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

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

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 C60 moiety were kept separated by a 310-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 C60 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 C60 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

Facile and <i>E</i>-Selective Intramolecular Ring-Closing Metathesis Reactions in 3₁₀-Helical Peptides: A 3D Structural Study

The ring-closing metathesis reaction can be used to cross-link allylated serine residues situated at the i and i + 3 positions in 310-helical peptides containing the helicogenic amino acid, α-aminoisobutyric acid (Aib). An octapeptide with the sequence Boc-Aib-Aib-Aib-Ser(Al)-Aib-Aib-Ser(Al)-Aib-OMe was found to undergo a facile and >20:1 E-selective ring-closing metathesis (RCM) reaction catalyzed by the Grubbs second-generation catalyst to yield an 18-membered macrocycle. The formation of this cross-link does not significantly disturb the peptide's native 310-helicity, as judged by an X-ray diffraction study of the acyclic diene, the E-olefin RCM product, and its hydrogenated derivative. A heptapeptide system with the sequence Boc-Val-Ser(Al)-Leu-Aib-Ser(Al)-Val-Leu-OMe also underwent an efficient RCM reaction, albeit with diminished E-selectivity. It is apparent from these studies that a minimal, RCM-derived, macrocyclic constraint can be readily incorporated into 310-helical peptides

Facile and <i>E</i>-Selective Intramolecular Ring-Closing Metathesis Reactions in 3₁₀-Helical Peptides: A 3D Structural Study

The ring-closing metathesis reaction can be used to cross-link allylated serine residues situated at the i and i + 3 positions in 310-helical peptides containing the helicogenic amino acid, α-aminoisobutyric acid (Aib). An octapeptide with the sequence Boc-Aib-Aib-Aib-Ser(Al)-Aib-Aib-Ser(Al)-Aib-OMe was found to undergo a facile and >20:1 E-selective ring-closing metathesis (RCM) reaction catalyzed by the Grubbs second-generation catalyst to yield an 18-membered macrocycle. The formation of this cross-link does not significantly disturb the peptide's native 310-helicity, as judged by an X-ray diffraction study of the acyclic diene, the E-olefin RCM product, and its hydrogenated derivative. A heptapeptide system with the sequence Boc-Val-Ser(Al)-Leu-Aib-Ser(Al)-Val-Leu-OMe also underwent an efficient RCM reaction, albeit with diminished E-selectivity. It is apparent from these studies that a minimal, RCM-derived, macrocyclic constraint can be readily incorporated into 310-helical peptides

Facile and <i>E</i>-Selective Intramolecular Ring-Closing Metathesis Reactions in 3₁₀-Helical Peptides: A 3D Structural Study

The ring-closing metathesis reaction can be used to cross-link allylated serine residues situated at the i and i + 3 positions in 310-helical peptides containing the helicogenic amino acid, α-aminoisobutyric acid (Aib). An octapeptide with the sequence Boc-Aib-Aib-Aib-Ser(Al)-Aib-Aib-Ser(Al)-Aib-OMe was found to undergo a facile and >20:1 E-selective ring-closing metathesis (RCM) reaction catalyzed by the Grubbs second-generation catalyst to yield an 18-membered macrocycle. The formation of this cross-link does not significantly disturb the peptide's native 310-helicity, as judged by an X-ray diffraction study of the acyclic diene, the E-olefin RCM product, and its hydrogenated derivative. A heptapeptide system with the sequence Boc-Val-Ser(Al)-Leu-Aib-Ser(Al)-Val-Leu-OMe also underwent an efficient RCM reaction, albeit with diminished E-selectivity. It is apparent from these studies that a minimal, RCM-derived, macrocyclic constraint can be readily incorporated into 310-helical peptides

Facile and <i>E</i>-Selective Intramolecular Ring-Closing Metathesis Reactions in 3₁₀-Helical Peptides: A 3D Structural Study

The ring-closing metathesis reaction can be used to cross-link allylated serine residues situated at the i and i + 3 positions in 310-helical peptides containing the helicogenic amino acid, α-aminoisobutyric acid (Aib). An octapeptide with the sequence Boc-Aib-Aib-Aib-Ser(Al)-Aib-Aib-Ser(Al)-Aib-OMe was found to undergo a facile and >20:1 E-selective ring-closing metathesis (RCM) reaction catalyzed by the Grubbs second-generation catalyst to yield an 18-membered macrocycle. The formation of this cross-link does not significantly disturb the peptide's native 310-helicity, as judged by an X-ray diffraction study of the acyclic diene, the E-olefin RCM product, and its hydrogenated derivative. A heptapeptide system with the sequence Boc-Val-Ser(Al)-Leu-Aib-Ser(Al)-Val-Leu-OMe also underwent an efficient RCM reaction, albeit with diminished E-selectivity. It is apparent from these studies that a minimal, RCM-derived, macrocyclic constraint can be readily incorporated into 310-helical peptides

Effect of the Charge State (<i>z</i> = −1, 0, +1) on the Nuclear Magnetic Resonance of Monodisperse Au₂₅[S(CH₂)₂Ph]₁₈^z Clusters

Monodisperse Au25L180 (L = S(CH2)2Ph) and [n-Oct4N+][Au25L18–] 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 [Au25L18+][C6F5CO2–]. 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 Au25L18 could be fully characterized by 1H and 13C 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 Au25L180 is particularly evident in the 1H and 13C NMR patterns of the inner ligands. The NMR behavior of radical Au25L180 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 Au25L18– and Au25L18+ were found to be quite similar, pointing to the latter cluster form as a diamagnetic species