Showing papers by "University of Coimbra published in 1979"
TL;DR: In this paper, the problem of finding a necessary and sufficient condition for a matrix A (λ) to be λ-imbeddable in B (λ), where B is equivalent to a regular λ matrix having A(λ) as a submatrix, was solved in terms of the invariant polynomials of A (δ) and B (ε).
143 citations
TL;DR: The permeability of liposome membranes is increased by organophosphorus and organochlorinated insecticides at concentrations of 10(-5)--10(-4) M and it is suggested that the insecticides may exert some of their toxic actions by modifying certain mechanisms in the cell membrane.
112 citations
TL;DR: In this paper, the mass transfer product, k L a, was determined using unsteady state measurements of oxygen dissolution rates in water, in 0.60 m 3 and 0.043 m 3 baffled mixing vessels fitted with geometrically similar flat-bladed turbines and spargers.
105 citations
TL;DR: The permeability of synaptosomes to non‐electrolytes is temperature dependent, and the low activation energies for the permeation of urea and erythritol indicate that the penetration of non‐ Electrolytes into the synaptsomes does not imply complete dehydration of the molecules.
Abstract: Synaptosomes swell rapidly in isosmotic solutions of glycerol or urea, but the swelling in solutions of larger non-electrolytes, such as erythritol, glucose or sucrose is slower. The permeability of synaptosomes to non-electrolytes is temperature dependent, and the low activation energies for the permeation of urea (13 kcal/mol) and erythritol (9.5 kcal/mol) indicate that the penetration of non-electrolytes into the synaptosomes does not imply complete dehydration of the molecules.
The relative permeability of synaptosomes to cations, as measured by the rate of swelling in isosmotic solutions of acetate salts is in the order: NH+4 > Na+ > Li+ > K+ > Ca2+. The ionophores, X-537A and nigericin, or valinomycin + FCCP, which promote exchange of cations for H+, cause swelling of synaptosomes in solutions of potassium salts of acetate or propionate, but not in KCI, whereas H+ release is higher in KCI medium. This suggests that the organic unions cross the membrane after combining with H+ to form the respective weak acids. The relative permeability to anions is in the order: acetate ≃ propionate > Cl− > SO2-4≃ maleate ≃ succinate.
The energies of activation for the permeability of synaptosomes to potassium acetate in the presence of X-537A or gramicidin D are 13 kcal/mol and 7.5 kcal/mol, respectively, which reflects different mechanisms of action for the two ionophores in the membranes.
30 citations
TL;DR: The results suggest that a transient alkalinization occurs rapidly inside the vesicles and reflects the formation of a transmembrane proton gradient responsible for sustaining the Ca2+ transport.
19 citations
TL;DR: In this paper, a conductimetric cell was used to measure the resistance of a solution inside the capillaries at recorded times, using the Onsager-Fuoss theory.
15 citations
TL;DR: Both rural and urban samples give a median age 0.5 year earlier than date from a national survey performed in 1950, which accords well with results obtained from urban samples in the years 1965-68.
Abstract: SummaryData upon age at menarche have been collected among 254 schoolgirls of a relatively endogamous rural population in central France. The median age at menarche estimated by probits was 13·05±0·18, SD1·42. This accords well with results obtained from urban samples in the years 1965–68. Both rural and urban samples give a median age 0·5 year earlier than data from a national survey performed in 1950.
13 citations
TL;DR: The nucleotidyl units of the unstacked form of ApA2'p have average conformations which are very similar to those of the corresponding mononucleotides in similar conditions, which agrees well with the results from vicinal coupling constants and dimerization shifts obtained for the metal-free system.
Abstract: The conformation of the dinucleotide adenylyl(3′-5′)adenosine 2′-phosphate (ApA2′p) in aqueous solution at different pH values and temperatures has been studied using the lanthanide(III) ethylenodiaminetetraacetate(EDTA) 1:1 complexes as shift and relaxation probes. The conformational analysis, based on mixing different conformations in fast exchange within the nuclear magnetic resonance time scale, agrees well with the results from vicinal coupling constants and dimerization shifts obtained for the metal-free system. The dinucleotide exists in a temperature-dependent and pH-dependent conformational equilibrium between unstacked and base-stacked forms. At neutral pH and low temperature, the stacked form predominates, and it is a predominantly right helical structure, characterized as an anti,3E, g−, g−g−, g′g′, gg, 3E, anti conformation. This situation contrasts with adenylyl(3′-5′)adenosine (ApA), where both right and left helices contribute to the stacked form. The nucleotidyl units of the unstacked form of ApA2′p have average conformations which are very similar to those of the corresponding mononucleotides in similar conditions.
9 citations
TL;DR: The results suggest that the mechanism of exchange of Ca 2+ for Na + in synaptosomes operates similarly for Sr 2+ , but not for Mg 2+ .
7 citations
TL;DR: A review of Wigner's time reversal is presented and some important aspects are emphasized in this article, where the subject is introduced via classical mechanics and comments are made on the roles of time and of the operatoriħ(∂/∂t) in quantum mechanics.
Abstract: A review of Wigner's time reversal is presented and some important aspects are emphasized. The subject is introduced via classical mechanics. Non-physical statements as “time running backwards” are avoided. Comments are made on the roles of time and of the operatoriħ(∂/∂t) in quantum mechanics. The role of symmetries and conservation laws and some properties of the time-reversed states are discussed.
7 citations
TL;DR: In this article, the Raman spectrum of bromoform is recorded at various temperatures in the pure compound and in solution in carbon tetrachloride and the features shown by some of the bands are considered in the light of previous spectroscopic data interpretations.
TL;DR: In this paper, a numerical method, using spline functions of degree five, for obtaining approximate solutions to initial value problems is presented, and it is shown that the method is stable and the convergence is analyzed.
Abstract: A numerical method, using spline functions of degree five, for obtaining approximate solutions to initial value problems is presented. It is shown that the method is stable and the convergence is analysed. Some numerical experiments are included.
TL;DR: In this paper, a consistent interpretation of the Raman spectra of liquid CS 2 is presented, based on the importance of intermolecular interactions in the liquid CS2, and it is assumed that the parallel alignment of CS 2 molecules with the carbon atom of one molecule close to the sulfur atom of another is an important structural arrangement in the time scale of a Raman experiment.
TL;DR: In this paper, an elementary treatment of a field theoretic many-body system of interacting fermions and bosons is presented, which follows closely the conventional Hartree-Fock and time-dependent Hartree Fock theories.
TL;DR: In this article, the generator coordinate method is applied to construct a boson representation of the energy of a S = 1 2 ferromagnet near its ground-state, and the resulting representation coincides with the one obtained starting from the Dirac-Heisenberg Hamiltonian, expanded to all orders inside the physical subspace.
Abstract: The Generator Coordinate Method is applied to construct a boson representation of the energy of a S = 1 2 ferromagnet near its ground-state. Considering one generator coordinate per lattice site the resulting representation coincides with the one obtained starting from the Dirac-Heisenberg Hamiltonian and using the Holstein-Primakoff transformation, expanded to all orders, inside the physical subspace. On the other hand, introducing generator coordinates to describe only the long-wavelength modes, we obtain a representation which can be consistently expanded until any desired order of approximation.
TL;DR: In this paper, the authors give a method for finding upper bounds for the absolute values of the differences between two latent roots of a lambda-matrix, that is to say, for the difference between two zeros of the determinant of a ε-approximation.
TL;DR: In this paper, the Raman spectrum of CCl3CN is recorded at various temperatures in the pure liquid and in solution in cyclopentane and it is shown that these effects can be understood as manifestations of intermolecular interactions operating in the liquid at room temperature.
01 Dec 1979
TL;DR: In this article, the characteristics of electromechanical filters for the suppression of harmonics and the supply of reactive power on the a.c. side of d.c.-link convertors are described.
Abstract: The characteristics of electromechanical filters for the suppression of harmonics and the supply of reactive power on the a.c. side of d.c.-link convertors are described. The problem in the design and construction of small-scale units are highlighted. Tests with the prototype filters on a model d.c. link show that stability margins are considerably improved when compared to operation with conventional static filters.
TL;DR: In this article, a generalization of the Bohn-staver formula for the sound velocity in metals is obtained, and the generator coordinate method is applied to the collective dynamics of a metal regarded as an electron system interacting dynamically with the phonons of the lattice.
Abstract: An application is made of the Generator Coordinate Method to a system of permions interacting with bosons. The problem treated in the present work is the collective dynamics of a metal regarded as an electron system interacting dynamically with the phonons of the lattice. A generalization of the Bohn-Staver formula for the sound velocity in metals is obtained.
TL;DR: In this paper, the authors give inequalities relating the given matrices and vectors, equivalent to the consistency of the system, where X is an n × m unknown real matrix and X is a real matrix.