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Showing papers by "Instituto Politécnico Nacional published in 1976"


Journal ArticleDOI
TL;DR: In this paper, the field dependence of the magnetic exchange energy associated with tunneling of electrons between neighboring Ni grains whose magnetic moments are not parallel was investigated and the effect was accounted for by the magnetic tunneling.
Abstract: Films consisting of fine Ni grains (\ensuremath{\sim}50 \AA{}) dispersed in ${\mathrm{SiO}}_{2}$ exhibit a large negative magnetoresistance. The effect is accounted for by the field dependence of the magnetic exchange energy, associated with tunneling of electrons between neighboring grains whose magnetic moments are not parallel.

376 citations


Journal ArticleDOI
TL;DR: The effect of critical fluctuations on the resistivity near magnetic and order-disorder phase transitions is discussed in this paper, where it is shown that in all magnetic and electronic systems the asymptotic high-momentum spin correlation functions dominate the temperature dependence of resistivity sufficiently close to the transition.
Abstract: The effect of critical fluctuations on the resistivity near magnetic and order-disorder phase transitions is discussed. It is shown that in all magnetic and electronic systems the asymptotic high-momentum spin correlation functions dominate the temperature dependence of the resistivity sufficiently close to the transition. The dependence on the parameters of the system of the critical behavior and the way in which this behavior is approached is discussed in detail. In particular, the importance of the transition from a classical to a proper critical behavior, for the interpretation of experimental results, is emphasized, and recent renormalization-group results for the form of the correlation functions are utilized. For semiconductors the effects of fluctuations on the band gap are also calculated. It is suggested that the Fisher-Langer relation between the temperature derivative of the resistivity and the specific heat should be valid over a considerable temperature range outside the critical region. Theoretical predictions are compared with available experimental results on the resistivity and band gaps. It is shown that these results can all be understood, at least qualitatively, in the Born approximation by using only the most general known properties of the system.

135 citations


Book ChapterDOI
TL;DR: The homolytic substitution acquires a more significant weight in the field of the aromatic substitutions, characterized by high selectivity and versatility.
Abstract: A common aspect characterizing both the homolytic animation and the substitution of protonated hetero-aromatic bases by nucleophilic free radicals is the presence of a nitrogen atom with a positive charge. This presence determines strong polar effects whether it characterizes the radical (\(R_2 \mathop N\limits_ + H\)) or the aromatic substrate (protonated base). The awareness that the global polar effects result from the polar characteristics of both the radical and the substrate has led to new homolytic aromatic substitutions, characterized by high selectivity and versatility. Thus the homolytic substitution acquires a more significant weight in the field of the aromatic substitutions.

125 citations


Journal ArticleDOI
TL;DR: For all complex space-times in which the self-dual part of the Weyl tensor is algebraically degenerate, Einstein's vacuum equations are reduced to a single differential equation of the second order and second degree as discussed by the authors.
Abstract: For all complex space-times in which the self-dual part of the Weyl tensor is algebraically degenerate, Einstein's vacuum equations are reduced to a single differential equation of the second order and second degree.

96 citations


Journal ArticleDOI
TL;DR: The structure and absolute configuration of ( 3 ) and ( 4 ) have been elucidated and a new α-pyrone is identified from the mycelium of Cercospora taiwanensis, grown on potato-agar.

44 citations


Journal ArticleDOI
TL;DR: In this article, six sesquiterpene lactones were isolated from Lactarius blennius and the structures of two of them were determined by spectroscopic methods and the structure of the seco-compound, blennin C was revised.

33 citations


Journal ArticleDOI
TL;DR: The dependence of the membrane conductance on pH and K+ concentration suggests a mechanism for the transfer of charge mediated by this ionophore based on a mobile dimer with both nigericin molecules protonated and complexed with one K+.
Abstract: Nigericin, in the concentration range (10−6M or higher) at which it uncouples intact mitochondria, was found to increase the conductance of black lipid membranes (BLM) by several orders of magnitude The dependence of the membrane conductance on pH and K+ concentration suggests a mechanism for the transfer of charge mediated by this ionophore based on a mobile dimer with both nigericin molecules protonated and complexed with one K+ This charged complex accounts for the uncoupling effect observed in intact mitochondria

31 citations


Journal ArticleDOI
TL;DR: Retinal rod disc membranes were labelled with the hydrophobic photoactive reagent (3H)Azidonaphthalene and Rhodopsin incorporated the label in regions which are probably buried in the lipid core of the bilayer.

27 citations


Journal ArticleDOI
TL;DR: In this article, the psychopharmacological activity of phenothiazine derivatives was investigated by 13C and 1H NMR spectroscopy in order to study the electronic properties of these heterocyclic systems and the conformation of ligands at N-10.

26 citations


Journal ArticleDOI
TL;DR: In this article, a general approach to Poisson brackets, based on the study of the Lie algebra of potential operators with respect to closed skew-symmetric bilinear forms, is proposed.

25 citations


Journal ArticleDOI
TL;DR: In this article, a relativistic three-particle theory of elastic scattering in the region of the 3, 3 7π N resonance is considered, where all spin and isospin effects are retained, as well as all partial waves which make significant contribution to the scattering.

Journal ArticleDOI
TL;DR: The purpose of this article is to examine and briefly summarize the experimental basis for current ideas about the mechanism of solute transport across epithelia, and to highlight the limits and inconsistencies of the experiments on which some of the theories rest.

Journal ArticleDOI
TL;DR: In this article, the visible fluorescence of gaseous UF 6 excited by a dye laser at 374 nm was measured at room temperature and a decay time of 500 ns at p = 0 and a quenching rate of 5.7 × 10 −12 cm 3 molec −1 s −1 have been measured.

Proceedings ArticleDOI
01 Apr 1976
TL;DR: Some methodologles based on fuzzy set theory are proposed for emitting and verifying lexical hypotheses in a Speech Understanding System.
Abstract: Some methodologles based on fuzzy set theory are proposed for emitting and verifying lexical hypotheses in a Speech Understanding System. Context-independent and context-dependent features are extracted from spectrogram patterns with a procedure that establishes a correspondence relation between a speech segment and its linguistic interpretation. The syllabic hypotheses are used for emitting lexical hypotheses by an algorithm that compensates for partial corruption of the syllabic information exploiting the lexical redundancy.

Journal ArticleDOI
01 Jun 1976
TL;DR: In this paper, the equivalence of commutative and matrix grammars is proven via Parikh mapping, and a new subclass of matrix commutativity is introduced which admits a positive answer to these problems.
Abstract: Commutative grammars are a formalism for generating bags, equivalent to vector addition systems and Petri nets. Known results are recalled and new one are presented on reachability and boundedness. In particular some subclasses of commutative grammars are introduced which admit a positive answer to these problems, and generate semilinear languages. Finally the equivalence, via Parikh mapping, of commutative and matrix grammars is proven.

Journal ArticleDOI
TL;DR: In this article, the retro 1,3-dipolar cycloaddition induced by electron impact on 3,5-diphenyl-1,2,4-oxadiazole is interpreted as a two-step process on the basis of the energetics and kinetics of the fragments [C7H5NO]+., [C6H5CN]+. and [C 6H5CO]+.
Abstract: The retro 1,3-dipolar cycloaddition induced by electron impact on 3,5-diphenyl-1,2,4-oxadiazole is interpreted as a two-step process on the basis of the energetics and kinetics of the fragments [C7H5NO]+., [C6H5CN]+. and [C6H5CO]+.

Journal ArticleDOI
TL;DR: In this paper, the complex [CH 3 SO(CH 2 ) 2 ·PdI] 2 which contains the new methylsulfoxonium dimethylide as a chelating ligand was described.

Journal ArticleDOI
TL;DR: In this paper, microcrystalline Se and Te overgrowths having an abnormal A-1-type structure were quench-condensed at 4.2 K on fine-grained base films of the space group oxides vapor-quenched at the same temperature on suitable glass substrates.
Abstract: Microcrystalline Se and Te overgrowths having an abnormal A-1-type structure were quench-condensed at 4.2 K on fine-grained base films ofO space group oxides vapor-quenched at the same temperature on suitable glass substrates. From measurements of the electrical resistance we found that in this metastable fcc phase Se and Te behave as metals and may become superconductors. On heating, one reaches a critical temperature at which the overgrowth breaks because of the increase in volume occurring when the metallic phase transforms to the less dense, ordinary A-8-type semiconducting structure. An analysis of the data shows that the smaller is the lattice constant of the base film oxide, the higher are the density, the bulk superconducting transition temperature, and the bulk conductivity of the metallic Se and Te and the lower is the breaking temperature of the overgrowth.

Book ChapterDOI
01 Jan 1976
TL;DR: The purpose of the present chapter is to analyze the information now available on the mechanism of coupling between the light-induced changes of the visual pigments and the electrical events in the photoreceptor cell, that is, the mechanisms of phototransduction.
Abstract: Photoreceptor cells in both vertebrate and invertebrate species are capable of responding electrically to the absorption of single quanta of light. Considerable effort has been directed to the understanding of the molecular mechanisms which could lead to such high sensitivity of the photoreceptors. Membrane-bound visual pigments, like rhodopsin, can absorb single quanta of light and initiate the electrophysiological response in the photoreceptors. The purpose of the present chapter is to analyze the information now available on the mechanism of coupling between the light-induced changes of the visual pigments and the electrical events in the photoreceptor cell, that is, the mechanisms of phototransduction. We will sacrifice in-depth discussion of the biochemical properties of the visual pigment and of the electrophysiological properties of the photoreceptors, which can be found separately in excellent recent reviews (Tomita, 1970; Hagins, 1972; Kropf, 1972; Fuortes and O’Bryan, 1972a,b; Daemen, 1973; Abrahamson and Fager, 1973), for the breadth of background which we consider important in order to bring both areas into a single discussion of phototransduction.

Journal ArticleDOI
TL;DR: The 13C shifts of the alkaloids roxburghine B, C, D and E were determined in this paper, and they confirmed the following configurations for the last three bases: C(18α)-normal, C( 18α)-pseudo, and C((18β)-epi-allo) respectively.
Abstract: The 13C shifts of the alkaloids roxburghine B, C, D and E are determined. They confirm the following configurations for the last three bases: C(18α)-normal, C(18α)-pseudo and C(18β)-pseudo, respectively. Roxburghine B is shown to be a C(18β)-epi-allo isomer.

Journal ArticleDOI
TL;DR: In this paper, the crystal growth of Co-P alloys chemically reduced with hypophosphites from aqueous solutions on the (0 0 1) faces of Ni and Cu single crystals has been studied in a wide range of thicknesses: 10 A to 12μm.
Abstract: The crystal growth of Co-P alloys chemically reduced with hypophosphites from aqueous solutions on the (0 0 1) faces of Ni and Cu single crystals has been studied in a wide range of thicknesses: 10 A to 12μm. Smooth deposits ofβ-Co and martensitically transformedα-Co are first obtained. Upon increasing the thickness, outgrowth is observed and the mechanism of formation is attributed to an anomalous (1 0. 1) twinning mode. At high thickness, outgrowing basal planes become perpendicular to the base, transform towards the growth mode imposed by the bath and form regions of equi-oriented planes. Electrodeposition of Co films at medium thickness yields similar results.

Journal ArticleDOI
TL;DR: Kinetic analysis of the sensitivity of the biosynthetic process to oxygen, chloramphenicol, actinomycin D and rifampicin gave results which are interpreted as evidence that oxygen (and possibly nitrate) affect simultaneously both the transcriptional and translational processes.


Book ChapterDOI
01 Jan 1976
TL;DR: A study is made of the laser-induced fluorescence properties of the complex formed by Quinacrine Mustard bound to Deoxyribonucleic Acid (DNA).
Abstract: A study is made of the laser-induced fluorescence properties of the complex formed by Quinacrine Mustard bound to Deoxyribonucleic Acid (DNA). Specific information is obtained on the binding mechanisms and on the base-pair sequences of the DNA. The relevance of these measurements in several fields from chemical physics to biology and medecine is discussed.

Journal ArticleDOI
TL;DR: In this paper, Cottle's criteria for regular progression in the plastic deformation response are given for proportional paths of generalized stresses and, subsequently, generalized strains at the element level; and proportional path of external actions (loads, imposed strains, or both) at the structure level.
Abstract: Piecewise-linear yield surfaces and hardening rules are assumed for structural constituents. Using results due to Cottle on parametric linear complementarity problems, criteria are given for regular progression (no local unloading) in the plastic deformation response: (1) To proportional paths of generalized stresses and, subsequently, generalized strains at the element level; and (2) to proportional paths of external actions (loads, imposed strains, or both) at the (assembled) structure level.

Journal ArticleDOI
TL;DR: In this paper, a general reaction of wide synthetic interest, involving homolytic δ-aminoalkylation of quinoxaline in an acidic medium, is described.

Journal ArticleDOI
TL;DR: Evidence is obtained by incorporating rhodopsin into vesicular and planar bilayer membranes and demonstrating that light increases the membrane conductance in a pattern consistent with the formation of a transmembrane channel that is not ohmic but voltage-dependent, which indicates that the bilayer conductance is cation-selective.
Abstract: Visual excitation is initiated with the absorption of light by the membrane-bound visual pigment rhodopsin. The effect of light on rhodopsin is now quite generally believed to be a photoisomerization of the chromophore, retinaldehyde, from the 1 1-cis to the all-rransconliguration(Hubbard & Kropf, 1957; Wald, 1968). This is followed by changes in the shape of the protein, and these lead to conductance changes in the retinal photoreceptor cell membrane (Hagins, 1972). Such changes could result from a simple mechanism based on rhodopsin functioning as a light-activated ion-translocator. We have obtained evidence in favour of this proposal by incorporating rhodopsin into vesicular and planar bilayer membranes and demonstrating that light increases the membrane conductance in a pattern consistent with the formation of a transmembrane channel (Montal, 1975; M. Montal, A. Darszon & H. W. Trissl, unpublished work). The rhodopsin/lipid vesicles were formed and loaded with radioactive markers by sonication. Exposure of the vesicles to light increased the permeability to Na+, Cs+, Caz+, glycerol and glucose, but not to C1- and sucrose. These results indicate that light induces the formation of a cation-selective wide permeability pathway with an apparent cut-off diameter of about 0.8-1 .Onm (8-lo&. Planar bilayers containing rhodopsin were formed by apposing two rhodopsin-lipid monolayers. Such monolayers are formed from an ether-soluble complex of rhodopsin and lipid (a proteolipid) that exhibits in the organic phase spectral characteristics similar to those of native rhodopsin in retinal rod discs. Exposure of the bilayer to light increased the membrane conductance. A distinct latency period between illumination and onset of conductance changes is characteristic: it is shorter the higher the rhodopsin content in the proteolipid extract and can vary from 1 s up to several hundred seconds. At high resolution the conductance of the rhodopsin membranes can be seen to fluctuate in discrete steps. Three main transition amplitudes are well identified, corresponding to conductances of 2 x and 6.1 xlO-'OR-' in symmetric media containing either 0.2~-NaCl or O.~M-KCI. By contrast, a conductance transition of 8 x 10-10i2-1 is most frequent in symmetric 0.2~-CaCl, solutions. Illumination does not change the fluctuation pattern, it merely increases the number of conductance events. The conductance transitions are ohmic; by contrast, the macroscopic membrane conductance is not ohmic but voltage-dependent: at voltages greater than 25mV the initially high conductance relaxes to lower steady-state values. The conductance events are reminiscence of those induced in lipid bilayers by molecules presently thought to act by forming transmembrane channels (Hladky et al., 1974). Further, concentrationpotential measurements indicate that the bilayer conductance is cation-selective. The results obtained with vesicles and planar bilayers are in remarkable agreement and establish that the effect of rhodopsin bleaching in bilayer membranes is to induce the formation of a transmembrane channel of about 0.8-1.0nm diameter. Since the light-induced permeability response can be elicited at 3"C, metarhodopsin I1 may be the first conducting form of rhodopsin. Although the data are not sufficient to enable us to propose a specific molecular picture, three models have been considered, namely a unimolecular channel, a channel made of a fixed aggregate (e.g. a tetramer) or an association-dissociation equilibrium process. On the basis of these findings a model of vertebrate visual phototransduction that considers CaZ+ as the transmitter (Hagins, 1972) can be proposed, as follows. In the dark, the channel is closed and the Caz+-dependent adenosine triphosphatase would pump Ca2+ into the discs; on bleaching, the channel would form and allow Caz+ to diffuse down its concentration gradient. The efflux of CaZ+ would change the disc membrane 4 x

Journal ArticleDOI
TL;DR: In this paper, a single general operator symmetry condition is derived by using the methods of nonlinear functional analysis and a simple and systematic procedure for finding symmetry conditions in particular problems is provided.
Abstract: By using the methods of nonlinear functional analysis a single general operator symmetry condition is derived. It supplies a simple and systematic procedure for finding symmetry conditions in particular problems. From it the Lie algebra structure associated with symmetries is easily displayed and the classical Lie theory of symmetries of differential equations is obtained without needing the technique of the group extension.

Journal ArticleDOI
TL;DR: In this article, the synthesis of polyfluorinated desmosterols by a Wittig condensation between a steroidal triphenylphosphonium ylid and polyfluoroacetone was described.