Global optimization of bimetallic cluster structures. II. Size-matched Ag-Pd, Ag-Au, and Pd-Pt systems

Genetic algorithm global optimization of Ag-Pd, Ag-Au, and Pd-Pt clusters is performed. The 34- and 38-atom clusters are optimized for all compositions. The atom-atom interactions are modeled by a semiempirical potential. All three systems are characterized by a small size mismatch and a weak tendency of the larger atoms to segregate at the surface of the smaller ones. As a result, the global minimum structures exhibit a larger mixing than in Ag-Cu and Ag-Ni clusters. Polyicosahedral structures present generally favorable energetic configurations, even though they are less favorable than in the case of the size-mismatched systems. A comparison between all the systems studied here and in the previous paper (on size-mismatched systems) is presented.     

Light-Lithophile Element Metasomatism of Finero Peridotite (W ALPS): A Secondary-Ion Mass Spectrometry Study

The high sensitivity of Secondary-Ion Mass Spectrometry (SIMS) in determining very low trace-element concentrations (down to few ppb wt) has been used to address the important issue of the Light-Lithophile Element (LLE) signatures of mantle peridotites and their potential utility as tracers of transfer of subducted material into the overlying mantle wedge operated either by slab-related fluids or melts. The effects of such metasomatic agents have been described and interpreted on the basis of geochemical and isotopic characteristics of peridotites. However, direct observation of metasomatic processes occurring in the mantle is limited because the most highly metasomatised peridotites are usually found as xenoliths which do not allow large-scale chemical information. In this context, the Finero phlogopite-peridotite massif (Ivrea-Verbano Zone, western Italian Alps) offers a unique opportunity because it contains hydrous mineral assemblages (amphibole + phlogopite ± apatite) and trace element enrichments, supposed to be related to the pervasive migration of slab-derived melts. In this study we report the results of texture-related SIMS investigations performed on a wide range of trace elements such as Li, B, Large-Ion Lithophile Elements (LILE), Rare Earth Elements (REE), Nb, Zr, Ti on clinopyroxenes from the Finero peridotite. SIMS analyses revealed unusual LLE signatures that highlight the role of crust-derived components and open new perspectives on the timing of metasomatic processes.     

Direct electrochemical oxidation of polyacrylates

A promising elimination treatment of non-biodegradable organic pollutants is the direct electro-oxidation. In this work has been proposed the electrochemical elimination of polyacrylates by using boron-doped diamond (BDD) as anodic material. The complete elimination of organic contaminants has been obtained and this is the first case of successful electrochemical treatment of polymeric and bio-refractory species. The tests of the electrochemical oxidation have been conducted at constant current conditions and a complete elimination of organic species has been reached. The decrease of the COD value with time follows the behaviour of an ideal anode as in the case of low molecular organic compounds.     

Production of MoF6 via electrochemical fluorination

A new process for the electrochemical preparation of the fluorinating agent MoF₆ is described. Solutions fo MoF₄ in anhydrous HF are electrolyzed in a multielectrode cell at 20°C, 5.4÷5.7V cell voltage and 50÷100 A/m² current density, using nickel anodes and cathodes. A new modified Simons cell has been designed, equipped with a stirring system for the efficient recirculation of the electrolyte and with a condenser held at ?15°C.The reaction product MoF₆ (b.p. 35°C) dissolves in the electrolyte, contributing to its conductivity and in part is volatilized together with HF and some byproduct fluorine.Gaseous MoF₆ passing beyond the condenser can be easily collected and separated in a nickel trap held at low temperature.The process presents substantial advantages in comparison with known alternatives, such as the synthesis of MoF₆ from the elements Mo and F₂ at 250°C or the dismutation reaction: 2MoCl₅ + 10HF → MoF₄ + MoF₆ + 10 HClPreparation of the starting material MoF₄, operating conditions and experimental results are illustrated.     

tert‐Butyl (3S,6R,9R)‐(5‐oxo‐3‐{[1(R)‐phenyl­ethyl]­carbamoyl}‐1,2,3,5,6,7,8,8a‐octa­hydro­indolizin‐6‐yl)­carbamate monohydrate at 95 K

The asymmetric unit of the title compound, C₂₂H₃₁N₃O₄·H₂O, incorporates one water mol­ecule, which is hydrogen bonded to the 3‐oxo O atom of the indolizidinone system. The two rings of the peptidomimetic mol­ecule are trans‐fused, with the six‐membered ring having a slightly distorted half‐chair conformation and the five‐membered ring having a perfect envelope conformation. The structure is stabilized by intermolecular O—H?O interactions between the water and adjacent peptide mol­ecules, and by N—H?O interactions between the peptide mol­ecules, which link the mol­ecules into infinite chains.     

Membrane localization and flexibility of a lipidated ras peptide studied by molecular dynamics simulations

Lipid-modified membrane-binding proteins are essential in signal transduction events of the cell, a typical example being the GTPase ras. Recently, membrane binding of a doubly lipid-modified heptapeptide from the C-terminus of the human N-ras protein was studied by spectroscopic techniques. It was found that membrane binding is mainly due to lipid chain insertion, but it is also favored by interactions between apolar side chains and the hydrophobic region of the membrane. Here, 10 explicit solvent molecular dynamics simulations for a total time of about 150 ns are used to investigate the atomic details of the peptide-membrane association. The 16:0 peptide lipid chains are more mobile than the 14:0 phospholipid chains, which is in agreement with (2)H NMR experiments. Peptide-lipid and peptide-solvent interactions, backbone and side-chain distributions, as well as the effects of lipidated peptide insertion onto the structure, and dynamics of a 1,2-dimyristoylglycero-3-phosphocholine bilayer are described. The simulation results validate the structural model proposed by the analysis of spectroscopic data and highlight the main aspects of the insertion mechanism. The peptide in the membrane is rather rigid over the simulation time scale of about 10 ns, but different partially extended conformations devoid of backbone hydrogen bonds are observed in different trajectories.     

Design and photochemical characterization of a biomimetic light-driven Z/E switcher

Protonated Schiff bases (PSBs) of polyenals constitute a class of light-driven switchers selected by biological evolution that provide model compounds for the development of artificial light-driven molecular devices or motors. In the present paper, our primary target is to show, through combined computational and experimental studies, that it is possible to approach the design of artificial PSBs suitable for such applications. Below, we use the methods of computational photochemistry to design and characterize the prototype biomimetic molecular switchers 4-cyclopenten-2'-enylidene-3,4-dihydro-2H-pyrrolinium and its 5,5'-dimethyl derivative both containing the penta-2,4-dieniminium chromophore. To find support for the predicted behavior, we also report the photochemical reaction path of the synthetically accessible compound 4-benzylidene-3,4-dihydro-2H-pyrrolinium. We show that the preparation and photochemical characterization of this compound (together with three different N-methyl derivatives) provide both support for the predicted photoisomerization mechanism and information on its sensitivity to the molecular environment.     

Ab initio prediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A', 2(2)A', and 1(2)A") potential-energy surfaces of the C2Cl radical, correlating at linear geometries with 2Sigma+ and 2Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the 12C12C35Cl isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous.     

QUANTUM YIELDS FOR LASER PHOTOCYCLIZATION OF BILIRUBIN IN THE PRESENCE OF HUMAN SERUM ALBUMIN. DEPENDENCE OF QUANTUM YIELD ON EXCITATION WAVELENGTH

The quantum yield for laser photocyclization of bilirubin to lumirubin in the presence of human serum albumin (phi LR) was measured at five monochromatic excitation wavelengths in the range 450-530 nm. Solutions used were optically thin throughout the wavelength range and precautions were taken to exclude contributions from photocyclization of bilirubin XIII alpha impurities. The values obtained (7.2-18 x 10(-4] were lower than those previously reported and showed the following wavelength dependence: 457.9 less than 488.0 less than 501.7 less than 514.5 approximately equal to 528.7. However, the rate of lumirubin formation, normalized to constant fluence, decreased with wavelength over the same wavelength range and no evidence was found that photoisomerization of bilirubin to lumirubin is faster with green (514.5 or 528.7 nm) than with blue (457.9 or 488.0 nm) light. The stereoselectivity of the configurational isomerization of bilirubin to 4Z,15E and 4E,15Z isomers also was studied. This reaction became less regioselective for the 4Z,15E isomer with increasing wavelength. The observed wavelength dependence of phi LR and of the [4E,15Z]: [4Z,15E] ratio at photoequilibrium are consistent with an exciton coupling model in which intramolecular energy transfer can occur between the two pyrromethenone chromophores of the bilirubin molecule in the excited state. Relative rates of lumirubin formation in vivo at different excitation wavelengths and constant fluence were estimated for different optical thicknesses and for different skin thicknesses. These estimates suggest that the recently reported clinical equivalence of blue and green phototherapy lights probably reflects the marked variation of skin transmittance with wavelength more than wavelength-dependent photochemistry. The calculations also indicated that the optimal wavelength for phototherapy is probably on the long wavelength side of the bilirubin absorption maximum.     

PHOTODYNAMIC THERAPY OF B16 PIGMENTED MELANOMA WITH LIPOSOME-DELIVERED Si(IV)-NAPHTHALOCYANINE

The possibility of extending photodynamic therapy to the treatment of highly pigmented neoplastic lesions was tested by using Si(IV)-naphthalocyanine (SiNc) as a tumor-localizing agent. Si(IV)-naphthalocyanine displays intense absorbance at 776 nm (ɛ= 5 × 10⁵ M⁻¹ cm⁻¹), where melanin absorption becomes weaker. As an experimental model we selected B16 pigmented melanoma subcutaneously transplanted to C57BL mice. Upon injection of 0.5 or 1 mg kg⁻¹ of liposome-incorporated SiNc, maximal accumulation of the photosensitizer in the tumor was observed at 24 h with recoveries of 0.35 and 0.57 μg g⁻¹, respectively. However, the tumor targeting by SiNc shows essentially no selectivity, since the photosensitizer concentrations in the skin (peritumoral tissue) were very similar to those found in the tumor at all postinjection times examined by us. Irradiation of SiNc-loaded melanoma with 776 nm light from a diode laser at 24 h postinjection induces tumor necrosis and delay of tumor growth. The effect appears to be of purely photochemical nature at dose rates up to 260 mW cm⁻²; at higher dose rates, thermal effects are likely to become important.