An iterative extended Hückel calculation for some amino acids containing sulphur and selenium

Semiempirical iterative extended Hückel calculations are performed for the dipolar ions of cystine, cysteine, methionine and their selenium analogues. An attempt has been made to interprete the wave function of cysteine in terms of localized molecular orbitals.

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Zusammenfassung Semiempirische iterative Rechnungen wurden nach der erweiterten Hückeltheorie für die dipolaren Ionen Cystin, Cystein, Methionin und ihre Selen-Homologen durchgeführt. Für den Fall des Cysteins wurde die Wellenfunktion auf lokalisierte Orbitale transformiert.

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Résumé On a analisé les ions dipolaires de la cystine, cystéine, methionine et leurs analogues séléniates avec la méthode semiempirique de Hückel étendue itérative. On a cherché à intérpreter la fonction d'onde pour la cystéine dans le schème des orbitales moleculaires localisées.

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Work performed with the financial support of the Consiglio Nazionale delle Ricerche.     

Force field parameters for molecular mechanical simulation of dehydroamino acid residues

Parameters suitable to describe the conformational behavior of α – β unsaturated aminoacids and peptides in the framework of an existing force field for nucleic acids and proteins (S.J. Weiner, P.A. Kollman, D.A. Case, U. Chandra Singh, C. Ghio, G. Alagona, S. Profeta, Jr., and P. Weiner, J. Am. Chem. Soc., 106 , 765 (1984)) are proposed and tested. Attention is primarily focused on dehydrophenylalanine and dehydroalanine containing peptides. The values of the parameters needed were obtained from experimental measurements available in the literature and from ad hoc quantum mechanical calculations. The selected values have subsequently been adapted and refined through molecular mechanical simulations on model compounds, such as Ac-ΔPhe-NMe, for which it was possible to carry out STO-3G/SCF calculations to check selected points on the ?,ψ conformational map, and Ac-ΔAla-NMe, where the comparison was carried out at the 4-31G level. The newly determined force field was then applied to Ac-ΔPhe-Ala-ΔPhe-NMe, whose minimal energy structures allowed us to explain the different circular dichroism behavior observed in CH₂Cl₂ and in dioxane. Starting from two minimum energy geometries of this tripeptide, the full optimization with AM1 produced an independent guess to their structure and stability in good agreement with the molecular mechanical one.     

Colour pigments of Trichoderma harzianum. Preliminary investigations with thin-layer chromatography-Fourier transform infrared spectroscopy and high-performance liquid chromatography with diode array and mass spectrometric detection

The colour pigments of Trichoderma harzianum fermentation broth were separated and the main fractions were tentatively identified by reversed-phase thin-layer chromatography-Fourier transform infrared spectroscopy (RP-TLC-FT-IR), RP-HPLC-diode array detection and RP-HPLC-MS. It was established that the multistep gradient elution developed for RP-TLC separation of pigments can be successfully used as a pilot method for the rational design of gradient elution in RP-HPLC for the separation of the same pigments. FT-IR and MS measurements were unable to identify the exact chemical structures of the main pigment fractions, the presence of OH, =CH and C=O (RP-TLC-FT-IR) and OH and NH, substructures (RP-HPLC-MS) was confirmed. It was assumed that the main pigment fractions are oxidation polymers originating from monomer molecules containing polar substructures and double bonds in the alkyl chain which are liable for oxidation during the aerobic fermentation process.     

Sol-Gel Nanosized Semiconducting Titania-Based Powders for Thick-Film Gas Sensors

Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO₂ nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO₂ powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO₂. The addition of Ta and Nb is effective to keep the TiO₂ grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity.     

New eco-friendly low-cost binders for Li-ion anodes

In the production of commercial Li-ion batteries, the active materials slurries are generally prepared using polyvinylidene fluoride (PVdF) as binder because of its good adhesion properties and electrochemical stability. Unfortunately, there are some disadvantages related to the use of PVdF: the most important is the use of toxic and environmentally unfriendly solvents, such as N-methyl-pyrrolidone (NMP), and the second is the high costs. In the light of these considerations, it seemed straightforward to investigate the suitability of some water-soluble, inexpensive, and eco-friendly materials to test as alternative binders (sodium alginate, chitosan tragacanth gum, gelatin). The rheological properties of these materials have been investigated in addition to the electrochemical characterization. Furthermore, graphite electrodes with PVdF, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) binders have been considered for sake of comparison. We found that some of these water-soluble binders, besides good electrochemical performances, showed a high adhesion to the current collector and a good electrochemical stability under the experimental conditions employed, which makes them interesting for the next generation of Li-ion batteries.     

Platinum Carbonyl Clusters Chemistry: Four Decades of Challenging Nanoscience

The state of art of the chemical, spectroscopic and structural characterization of platinum carbonyl clusters is reviewed. We begin by enlightening the fundamental contribution given to this chemistry of two great scientists: Paolo Chini and Larry Dahl, two without equal maestros of science and life. We then focus the main body of this review on the challenge represented by studying molecular ions already belonging to the nano regime by size almost 50 years ago, and the challenges their chemistry continues to present also nowadays. In detail, the possible reasons which enable the [Pt_3n (CO)_6n ]^2? oligomers to grow up to n = 10, and why the oligomers with n > 5 may self-assemble in infinite molecular conductor wires are suggested. The interplay between the CO/Pt_surface ratios and electronic effects in driving the platinum cluster from pseudo one- to tri-dimensional globular structures, often representing chunks of the fcc metal lattice or interpenetrated pentagonal prisms and icosahedra, is examined by means of two significant examples. The nanocapacitor behavior of most high-nuclearity carbonyl clusters is briefly recalled and is confirmed by most recent results. The size-induced transition of their metal kernels from insulator to semiconductor and the expected consequences of their eventual transition to a metallic state are also discussed. Finally, we conclude by commenting the present lack of Pt cluster interstitially lodging a main group element and not yet quantitatively-assessed aggregation phenomena in solution, perhaps peculiar of ionic salts of carbonyl clusters, which could make undetectable the NMR signal of any spin-active nuclei beyond a cluster nuclearity of ca. 20–25.     

Theoretical investigation of histidine-tryptophan preferential interactions

Several histidine-tryptophan complexes (either stacked or T-shaped), derived from the crystal structures available in the Brookhaven Protein Data Bank, have been examined with molecular mechanics (MM), using the Tripos force field with Gasteiger-Hückel charges, whose trend was found to be analogous to the AMBER or CHARMM ones. The MM results were compared to the ab initio MP2 results, with and without counterpoise (CP) correction, previously obtained using extended basis sets on 5-methylimidazole and indole as model systems. MM seems to underestimate the interaction energy between the two monomers when compared to the uncorrected MP2 results, while the agreement is much better after including the CP correction at the MP2 level in all cases. MM was thus used to qualitatively analyse the dependence of the stacking energy on the ring rotation at a variable distance and ring centroid displacement for these systems, while keeping the rings in parallel planes. An analogous study was carried out for a T-shaped adduct. Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 1 February 1999     

PHOTOBEHAVIOR OF EUGLENA GRACILIS: ACTION SPECTRUM FOR THE STEP-DOWN PHOTOPHOBIC RESPONSE OF INDIVIDUAL CELLS

Abstract—Light-induced behavioral responses of Euglena gracilis have been investigated in single cells by means of a video system coupled to an optical microscope. Light intensity-effect curves at different wavelengths in the near UV and visible range have been determined. From these curves the action spectrum for the step-down photophobic response of Euglena has been calculated. From a comparison with the results obtained using a population method by means of a phototaxigraph, it is concluded that a single photomotile reaction is responsible for cell accumulation, brought about by trapping in the light spot and possibly by phototaxis towards scattered light from organisms already in the light field.