Spectroscopic investigation of the adsorption and oxidation of thiourea on polycrystalline Au and Au(111) in acidic media

In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm(-1). It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 VRHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 VRHE and implies the formation of [Au(I)-(TU)2]+, which is oxidized to NH2CN and S0 at E > 0.80 VRHE. In a following oxidation step at E > 1.20 V, N2, CO2, and HSO4-/SO4(2-) were produced. The second parallel reaction occurs from TU in solution at E > 0.50 VRHE to form (TU)2(2+). All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.     

ATRP grafting of styrene from benzyl chloride functionalized polysiloxanes: An AFM and TGA study of the Cu(0)/bpy catalyst

Various combinations of Cu(0), CuCl, 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) were used as catalysts for the grafting polymerizations of styrene from polysiloxane macroinitiators functionalized with benzyl chloride. While Cu(0)/bpy alone promotes the grafting, narrower polydispersities were obtained in the presence of CuCl. Analysis of the Cu(0) surface before and after polymerization by a combination of AFM, TGA and FTIR investigations reveals the formation of bpy or phen films on Cu(0). In the presence of CuCl, the ligand film appears decorated with CuCl particles which increase in size with increasing the CuCl concentration. The initial layer occurs most likely as a result of complexation between the ligands and the Cu(0) surface and acts as a support for the rest of the film. These observations are consistent with the film formation on Cu(0) from related nitrogen donors and indicate that the reactivity of the Cu surface may depend not only on its prior treatment but also on the deposition of ligands from the reaction mixture.     

Characterization of the Photosynthetic Activity of Platinized Chloroplasts: A Study Using Fluorescence and Photoacoustic Techniques

In this report, the effect of platinization on the photosynthetic activity of the chloroplast membranes is studied. Oxygen evolution, fluorescence emission and thermal de-activation processes are modified after platinization. It is shown that photosystem II activity is affected by the hydrogen purging involved in the platinization procedure as seen by the reduced rates of oxygen evolution and a decrease in variable fluorescence. Depletion of bicarbonate from photosystem II during purging is suggested to be responsible partly for the decreased electron transfer rates and for a lower half-saturation light intensity required for energy storage as measured by photoacoustic spectroscopy. On the other hand, the electron sink created by the reduction of hydrogen at the acceptor side of photosystem I is shown to reoxidize efficiently the plas-toquinone pool of photosystem II.     

Kinetics of lamellar-to-cubic and intercubic phase transitions of pure and cytochrome c containing monoolein dispersions monitored by time-resolved small-angle X-ray diffraction

We investigated the effect of incorporation of a small aqueous peripheral membrane protein (cyt c) into the three-dimensional periodic nanochannel structures formed by the lipid monoolein (MO) on its rich phase behavior as a function of temperature, pressure, and protein concentration using synchrotron X-ray small-angle diffraction. By simultaneous use of the pressure-jump relaxation technique and time-resolved synchrotron X-ray diffraction, we also studied the kinetics of various lipid mesophase transformations of the system for understanding the mechanistic pathways of their formation influenced by the protein-lipid interactions. Cyt c incorporated into the bicontinuous cubic phase Ia3d of MO has a significant effect on the lipid structure and the pressure stability of the system already at low protein concentrations. Concentrations higher than 0.2 wt % of cyt c led to an increase in interfacial curvature due to interaction of the protein with the lipid headgroups. This promotes the formation of a new, probably partially micellar cubic phase of crystallographic space group P4(3)32. Upon pressurization, the P4(3)32 phase undergoes a phase transition to a cubic Pn3m phase with smaller partial specific volume. Increase in protein concentration increases the pressure stability of the P4(3)32 phase. The formation of this phase from the cubic phase Pn3m is a slow process taking many seconds and having a time lag in the beginning. It seems to occur as a two-state process without ordered intermediate states. At temperatures above 60 degrees C, the P4(3)32 phase is unable to accommodate the unfolded protein and transforms to a bicontinuous cubic Ia3d phase. Time-resolved small-angle X-ray scattering studies show that the L(alpha) --> Ia3d transition in pure MO dispersions under limited hydration conditions occurs within a time interval of 1 s at 35 degrees C preceded by a lag phase of 1.5 s. The Ia3d cubic phase initially forms with a much larger lattice constant due to hydration and experiences an initially lower curvature that relaxes within about 1 s. Interestingly, no other cubic phases are involved as intermediates in the transition, i.e., the gyroid cubic phase is able to form directly from the L(alpha) phase. The mechanism behind the L(alpha) --> Ia3d transition in pure MO dispersions has been discussed within the framework of recent stalk models for membrane fusion. In the presence of cyt c, the L(alpha) --> Ia3d transition is much slower. The rather long relaxation times of the order of seconds are probably due to a kinetic trapping of the system and limitation by the transport and redistribution of water and lipid in the evolving new lipid phases. We also studied the transition from the pure lamellar L(alpha) phase to the Ia3d-P4(3)32 two phase region and observed a rather complex transition behavior with transient lamellar and cubic intermediate states.     

Amides as precursors of imidoyl radicals in cyclisation reactions

Amides have been successfully used as precursors of imidoyl radicals for radical cyclisation. The amides have been converted to imidoyl selanides via reaction with phosgene to yield imidoyl chlorides followed by reaction with potassium phenylselanide. Imidoyl selanides were reacted with tributyltin hydride (Bu₃SnH) as the radical mediator with triethylborane or AIBN as initiators to yield imidoyl radicals for cyclisation reactions. Imidoyl radicals have been cyclised onto alkenes to yield 2,3-substituted-indoles and -quinolines and also onto pyrroles and indoles to give bi- and tricyclic heteroarenes.     

Gas-phase chemistry of the negative ions of fully-protected peptides by high-resolution electrospray ionization tandem mass spectrometry

Fully-protected C-terminal free peptides can be conveniently analyzed by high-resolution electrospray tandem mass spectrometry (ESI-MS/MS) in a quadrupole quadrupole time-of-flight tandem hybrid mass spectrometer, operated in the negative (-) ionizaionization mode. The unusual choice of negative ions in mass spectrometry applications to peptide analysis was needed to obtain exhaustive sequence and structural data. The low-energy collision-induced dissociation (CID) experiments provided, in fact, tandem mass spectra displaying highly diagnostic fragments with a good signal-to-noise ratio. The method is applied to segments of porcine calcitonin (Cal), Cal (1016, 1), Cal (1724, 2) and Cal (2528, 3) whose [M H]- deprotonated molecular ions provided low-energy CID mass spectra which allow the evaluation either of the primary structure of the peptide and of the location of the side-chain protective groups. ESI (+) MS can be conveniently used, in the high resolution mode, to achieve precise information on the elemental composition of the examined peptides.     

A convenient synthesis of novel pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazinones from imidazo[1,2‐a]pyridines

The imidazo[1,2‐a]pyridine system was investigated as a synthon for the building of very attractive fused triazines, a planar, angular tri‐heterocycle with potential biological activity. Thus ethyl 3‐nitroimidazo[1,2‐a]pyridine‐2‐carboxylate was treated with ammonia or with an excess of primary amines to generate the corresponding substituted nitro carboxamidoimidazopyridines. The nitro substituent in the latter products, was reduced to yield 3‐amino‐2‐carboxamidoimidazo[1,2‐a]pyridine derivatives, which in turn were treated with nitrous acid to furnish 1‐oxo‐2‐substituted pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazines.     

Atmospheric degradation of 2-butanol, 2-methyl-2-butanol, and 2,3-dimethyl-2-butanol: OH kinetics and UV absorption cross sections

The absolute rate coefficients for the reactions of hydroxyl radical (OH) with 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), and 2,3-dimethyl-2-butanol (k(3)) were measured as a function of temperature (263-354 K) and pressure (41-193 Torr of He, Ar, and N(2)) by the pulsed laser photolysis/laser-induced fluorescence technique. This work represents the first absolute determination of k(1)(-)k(3) and their temperature dependence. No pressure dependence of the rate coefficients was observed in the range studied. Thus, k(i)(298 K) values (x10(-12) cm(3) molecule(-1) s(-1) with an uncertainty of +/-2sigma) were averaged over the pressure range studied yielding 8.77 +/- 1.46, 3.64 +/- 0.60, and 9.01 +/- 1.00 for 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), and 2,3-dimethyl-2-butanol (k(3)), respectively. k(1) and k(3) exhibit a slightly negative temperature dependence over the temperature range studied. In contrast, the rate coefficient for the reaction of OH with 2-methyl-2-butanol (k(2)) did not show any temperature dependence. Some deviation of the conventional Arrhenius behavior was clearly observed for k(3). In this case, the best fit to our data was found to be described by the three-parameter expression k(T) = A + B exp(-C/T). The UV absorption cross sections of 2-butanol, 2-methyl-2-butanol, and 2,3-dimethyl-2-butanol have also been measured at room temperature between 208 and 230 nm. The values reported constitute the first determination of the UV cross sections of those alcohols. Our results are compared with previous studies, when possible, and are discussed in terms of the H-abstraction by OH radicals. The atmospheric implications of these reactions and the photochemistry of these alcohols are also discussed.     

Hyperbranched polycyclocarbosiloxane

A new cross-linkable naphthalene-cyclocarbosiloxane was synthesised starting from bis[1,4-bis(methylhydrosiloxanyl)-dihydronaphthalene] via catalytic addition of the Si-H reactive groups to the unsaturated bond of triethoxyvinylsilane. Hyperbranched polycyclocarbosiloxane networks were formed by condensation of the alcoxyl groups in the presence of dibutyltin dilaurate as catalyst. The cross-linking process was studied in order to obtain information concerning the values of the reaction parameters and to determine the position of the gelation point relative to conversion.GPC analysis, IR, and 1H-NMR spectral analysis techniques were used to investigate the chemical structures of the intermediates and of the product.     

Unsupported gold(I)-copper(I) interactions through eta1 Au-[Au(C6F5)2]- coordination to Cu+ Lewis acid sites

The reaction of the complex [Au2Ag2(C6F5)4)N[triple bond]CCH3)2]n (1) with 1 equiv of CuCl in the presence of 1 equiv of pyrimidine ligand leads to the formation of the heteronuclear Au(I)-Cu(I) organometallic polymer [Cu{Au(C6F5)2}(N[triple bond]CCH3)(mu2-C4H4N2)]n (2) through a transmetalation reaction. Complex 2 displays unprecedented unsupported Au(I)...Cu(I) interactions of [Au(C6F5)2]- units with the acid Cu(I) sites in a [Cu(N[triple bond]CCH3)(mu2-pyrimidine)]n+(n) polymeric chain. Complex 2 has a rich photophysics in solution and in the solid state.