Showing papers by "University of Coimbra published in 1978"

Analytical potentials for triatomic molecules from spectroscopic data: IV. Application to linear molecules

Abstract: Analytic functions have been derived for the potential energy surfaces of the ground states of HCN (HNC) and HSiN(HNSi) using a combination of spectroscopic data and ab initio calculations. The potentials derived give no barrier to the approach of H to CN at any angle, but there is a barrier when H approaches SiN, this being larger at the Si end.

Nuclear-Magnetic-Resonance Studies of 5′-Ribonucleotide and 5′-Deoxyribonucleotide Conformations in Solution Using the Lanthanide Probe Method

Abstract: The conformations of the metal-bound 5′-ribonucleotides and 5′-deoxyribonucleotides in aqueous solution at different pH values have been studied using the lanthanide probe method. The conformational analysis, based on mixing different conformations in fast exchange within the nuclear magnetic resonance time scale, agrees well with the results from coupling constants, nuclear Overhauser effects and spin-lattice relaxation times, obtained for the metal-fixed systems. The equilibrium between the two basic conformational combinations for the 5′-nucleotides, anti-(N⇄S)-gg-g'g′ and syn-(N⇄S)-gt-g'g′ depends on the nature of the furanose ring, the base and also on the state of base protonation and phosphate ionization. The effect of base protonation is particularly strong for the guanine nucleotides.

Chlorotetracycline as an indicator of the interaction of calcium with brain membrane fractions.

Abstract: — The fluorescence of chlorotetracycline (CTC) in the presence of synaptosomes isolated from sheep brain is selectively increased by Ca2+ under conditions in which Mg2+, Na+, K+, Li+ or choline have only a small effect. The monovalent cations release bound Ca2+ from synaptosomes, and this effect is reflected by a decrease in the CTC fluorescence. Under optimal conditions there is a near parallelism between Ca2+ and CTC binding to the synaptosomes membranes, and Li+ is the monovalent cation tested which interferes the most with the binding of both substances. These results obtained in a predominantly sucrose medium become less distinct when media simulating physiological composition are utilized, which limits the usefulness of the method. Brain mitochondria and myelin also bind Ca2+ and CTC. The ratio of the fluorescence signal (or CTC bound) to Ca2+ bound is highest of all for mitochondrial membranes, and the apparent fluorescence quantum yield of CTC is also the highest in these membranes, which suggests that the Ca2+ in these membranes is localized in a more apolar region than is the case for synaptosomes and myelin.

Nucleotide Torsional Flexibility in Solution and the Use of the Lanthanides as Nuclear‐Magnetic‐Resonance Conformational Probes

Abstract: In this article the single family of conformations of adenosine 5′-monophosphate (5′AMP) in aqueous solution previously obtained using the lanthanide ions as nuclear magnetic resonance (NMR) probes is shown to disagree with the conformational data available from independent NMR methods, such as coupling constants and nuclear Overhauser enhancement analysis. The original lanthanide experimental data is reinterpreted assuming the contribution of different conformers to the observed magnetic perturbations. The average conformation obtained for 5′ AMP agrees quite satisfactorily with the other independent conformational information.

Cubic spline functions and initial value problems

Abstract: A numerical process is presented which provides a cubic spline function approximation for the solution of initial value problems in ordinary differential equations. With interpolate cubic spline functions we can achieveO(h4) convergence.

Pion-nucleon resonances and the Peierls-Yoccoz projection

Abstract: An attempt is made to provide a microscopic interpretation of the pion-nucleon resonances in terms of the Peierls-Yoccoz projection applied to a coherent state of satellite mesons.

Conformational Studies of Some 2′:3′‐Cyclic Mononucleotides in Solution by Different Nuclear‐Magnetic‐Resonance Methods

Abstract: The conformations of the 2':3'-cyclic mononucleotides of adenosine and cytidine in deuterium oxide has been studied at pH 2.3, using lanthanide ions as paramagnetic nuclear magnetic resonance (NMR) probes. It was not possible to find any single conformation for these molecules which accounts for the observed shift and relaxation data. This situation is in agreement with the interpretation of vicinal 1H-1H and 1H-31P coupling constants, which indicate that the ribofuranose and cyclic phosphate rings are in rapid equilibrium between different puckered forms. The interpretation of the lanthanide data in terms of an equilibrium between different conformations give average rotamer populations in good agreement with the coupling constant analysis. The conformations of these systems in aqueous solutions were found to be more flexible than in the solid state, where rigid planar ribofuranose rings have been observed. Adenosine 2':3'-monophosphate differs from cytidine 2':3'-monophosphate at the glycosidic link.

The generator coordinate method and the electron gas

Abstract: A previously developed restrictive RPA (random-phase-approximation) method is re-examined and related to the generator coordinate method of Hill and Wheeler. It is found that correlation corrections to the energy, the plasmon dispersion, the compressibility and the pair-correlation function, calculated on the basis of the restrictive RPA, compare rather favourably with the results of more sophisticated methods. Also, by an appropriate extension of the generator coordinate method, the electron gas dielectric function can be derived.

Pressure dependence of triplet formation in acridine and phenazine vapours

Abstract: The pressure dependence of T 1 formation in acridine and phenazine vapours was studied between 0.05 torr and 40 torr of an added buffer gas, by microsecond flash photolysis. In contrast to the pressure dependence of anthracene triplet formation, in the heterocyclic compounds the triplet absorption increases slightly with pressure up to 1 torr and then decreases strongly with an increase in pressure up to 30–40 torr. These effects were confirmed by the slight increase followed by a decrease with time of the integrated areas of the T 1 acridine spectra observed at much shorter times by means of nanosecond flash photolysis. The absorption of the lowest triplet was found to have an induction period more than one order of magnitude longer than the laser pulse or the singlet lifetimes revealing that the T 1 state is not populated directly from S 1 but via other electronic triplet states. The pressure effects on triplet formation are interpreted in terms of a reversible intersystem crossing S 1 ⇌ T 1 associated with vibrational relaxation in the triplet states.

From the Classical Field with Sources to the Multipole Matrix Elements

Abstract: Starting with the classical interaction of the electromagnetic field with sources and then quantizing, the interaction Hamiltonian is expanded in multipoles. Explicit expressions for the multipole operators are obtained. Transformation properties, under time reversal and Hermitian conjugation, of nuclear states and multipole operators are studied.

Simple High Performance General Purpose Single Channel Analyser

Abstract: A simple single channel analyser using standard components is described. High temperature stability (0.002% / °C) and resolution (? 100 ?V) is achieved. The two-level generator circuits described allow full DC coupling of the input pulses. The performance of these circuits is discussed in detail and experimental results presented.