Ion Transport across Biological Membranes by Carborane-Capped Gold Nanoparticles

Marcin P. Grzelczak*, Stephen P. Danks, Robert C. Klip, Domagoj Belic, Adnana Zaulet, Casper Kunstmann-Olsen, Dan F Bradley, Tatsuya Tsukuda, Clara Viñas, Francesc Teixidor, Jonathan J. Abramson, and Mathias Brust
ACS Nano 11, 12492-12499 (2017)

2-3 nm carborane-capped gold nanoparticles (Au/carborane NPs) can act as artificial ion transporters across biological membranes. The particles themselves are large hydrophobic anions that have the ability to disperse in aqueous media and to partition over both sides of a phospholipid bilayer membrane. Their presence therefore causes a membrane potential that is determined by the relative concentrations of particles on each side of the membrane according to the Nernst-Equation. The particles tend to adsorb to both sides of the membrane and can flip across if changes in membrane potential require their repartitioning. Such changes can be made either with a potentiostat in an electrochemical cell, or by competition with another partitioning ion, for example potassium in the presence of its specific transporter valinomycin. Carborane-capped gold nanoparticles have a ligand shell full of voids, which stem from the packing of near spherical ligands on a near spherical metal core. These voids are normally filled with sodium or potassium ions and the charge is overcompensated by an excess of electrons in the metal core. The anionic particles are therefore able to take up and release a certain payload of cations and to adjust their net charge accordingly. It is demonstrated by potential dependent fluorescence spectroscopy that polarised phospholipid membranes of vesicles can be depolarised by ion transport mediated by the particles. It is also shown that the particles act as alkali ion specific transporters across free standing membranes under potentiostatic control. Magnesium ions are not transported.

Structure Determination of a Water-Soluble 144-Gold Atom Particle at Atomic Resolution by Aberration-Corrected Electron Microscopy

Maia Azubel*, Ai Leen Koh, Kiichirou Koyasu, Tatsuya Tsukuda, and Roger David Kornberg
ACS Nano 11, 11866-11871 (2017)

Structure determination by transmission electron microscopy has revealed the long sought 144-gold atom particle. The structure exhibits deviations from face centered cubic (fcc) packing of the gold atoms, similar to the solution structure of another gold nanoparticle, and in contrast to a previous X-ray crystal structure. Evidence from analytical methods points to a low number of 3-mercaptobenzoic acid ligands covering the surface of the particle.

Photoassisted Homocoupling of Methyl Iodide Mediated by Atomic Gold in Low-Temperature Neon Matrix

Satoru Muramatsu, Xuan Wu, Mohua Chen, Mingfei Zhou*, and Tatsuya Tsukuda*
J. Phys. Chem. A, 121, 8408-8413 (2017).

Infrared spectroscopy and density functional theory calculations showed that gold complexes [CH3–Au–I] and [(CH3)2–Au–I2], in which one and two CH3I molecule(s), respectively, are oxidatively adsorbed on the Au atoms, are formed in a solid neon matrix via the reactions between the laser-ablated gold atoms (Au) and methyl iodide (CH3I). Global reaction route mapping calculations revealed that the heights of the activation barriers for the sequential oxidative addition to produce [CH3–Au–I] and [(CH3)2–Au–I2] are 0.53 and 1.00 eV, respectively, suggesting that the reactions proceed via electronically excited states. The reductive elimination of ethane (C2H6) from [(CH3)2–Au–I2] leaving AuI2 was hindered by an activation barrier as high as 1.22 eV, but was induced by visible light irradiation on [(CH3)2–Au–I2]. These results demonstrate that photoassisted homocoupling of CH3I is mediated by Au atom, via [(CH3)2–Au–I2] as an intermediate.

Anion Photoelectron Spectroscopy of Free [Au25(SC12H25)18]

Keisuke Hirata, Keishiro Yamashita, Satoru Muramatsu, Shinjiro Takano, Keijiro Ohshimo, Toshiyuki Azuma, Ryuzo Nakanishi, Takashi Nagata, Seiji Yamazoe, Kiichirou Koyasu and Tatsuya Tsukuda*
Nanoscale, 9, 13409-13412 (2017).

Previous theoretical studies have shown that the thiolated gold cluster compound [Au25(SR)18] can be viewed as a prototypical superatom with a closed electronic structure. The quantized electronic structure of [Au25(SR)18] has been experimentally demonstrated by optical and electrochemical methods in the dispersed state. Nevertheless, no direct information is available on the energy levels and densities of occupied states. Here, we report the photoelectron spectrum of [Au25(SC12H25)18] isolated in vacuum for the first time. The spectrum exhibits two distinct peaks, corresponding to electron detachment from the superatomic 1P orbitals and Au 5d orbitals of the Au13 core. The adiabatic electron affinity of [Au25(SC12H25)18]0 was experimentally determined to be 2.2 eV, which is significantly smaller than those of [Au25(SCH3)18]0 predicted theoretically.

Lewis Base Catalytic Properties of [Nb10O28]6− for CO2 Fixation to Epoxide: Kinetic and Theoretical Studies

Shun Hayashi, Seiji Yamazoe*, Kiichirou Koyasu, and Tatsuya Tsukuda*
Chem. Asian J. 12, 1635-1640 (2017).
Selected in the Spotligihts on our sister journals of Angewandte Chimie International Edition

The decaniobate cluster (TBA)6[Nb10O28] (TBA+ = tetrabutylammonium cation) was found to act as a Lewis base catalyst for fixation of carbon dioxide (CO2) to styrene oxide (SO). Kinetic study showed that the cycloaddition of CO2 adsorbed on [Nb10O28]6− with SO corresponds to the rate-determining step in the Eley-Rideal mechanism. Density functional theory calculation predicted that CO2 on the corner and edge O atoms of [Nb10O28]6− is negatively charged and thus activated for nucleophilic attack on SO.

Suppressing Isomerization of Phosphine–Protected Au9 Cluster by Bond Stiffening Induced by Single Pd Atom Substitution

Seiji Yamazoe, Shota Matsuo, Satoru Muramatsu, Shinjiro Takano, Kiyofumi Nitta, and Tatsuya Tsukuda*
Inorg. Chem. 56, 8319-8325 (2017).

The fluxional nature of small gold clusters has been exemplified by reversible isomerization between [Au9(PPh3)8]3+ with a crown motif (Au9(C)) and that with a butterfly motif (Au9(B)) induced by association and dissociation with compact counter anions (NO3, Cl). However, structural isomerization was suppressed by substitution of the central Au atom of the Au9 core in [Au9(PPh3)8]3+ with a Pd atom: [PdAu8(PPh3)8]2+ with a crown motif (PdAu8(C)) did not isomerize to that with a butterfly motif (PdAu8(B)) upon association with the counter anions. Density functional theory calculation showed that the energy difference between PdAu8(C) and PdAu8(B) is comparable to that between Au9(C) and Au9(B), indicating that the relative stabilities of the isomers are not a direct cause for the suppression of isomerization. Temperature dependence of Debye–Waller factors obtained by X–ray absorption fine–structure analysis revealed that the intracluster bonds of PdAu8(C) were stiffer than the corresponding bonds in Au9(C). Natural bond orbital analysis suggested that the radial Pd–Au and lateral Au–Au bonds in PdAu8(C) are stiffened due to the increase in ionic nature and decrease in electrostatic repulsion between the surface Au atoms, respectively. We conclude that the formation of stiffer metal–metal bonds by Pd atom doping inhibits the isomerization from PdAu8(C) to PdAu8(B).

Hydrogen-Mediated Electron Doping of Gold Clusters as Revealed by In Situ X-ray and UV-Vis Absorption Spectroscopy

Ryo Ishida, Shun hayashi, Seiji Yamazoe, Kazuo Kato, and Tatsuya Tsukuda*
J. Phys. Chem. Lett, 8, 2368-2372 (2017).
Selected as ACS Editors' Choice
Selected in Spotlights of JPCL.

We previously reported that small (~1.2 nm) gold clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) exhibited a localized surface plasmon resonance (LSPR) band at ~520 nm in the presence of NaBH4. To reveal the mechanism of this phenomenon, the electronic structure of Au:PVP during the reaction with NaBH4 in air was examined by means of in situ X-ray absorption spectroscopy at Au L3-edge and UV-Vis spectroscopy. These measurements indicated that the appearance of the LSPR band is not associated with the growth in size, but is ascribed to electron doping to the Au sp band by the adsorbed H atoms.

Observation and the Origin of Magic Compositions of ConOm Formed in Oxidation of Cobalt Cluster Anions

Ryohei Tomihara, Kiichirou Koyasu, and Tatsuya Tsukuda*
J. Phys. Chem. C, 121 (20), 10957-10963 (2017)

To obtain atomistic insights into the early stage of oxidation process of free cobalt cluster anions Con the reaction of Con (n ≤ 10) with varied pressure of O2 was studied experimentally and theoretically. Population analysis of the oxidation products ConOm as a function of m revealed two types of magic compositions: the population decreases abruptly upon addition of a single O atom to and removal of single O atom from the magic compositions. Magic compositions of the former type were further divided into oxygen-rich (n:m ~ 3:4) and oxygen-poor (n:m ~ 1:1) series. The oxygen-rich compositions most likely correspond to fully oxidized states since the compositions are comparable to those of Co3O4 in the bulk. Their appearance is ascribed to the significant reduction of binding energies of O atoms to fully oxidized clusters. In contrast, oxygen-poor compositions correspond to the intermediates of the full oxidation states in which only the surface is oxidized based on theoretical prediction that oxidation proceeds by bonding O atoms sequentially on the surface of Con while retaining its morphology. Their appearance is ascribed to the kinetic bottleneck against internal oxidation owing to significant structural change of the Con moiety. In contrast, magic compositions of the latter type are associated to the abrupt increase of survival probability as anionic states during the relaxation of internally-hot Co oxide clusters based on the m-dependent behaviors of adiabatic electron affinities determined by photoelectron spectroscopy.

Monodisperse Iridium Clusters Protected by Phenylacetylene: Implication for Size-Dependent Evolution of Binding Sites

Hiroki Yamamoto, Prasenjit Maity, Ryo Takahata, Seiji Yamazoe,
Kiichirou Koyasu, Wataru Kurashige, Yuichi Negishi, and Tatsuya Tsukuda*
J. Phys. Chem. C, 121 (20), 10936-10941 (2017)

Terminal alkynes form a variety of interfacial structures with the clusters and nanoparticles of Au, Ag, Cu, Ru, Pd, and Pt. In order to extend the scope of modification by alkynes, we herein report the synthesis and structure characterization of Ir clusters protected by phenylacetylene (PA). Small, monodisperse PA-protected Ir clusters (1.3 ± 0.2 nm) were obtained by biphasic ligand exchange from ethylene glycol-stabilized Ir clusters (1.5 ± 0.2 nm). High-resolution transmission electron microscopy, extended X-ray absorption fine structure analysis, and powder X-ray diffraction analysis indicated that Ir clusters exhibit a face-centered cubic structure. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy indicated Ir atoms are almost in the zero valence state, which is in sharp contrast to the partial oxidation observed for Ir clusters stabilized by polymers. Absence of the terminal hydrogen of PA and red-shift of the CC stretching mode upon ligation observed by Fourier-transform infrared spectroscopy showed that the PA ligands are bound to Ir clusters via Ir−C bonds. Mass spectrometry revealed that the number of the Ir atoms in Irn(PA)m has a much narrower distribution (n = 46-53) than that of the PA ligands (m = 19-33). This finding suggests a drastic change in the binding sites of PA ligands even with a small change of the size of Ir clusters. Surprisingly, the number of PA ligands (m) decreased with increase in the size of Ir clusters (n). Within the framework of the upright binding model of PA, this counterintuitive correlation between n and m values suggests that binding sites of PA are shifted dramatically from on-top sites to bridged sites with increase in the cluster size.

Formation of Grignard Reagent-like Complex [CH3−M−I] via Oxidative Addition of CH3I on Coinage Metal Anions M (M = Cu, Ag, Au) in the Gas Phase

Satoru Muramatsu, Kiichirou Koyasu, and Tatsuya Tsukuda*
Chem. Lett. 46, 676-679 (2017).

Gas-phase reactions of coinage metal anions M (M = Cu, Ag, Au) with methyl iodide (CH3I) yielded the adduct product MCH3I. Photoelectron spectroscopy and density functional theory (DFT) calculations revealed a Grignard reagent-like structure [CH3−M−I] regardless of M. Global reaction route mapping clarified that [CH3−M−I] is formed in a highly exothermic process via SN2 attack by M on CH3I followed by I migration to M.

Structural Model of Ultrathin Gold Nanorods Based on High-Resolution Transmission Electron Microscopy: Twinned 1D Oligomers of Cuboctahedrons

Ryo Takahata, Seiji Yamazoe, Kiichirou Koyasu, and Tatsuya Tsukuda*
J. Phys. Chem. C, 121 (20), 10942-10947 (2017)

Recently, we have developed a synthetic method of ultrathin gold nanorods (AuUNRs) with a fixed diameter of ~1.8 nm and variable lengths in the range of 6-400 nm. It was reported that these AuUNRs exhibited intense IR absorption assigned to the longitudinal mode of localized surface plasmon resonance and broke up into spheres owing to Rayleigh-like instability at reduced surfactant concentration and at elevated temperatures. In order to understand the structure-property correlation of AuUNRs, their atomic structures were examined in this work using aberration-corrected high-resolution transmission electron microscopy. Statistical analysis revealed that the most abundant structure observed in the AuUNRs (diameter ≈ 1.8; length ≈ 18 nm) was a multiply twinned crystal, with a periodicity of ~1.4 nm in length. We propose that the AuUNRs are composed of cuboctahedral Au147 units, which are connected one-dimensionally through twin defects.

A Gold Superatom with 10 Electrons in Au13(PPh3)8(p-SC6H4CO2H)3

Shinjiro Takano, Seiji Yamazoe, and Tatsuya Tsukuda*
APL. Mater. 5, 053402 (2017).

The title compound Au13(PPh3)8(p-MBA)3 (1) was synthesized by chemical reduction of the neutral complex Au(PPh3)(p-MBA). Single-crystal X-ray diffraction analysis of 1 showed that the Au11 core is protected by seven PPh3 ligands and an Au2(p-MBA)3(PPh3)1 assembled ligand. Optical spectroscopy indicated that the electronic structure of the Au11 core of 1 is significantly different from that of the conventional Au11 superatom with an electron configuration of (1S)2(1P)6. Density-functional theory calculations demonstrated that the Au11 core can be viewed as a non-rare-gas-like superatom with an electron configuration of (1S)2(1P)6(1D)2.

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