Donor acceptor interaction in metalloproteins

This work addresses some electronic aspects of electron transfer at large distances when the donor and acceptor are attached to a polymeric structure. The transfer-matrix approach and a Green's function formalism are used to obtain an expression for the transition matrix element T _ab. The results are analyzed and compared with previous results obtained from related models.Work partially supported by the Brazillian Agency CNPqOn leave of absence from Departamento de Física, Universidade Federal de Pernambuco, 50000, Recife, Pe, Brasil     

Bond index: relation to second-order density matrix and charge fluctuations

It is shown that, in the same way as the atomic charge is an invariant built from the first-order density matrix, the closed-shell generalized bond index is an invariant associated with the second-order reduced density matrix. The active charge of an atom (sum of bond indices) is shown to be the sum of all density-density correlation functions between it and the other atoms in the molecule; similarly, the self-charge is the fluctuation of its total charge.On leave of absence from Depto. de Física e Química, UFES, 29000 Vitória, ES, Brasil     

An ab initio 4-21G gradient study of clavulanic acid

The structure of five conformations of the-lac inhibitor clavulanic acid have been optimized using ab initio gradient methods at the 4-21G level. The conformations of lowest energy possess an intramolecular H bond and also have the lowest pyramidization of N1. Different side-chain conformations lead to (i) differences in anomeric interactions and variations of the hydroxyethylidene geometry and (ii) changes in the geometry of the ring skeleton, which are smaller in the four- than in the five-membered ring.On leave from Universidad de Santiago de Compostela, Departamento de Química Física, Facultad de Química, Galicia (Spain).     

Formation of negative ions by electron impact on SiF4 and CF4

First measurements of cross sections for the formation of negative ions by electron attachment to SiF₄ and CF₄ are reported for an electron impact energy range of 0 to 50 eV. The energies at which the various ions appear and the positions at which the ionization efficiency curves peak have been obtained and compared with previous measurements. Thermochemical data have been used to predict and identify the various channels of dissociation. Cross sections for the production of ion pairs through the process of polar dissociation have also been measured.Visiting fellow, on leave of absence from Universidade Federal de Sao Carlos, Departamento de Quimica, 13560, Sao Carlos, SP, Brazil     

Definition of a multicenter bond index

The tensor character of the first-order density matrix leads to the definition of an MO multicenter bond index for closed-shell systems. It is here applied to three-center bonds. Satisfactory results are obtained for compounds involving secondary bonds, strong and normal hydrogen bonds; the index for the peptide bond is found to be similar to that of strong hydrogen bonds.On leave of absence from Inst. de Física, UFBA, 40000 Salvador, BA, Brasil.     

Characterization of carbonaceous products by TG and DTA

Thermogravimetry and differential thermal analysis, using only a combustion step with oxygen, were used to characterize carbonaceous products. Binary composition of alumina and wood charcoal, coke, carbon black, petroleum coke or carbon graphite were prepared containing 5 wt% of each. They were characterized by the burnout onset and DTA and DTG peak temperatures, which range from 476°C for wood charcoal to 790°C for carbon graphite.Complementary characterization of each product was also performed by estimating the ash content from the TG curves, and the calorific value from the DTA curves. The results indicate that these thermal analysis techniques, with only one oxygen burnout step, can be applied both for quality control of each raw material, and to determine the appropriate processing temperatures of the ceramic compositions in which they are used.Funding for this work was provided by the Brazilian PADCT-GTM Project. Participation at the 11th ICTAC was sponsored by Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior — CAPES — and Fundação de Amparo à Pesquisa do Rio de Janeiro — FAPERJ.     

Potential energy surface for the Pd-H2 system: Comparison with matrix isolation experiments

Summary We present here those aspects of the Pd-H₂ potential energy surface that are most directly related to the questions raised by matrix isolation experiments for the formation of Pd(¹-H₂) and Pd(²-H₂) complexes. 125 points of this potential energy surface were obtained at a CI level using the CIPSI Scheme and including the order of 10⁵ configurations. Relativistic effects are included and shown not to be crucial in understanding the main features of the surfaces. The theoretical results serve to explain many features of the low-temperature experiments on the Pd-H₂ reaction, especially those concerning the spectroscopic changes observed when different noble gas supports, Kr or Xe, are utilized.On Sabbatical leave from Instituto de Física, UNAM     

Limestone doses affecting mineral contents in tropical grass forage

Summary Field trial was performed at the experimental farm of Southeast Embrapa Cattle, São Carlos - SP, Brazil, on a 16 year old Brachiaria decumbens pasture, grown on a distrophic Hapludox (Oxisol), recovered by the use of limestone and fertilizer. The experiments were carried out in random blocks, with 6 replications and 5 treatments. The 100 m² blocks were established in the pasture. Each block received a sequence of limestone doses of 0, 1, 2, 4 and 8 t/ha. The forage samples were taken one year after limestone application on soil surface. Instrumental neutron activation analysis (INAA) followed by gamma-ray spectrometry was the analytical method used to determine mineral contents. The statistical analysis showed a negative linear correlation of Br, Co, Cr, Mn and Zn contents in forage with the limestone doses, while the uptake of Mg was affected in a positive way.     

Evaluation of thermal behavior of latex membranes from genetically improved rubber tree (Hevea brasiliensis)

Thermal stability, thermal decomposition process, residual mass, temperature of glass transition (T_g) and temperature dependence of storage modulus (E’), were determined for latex membranes prepared from six clones of Hevea brasiliensis: IAC 331, IAC 332, IAC 333 and IAC 334 grown at experimental plantations of Instituto Agronômico de Campinas (IAC) in Votuporanga, São Paulo State, Brazil. Latex membranes from GT1 and RRIM 600 Asian matrix clones were used as references. The thermal behavior of latex membranes from genetically improved rubber trees was characterized using thermogravimetry/derivative thermogravimetry (TG/DTG), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The thermal behavior of latex from clones studied in the present work showed similar features of the clones previously reported (IAC 40, IAC 300, IAC 301, IAC 328, IAC 329 and IAC 330), with mass loss in four consecutive steps, except IAC 333, which showed an additional mass loss step.     

Multielemental analysis of Brazilian milk powder and bread samples by neutron activation

The concentrations of Na, Cl, Mn, Br, Fe, Zn, Rb, Sb, Sc, Cr, Al and Mg were determined in some types of bread and in some brands of milk powder consumed in the city of São Paulo (SP—Brasil), by instrumental neutron activation analysis. Radiochemical separations were carried out by means of retention of²⁴Na on hydrated antimony pentoxide (HAP) from a 8N HCl solution, after digestion of the organic matter. It was possible in this way to determine the radioisotopes⁶⁴Cu,^69mZn and¹⁴⁰La in the effluent solution. The detection limits of the trace elements analyzed in bread and milk powder samples were determined using the Currie and Girardi criterions.From a dissertation submitted by V. A. MAIHARA to the University of São Paulo in partial fullfillment of the requirements for a Master of Science Degree in Nuclear Technology.     

An improved flow system for chloride determination in natural waters exploiting solid-phase reactor and long pathlength spectrophotometry

A single and sensitive spectrophotometric method for chloride ions determination based on a commuted flow system with a 100 cm optical path flow cell and a solid-phase reactor containing immobilized silver chloranilate was proposed. This procedure exploited the AgCl formation in the solid-phase reactor leading the chloranilate ions, monitored spectrophotometrically at 530 nm. The analytical signals were 75-fold higher and the sensitivity was 12-fold than that achieved with a 1 cm flow cell, allowed a chloride determination in the 0.5-100 mg l⁻¹ range. The R.S.D. was 1.1% (n = 20) with a sample throughput of 80 h⁻¹ and a waste generated of ca. 100 ng of chloranilate ions per determination. Four samples of natural waters from São Carlos and Araraquara cities were evaluated using the proposed method. Results agreed with the obtained by a reference method at the 95% confidence level.     

Theoretical study of the electronic spectra ofcis-1,3,5-hexatriene andcis-1,3-butadiene

Summary The electronic spectra forcis-1,3-butadiene andcis-1,3,5-hexatriene have been studied using multiconfiguration second-order perturbation theory (CASPT2) and extended ANO basis sets. The calculations comprise all singlet valence excited states below 8.0 eV, the first 3s, 3p, 3d Rydberg states, and the second 3s state. The four lowest triplet states were also studied. The resulting excitation energies forcis-hexatriene have been used in an assignment of the experimental spectrum, leading to a maximum deviation of 0.13 eV for the vertical transition energies. The calculations place the 1¹ B ₂ state 0.04 eV below the 2¹ A ₁ state. 16 excited states were studied incis-butadiene, using a CASPT2 optimized ground state geometry. The 1¹ B ₂ state was located at 5.58 eV, 0.46 eV below the 2¹ A ₁ state and 0.09 eV above the experimental value. No experimental assignments are available for the 15 other transitions. On leave from: Departmento de Quimica Física, Universidad de Valencia, Dr. Moliner 50, Burjassot, E-46100-Valencia, Spain     

Modelling the kinetics of solute segregation to partial dislocations for isothermal microcalorimetric evaluations

A model is proposed to describe the kinetics of solute segregation to partial dislocations in solid solutions of cold-rolled alloys. The case when half edge and half screw dislocations are present is considered. The model gives account of the kinetic behaviour observed in a deformed Cu-19 at% Al alloy where two unknown processes could be assessed during calorimetric isothermal experiments. The faster process corresponds to segregation to screw dissociated dislocations while the slower one corresponds to segregation to edge dissociated dislocations. Experimental activation energies, larger for edge dislocations, are close to that for pipe diffusion along the partials corrected by pinner binding energy terms. It is also predicted that segregation occurs faster as the dislocation density is increased. A quantitative comparison of experimental results with model predictions is given.The authors would like to thank the Fondo de Desarrollo Científico y Tecnológico (FONDECYT) for financial support through Project 1950566, and the Institute de Investigaciones y Ensayes de Materiales (IDIEM), Facultad de Ciencias Físicas y Matemticas, Universidad de Chile, for the facilities provided for this research.     

Chemical Characterization by INAA of Brazilian Ceramics and Cultural Implications

Archaeological ceramic fragments from Água Limpa site, in São Paulo, Brazil, were analyzed using instrumental neutron activation analysis. Multivariate statistical methods including Pearson correlation coefficient, cluster and principal components analysis were used to interpret the concentration data. Rare earth and alkaline elements were highly correlated. Six principal components explained 74.9% of the total variance and five clusters were found. The sample chemical composition showed that all samples have the same provenance.     

About evaluation of many-center molecular integrals

Studies of truncated expansions of Slater s-orbitals, in terms of associated Laguerre functions and spherical harmonics at another center, are carried out. The possibility of using such an expansion to calculate many-center molecular integrals is considered. The procedure is applied to solve three-center nuclear attraction integrals; it is shown that in some cases this expansion can provide relatively good results.

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Zusammenfassung Slater-s-Orbitale werden an einem anderen Zentrum in Reihen von Produkten aus Kugelfunktionen und zugeordneten Laguerreschen Funktionen entwickelt. Untersucht wird die Konvergenz der Reihen und ihre Eignung zur Berechnung von Mehrzentren-, insbesondere von Dreizentren-Kernwechselwirkungs-Integralen. Die Ergebnisse sind zum Teil recht gut.

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Résumé On étudie des développements tronqués d'orbitales s de Slater, en série de fonctions associées de Laguerre et d'harmoniques sphériques centrés en un autre point. On examine les possibilités d'employer ce genre de développement dans le calcul des intégrales moléculaires polycentriques. On applique le procédé à la résolution des intégrales tricentriques d'attraction nucléaire et l'on montre que dans certains cas cette expansion peut conduire à des résultats relativement bons.

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A preliminary report of this work was read at the XII. meeting of the Real Sociedad Española de Física y Química, Salamanca, June 1965.     

Neutron activation analysis of thorium after separation of233Th by isotopic ion exchange

The procedure for thorium determination in ammonium diuranate (ADU) and rocks, by neutron activation analysis after chemical separation of²³³Th, is presented. The separation of²³³Th from the interfering radioisotopes is based on the retention of²³³Th by a resin saturated with thorium (isotopic exchange) and on the elution of the interfering radioisotopes by a dilute solution of thorium in 0.5M HCl (ion exchange). The determination limit of thorium in rocks and ADU was found to be 0.56 and 9.3 g, respectively, when a 20% relative standard deviation was assumed as acceptable. The highest value obtained for the determination limit of thorium in uranium compounds, on account of the²³⁴Th activity present, is discussed.From a thesis submitted by C. S. Munita to the Instituto de Pesquisas Energéticas e Nucleares (CNEN/SP) University of São Paulo, in partial fulfillment for a Doctor of Science's Degree. Work supported by the Brazilian Atomic Energy Commission.     

Radiochemical separation of thorium by isotope ion exchange

The aim of the present work is to obtain the separation of²³³Th from the radioisotopes formed in the irradiation of Mn, U, Ba, Cs, Co and the lanthanide elements with thermal neutrons, because they may interfere in the neutron activation analysis of Th, when the activity of²³³Th is used. The experiments were performed with the resin Bio-Rad AG 50W X-4 and X-8 (100–200 mesh) in the thorium form. The separation of²³³Th from the interfering radioisotopes is based on the retention of²³³Th by the resin (isotope exchange) and the elution of the interfering radioisotopes with a dilute solution of Th in 0.5M HCl. Batch experiments were made in order to determine the equilibrium time for the isotopic ion exchange of thorium and also the distribution coefficients of the interfering elements between the solution and the resin. Column experiments were carried out with the purpose of establishing the conditions that allow the maximum isotope exchange of²³³Th and the minimum retention of the interfering radioisotopes in the resin. With this purpose, a statistical interpretation of a four variable experimental design is presented.From a thesis submitted by C. Sepúlveda Munita to the Instituto de Pesquisas Energéticas e Nucleares (CNEN/SP) University of São Paulo, in partial fulfillment of a Doctor of Science's Degree. Work supported by the Brazilian Atomic Energy Commission.     

Determination of mercury in head hair of Brazilian populational groups by neutron activation analysis

In the present work, determination of mercury in the head hair of populational groups living near a heavily industrialized region in São Paulo and of Indians living in the Xingu park in the Amazonic region, was carried out by instrumental neutron activation analysis. A control group of people with no suspicion of contamination by mercury was also studied. The range of mercury concentrations found up to now were: for the control group from 0.26 to 02.5 ppm; for the Indians from 6.9 to 34 ppm, and for the industrialized region inhabitants: from 0.30 to 3.0 ppm.     

Solvent extraction studies using tetracycline as a complexing agent. XIII. Application of tetracycline for separation of interfering elements in activation analysis of uranium

Tetracycline in solution of benzyl alcohol was used as an extracting agent to separate uranium from interfering elements in the determination of uranium and of isotopic ratio²³⁵U/²³⁸U by neutron activation analysis. Separation gives a recovery of 97% for uranium and the interferences from matrices of pitchblende and monazite are eliminated.From a dissertation submitted by R. Petrauskas to the University of São Paulo in partial fulfilment for a Master of Science Degree in Nuclear Technology.     

An analysis of the polarizabilities in some conjugated molecules

Atom-atom, atom-bond and bond-bond polarizabilities are calculated with a previously used LCAO approximation. Some properties of naphtalene, assumed as general for alternant hydrocarbons, seem to depend on the approximation employed. Atom-atom polarizabilities in azines are related to other theoretical and experimental quantities. In all the molecules studied, the part played by formal bonds is highly stressed.

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Zusammenfassung Atom-Atom-, Atom-Bindungs- und Bindungs-Bindungs-Polarisierbarkeiten werden mit einem schon früher benutzten LCAO-Verfahren berechnet. Einige Eigenschaften von Naphthalin, die für alternierende Kohlenwasserstoffe als charakteristisch angesehen werden, scheinen von der benutzten Näherung abzuhängen. Atom-Atom-Polarisierbarkeiten in Azinen werden auf andere theoretische und experimentelle Größen zurückgeführt. In allen betrachteten Molekülen spielen formale Bindungen eine bedeutende Rolle.

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Résumé Calcul des polarisabilités atome-atome, atome-liaison et liaison-liaison dans une approximation LCAO utilisée auparavant. Certaines propriétés du naphtalène, supposées générales pour les hydrocarbures alternants, semblent dépendre de l'approximation employée. Les polarisabilités atome-atome des azines sont reliées à d'autres grandeurs théoriques et expérimentales. Dans toutes les molécules étudiées, le rôle joué par les liaisons formelles est mis en relief.

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With a fellowship from the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.

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With a fellowship from the Centro Latino Amerioano de Física.

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The authors gratefully acknowledge the helpful discussions with Dr. J. Brieux and Lic. H. C. González, who has besides collaborated in part of the calculations.