 67 (ECL) nm, respectively. For all compounds, we observed a very high PL quantum yield (79-89%) and formation of stable radical ions. The ECL properties were investigated by direct annihilation of the electrogenerated radical anion and radical cation. The radical-ion annihilation process is very efficient and causes an intense greenish-blue ECL emission, easily observable even by naked eye, with quantum yield higher than the standard 9,10-diphenylanthracene. The ECL spectra show one single band that almost matches the PL band. Because the energy of the annihilation reaction is higher than that required to form the singlet excited state, the S-route is considered the favored pathway followed by the ECL process in these molecules. All these features point to this type of molecular system as promising for ECL applications.

Molecular resolution visualization of a pore formed by trichogin, an antimicrobial peptide, in a phospholipid matrix.

Electrochemical scanning tunneling microscopy (EC-STM) was employed to study the aggregation of trichogin OMe (TCG), an antimicrobial peptide, incorporated into a lipid monolayer. High-resolution EC-STM images show that trichogin molecules aggregate to form channels in the lipid monolayer. Two types of aggregates were observed in the images. The first consisted of a bundle of six TCG molecules surrounding a central pore. The structure and dimensions of this channel are similar to aggregates that in bilayers are described by the barrel-stave model. The EC-STM images also reveal that channels aggregate further to form a hexagonal lattice of a two dimensional (2D) nanocrystal. The model of 2D lattice was built from trimers of TCG molecules that alternatingly are oriented with either hydrophilic or hydrophobic faces to each other. In this way each TCG molecule is oriented partially with its hydrophilic face towards the hexameric pore allowing the formation of the column of water inside this pore.

Influence of surface structure on single or mixed component self-assembled monolayers via in situ spectroelectrochemical fluorescence imaging of the complete stereographic triangle on a single crystal Au bead electrode.

The use of a single crystal gold bead electrode is demonstrated for characterization of self-assembled monolayers (SAM)s formed on the bead surface expressing a complete set of face centered cubic (fcc) surface structures represented by a stereographic projection. Simultaneous analysis of many crystallographic orientations was accomplished through the use of an in situ fluorescence microscopic imaging technique coupled with electrochemical measurements. SAMs were prepared from different classes of molecules, which were modified with a fluorescent tag enabling characterization of the influence of electrical potential and a direct comparison of the influence of surface structure on SAMs adsorbed onto low index, vicinal and chiral surfaces. The assembly of alkylthiol, Aib peptide and DNA SAMs are studied as a function of the electrical potential of the interface revealing how the organization of these SAMs depend on the surface crystallographic orientation, all in one measurement. This approach allows for a simultaneous determination of SAMs assembled onto an electrode surface onto which the whole fcc stereographic triangle can be mapped, revealing the influence of intermolecular interactions as well as the atomic arrangement of the substrate. Moreover, this method enables study of the influence of the Au surface atom arrangement on SAMs that were created and analyzed, both under identical conditions, something that can be challenging for the typical studies of this kind using individual gold single crystal electrodes. Also demonstrated is the analysis of a SAM containing two components prepared using thiol exchange. The two component SAM shows remarkable differences in the surface coverage, which strongly depends on the surface crystallography enabling estimates of the thiol exchange energetics. In addition, these electrode surfaces enable studies of molecular adsorption onto the symmetry related chiral surfaces since more than one stereographic triangle can be imaged at the same time. The ability to observe a SAM modified surface that contains many complete fcc stereographic triangles will facilitate the study of the single and multicomponent SAMs, identifying interesting surfaces for further analysis.

Interaction of mixed-ligand monolayer-protected Au144 clusters with biomimetic membranes as a function of the transmembrane potential.

Understanding the interaction of nanoparticles with cell membranes is a high-priority research area for possible biomedical applications. We describe our findings concerning the interaction of Au144 monolayer-protected clusters (MPCs) with biomimetic membranes and their permeabilizing effect as a function of the transmembrane potential. We synthesized Au144(SCH2CH2Ph)60 and modified the capping monolayer with 8-mercaptooctanoic acid (Au144OctA) or thiolated trichogin (Au144TCG), a channel-forming peptide. The interactions of these MPCs with mercury-supported lipid mono- and bilayers were studied with a combination of electrochemical techniques specifically sensitive to changes in the properties of biomimetic membranes and/or charge-transfer phenomena. Permeabilization effects were evaluated through the influence of MPC uptake on the reduction of cadmium(II) ions. The nature and properties of the Au144 capping molecules play a crucial role in controlling how MPCs interact with membranes. The native MPC causes a small effect, whereas both Au144OctA and Au144TCG interact significantly with the lipid monolayer and show electroactivity. Whereas Au144OctA penetrates the membrane, Au144TCG pierces the membrane with its peptide appendage while remaining outside of it. Both clusters promote Cd(2+) reduction but with apparently different mechanisms. Because of the different way that they interact with the membrane, Au144OctA is more effective in Cd(2+) reduction when interacting with the lipid bilayer and Au144TCG performs particularly well when piercing the lipid monolayer.

Probing membrane permeabilization by the antibiotic lipopeptaibol trichogin GA IV in a tethered bilayer lipid membrane.

The lipopeptaibol trichogin GA IV (TCG) can be incorporated in the lipid bilayer moiety of a mercury-supported tethered bilayer lipid membrane (tBLM) at a non-physiological transmembrane potential of about -240 mV, negative on the trans side of the bilayer. Once incorporated in the tBLM, TCG is stable over the range of physiological transmembrane potentials and permeabilizes the membrane at transmembrane potentials negative of -80 / -90 mV. The chronocoulometric behavior is consistent with a kinetics of nucleation and growth of bundles of TCG building blocks with ion-channel properties. The TCG building blocks also permeabilize the lipid bilayer, albeit at more negative transmembrane potentials, and can be tentatively regarded as dimers of aligned TCG helical monomers. The cyclic voltammograms of tBLMs incorporating TCG point to a voltage-gated behavior of the TCG channel, similar to that exhibited by the peptaibol alamethicin.

Interplay of charge state, lability, and magnetism in the molecule-like Au25(SR)18 cluster.

Au25(SR)18 (R = -CH2-CH2-Ph) is a molecule-like nanocluster displaying distinct electrochemical and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepared monodisperse Au25(SR)18(0) and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its reduction or oxidation to a series of charge states, -2, -1, +1, +2, and +3. For each electrode process, we determined the standard heterogeneous electron-transfer (ET) rate constants and the reorganization energies. The latter points to a relatively large inner reorganization. Reduction to form Au25(SR)18(2-) and oxidation to form Au25(SR)18(2+) and Au25(SR)18(3+) are chemically irreversible. The corresponding decay rate constants and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au25 magnetism was addressed by comparing the continuous-wave electron paramagnetic resonance (cw-EPR) behaviors of radical Au25(SR)18(0) and its oxidation product, Au25(SR)18(+). As opposed to recent experimental and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and density of states of the -1, 0, and +1 charge states nicely reproduced the experimentally estimated dependence of the HOMO-LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: In particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochemical, EPR, and computational results thus provide a self-consistent picture of the properties of Au25(SR)18 as a function of its charge state and may furnish a methodology blueprint for understanding the redox and magnetic behaviors of similar molecule-like gold nanoclusters.

Conformationally constrained functional peptide monolayers for the controlled display of bioactive carbohydrate ligands.

In this study, we employed thiolated peptides of the conformationally constrained, strongly helicogenic α-aminoisobutyric acid (Aib) residue to prepare self-assembled monolayers (SAMs) on gold surfaces. Electrochemistry and infrared reflection absorption spectroscopy support the formation of very well packed Aib-peptide SAMs. The immobilized peptides retain their helical structure, and the resulting SAMs are stabilized by a network of intermolecular H bonds involving the NH groups adjacent to the Au surface. Binary SAMs containing a synthetically defined glycosylated mannose-functionalized Aib-peptide as the second component display similar features, thereby providing reproducible substrates suitable for the controlled display of bioactive carbohydrate ligands. The efficiency of such Aib-based SAMs as a biomolecular recognition platform was evidenced by examining the mannose-concanavalin A interaction via surface plasmon resonance biosensing.

Effect of orientation of the peptide-bridge dipole moment on the properties of fullerene-peptide-radical systems.

We synthesized two series of compounds in which a nitroxide radical and a fullerene C(60) moiety were kept separated by a 3(10)-helical peptide bridge containing two intramolecular C═O···H-N hydrogen bonds. The direction of the resulting molecular dipole moment could be reversed by switching the position of fullerene and nitroxide with respect to the peptide nitrogen and carbon termini. The resulting fullerene-peptide-radical systems were compared to the behaviors of otherwise identical peptides but lacking either C(60) or the free radical moiety. Electrochemical analysis and chemical nitroxide reduction experiments show that the dipole moment of the helix significantly affects the redox properties of both electroactive groups. Besides providing evidence of a folded helical conformation for the peptide bridge, IR and NMR results highlight a strong effect of peptide orientation on the spectral patterns, pointing to a specific interaction of one of the helical orientations with the C(60) moiety. Time-resolved EPR spectra show not only that for both systems triplet quenching by nitroxide induces spin polarization of the radical spin sublevels, but also that the coupling interaction can be either weak or strong depending on the orientation of the peptide dipole. As opposed to the concept of dyads, the molecules investigated are thus better described as fullerene-peptide-radical systems to stress the active role of the bridge as an important ingredient capable of tuning the system's physicochemical properties.

Effect of the charge state (z = -1, 0, +1) on the nuclear magnetic resonance of monodisperse Au25[S(CH2)2Ph]18(z) clusters.

Monodisperse Au(25)L(18)(0) (L = S(CH(2))(2)Ph) and [n-Oct(4)N(+)][Au(25)L(18)(-)] clusters were synthesized in tetrahydrofuran. An original strategy was then devised to oxidize them: in the presence of bis(pentafluorobenzoyl) peroxide, the neutral or the negatively charged clusters react as efficient electron donors in a dissociative electron-transfer (ET) process, in the former case yielding [Au(25)L(18)(+)][C(6)F(5)CO(2)(-)]. As opposed to other reported redox methods, this dissociative ET approach is irreversible, easily controllable, and clean, particularly for NMR purposes, as no hydrogen atoms are introduced. By using this approach, the -1, 0, and +1 charge states of Au(25)L(18) could be fully characterized by (1)H and (13)C NMR spectroscopy, using one- and two-dimensional techniques, in various solvents, and as a function of temperature. For all charge states, the NMR results and analysis nicely match recent structural findings about the presence of two different ligand populations in the capping monolayer, each resonance of the two ligand families displaying distinct NMR patterns. The radical nature of Au(25)L(18)(0) is particularly evident in the (1)H and (13)C NMR patterns of the inner ligands. The NMR behavior of radical Au(25)L(18)(0) was also simulated by DFT calculations, and the interplay between theory and experiments revealed a fundamental paramagnetic contribution coming from Fermi contact shifts. Interestingly, the NMR patterns of Au(25)L(18)(-) and Au(25)L(18)(+) were found to be quite similar, pointing to the latter cluster form as a diamagnetic species.

Isolation of the Au145(SR)60X compound (R = n-butyl, n-pentyl; X = Br, Cl): novel gold nanoclusters that exhibit properties subtly distinct from the ubiquitous icosahedral Au144(SR)60 compound.

We report the identification and quantitative isolation of Au145(SR)60X (R = n-butyl, n-pentyl; X = halide) along with elucidation of key properties as compared to the corresponding ubiquitous chiral-icosahedral Au144(SR)60 cluster known to have a central vacancy. The stoichiometries were assessed by electrospray mass spectrometry (ESI-MS) at isotopic resolution, and induced dissociation patterns indicate the 'extra' (Au,Br) atoms are strongly bound components of these structures. Voltammetric and spectroscopic characterization reveals Au145(SR)60X behaviors that are qualitatively similar to yet fascinatingly distinct from those of Au144(SR)60. (1H,13C)-NMR spectra clearly show how both Au145(SR)60X and Au144(SR)60 are capped by 12 distinct ligand types of 5-fold equivalence, as was recently established for Au144(SR)60 capped by shorter ligands, demonstrating that this novel cluster shares the same chiral-icosahedral motif. Intriguingly, Au145(SR)60X is strongly near-IR luminescent, whereas under comparable conditions Au144(SR)60 barely emits. The photoluminescence pattern of Au145(SR)60X is very similar to that observed for Au25(SR)18, which contains the Au13 core. The combined results are interpreted as consistent with neutral Au145(SR)60X as a diamagnetic species, electronically and structurally similar to the corresponding Au144(SR)60 compounds.

Effect of a strong interfacial electric field on the orientation of the dipole moment of thiolated aib-oligopeptides tethered to mercury on either the N- or C-terminus.

Four oligopeptides consisting of a sequence of alpha-aminoisobutyric acid (Aib) residues, thiolated at either the N- or C-terminus by means of a -(CH(2))(2)-SH anchor, were self-assembled on mercury, which is a substrate known to impart a high fluidity to self-assembled monolayers (SAMs). The surface dipole potential of these peptide SAMs was estimated in 0.1 M KCl aqueous solution at a negatively charged electrode, where the interfacial electric field is directed toward the metal. To the best of our knowledge, this is the first estimate of the surface dipole potential of peptide SAMs in aqueous solution. The procedure adopted consisted in measuring the charge involved in the gradual expansion of a peptide-coated mercury drop and then combining the resulting information with an estimate of the charge density experienced by diffuse layer ions. The dipole moment of the tethered thiolated peptides was found to be directed toward the metal, independent of whether they were thiolated at the C- or N-terminus. This result was confirmed by the effect of these SAMs on the kinetics and thermodynamics of the Eu(III)/Eu(II) redox couple. The combined outcome of these studies indicates that a strong interfacial electric field orients the dipole moment of peptide SAMs tethered to mercury, even against their "natural" dipole moment.