34 research outputs found
Order-disorder layering transitions of a spin-1 Ising model in a variable crystal field
The magnetic order-disorder layering transitions of a spin-1 Ising model are
investigated, under the effect of a variable surface crystal field
, using the mean field theory. Each layer , of the film formed
with layers, disorders at a finite surface crystal field distributed
according to the law , and
being a positive constant. We have established the temperature-crystal field
phase diagrams and found a constant tricritical point and a reentrant
phenomenon for the first layers. This reentrant phenomenon is absent for
the remaining layers, but the tricritical points subsist and depend
not only on the film thickness but also on the exponent . On the other
hand, the thermal behaviour of the surface magnetisation for a fixed value of
the surface crystal field and selected values of the parameter
are established.Comment: 10 Pages Latex, 9 Figures Postscript. To appear in JMMM (2002
Nuclear Magnetic Resonance and Hyperfine Structure
Contains reports on six research projects
On the dynamics of the adenylate energy system: homeorhesis vs homeostasis.
Biochemical energy is the fundamental element that maintains both the adequate turnover of the biomolecular structures and the functional metabolic viability of unicellular organisms. The levels of ATP, ADP and AMP reflect roughly the energetic status of the cell, and a precise ratio relating them was proposed by Atkinson as the adenylate energy charge (AEC). Under growth-phase conditions, cells maintain the AEC within narrow physiological values, despite extremely large fluctuations in the adenine nucleotides concentration. Intensive experimental studies have shown that these AEC values are preserved in a wide variety of organisms, both eukaryotes and prokaryotes. Here, to understand some of the functional elements involved in the cellular energy status, we present a computational model conformed by some key essential parts of the adenylate energy system. Specifically, we have considered (I) the main synthesis process of ATP from ADP, (II) the main catalyzed phosphotransfer reaction for interconversion of ATP, ADP and AMP, (III) the enzymatic hydrolysis of ATP yielding ADP, and (IV) the enzymatic hydrolysis of ATP providing AMP. This leads to a dynamic metabolic model (with the form of a delayed differential system) in which the enzymatic rate equations and all the physiological kinetic parameters have been explicitly considered and experimentally tested in vitro. Our central hypothesis is that cells are characterized by changing energy dynamics (homeorhesis). The results show that the AEC presents stable transitions between steady states and periodic oscillations and, in agreement with experimental data these oscillations range within the narrow AEC window. Furthermore, the model shows sustained oscillations in the Gibbs free energy and in the total nucleotide pool. The present study provides a step forward towards the understanding of the fundamental principles and quantitative laws governing the adenylate energy system, which is a fundamental element for unveiling the dynamics of cellular life
Geometry of the inverted Cretaceous Chañarcillo Basin based on 2-D gravity and field data – an approach to the structure of the western Central Andes of northern Chile
This paper discusses an integrated approach that provides new ideas about the
structural geometry of the NNE-striking, Cretaceous Chañarcillo Basin
located along the eastern Coastal Cordillera in the western Central Andes of
northern Chile (27–28° S). The results obtained from the
integration of two transverse (E–W) gravity profiles with previous geological
information show that the architecture of this basin is defined by a large
NNE–SSE-trending and east-vergent anticline ("Tierra Amarilla
Anticlinorium"), which is related to the positive reactivation of a former
Cretaceous normal fault (Elisa de Bordos Master Fault). Moreover,
intercalations of high and low gravity anomalies and steep gravity gradients
reveal a set of buried, west-tilted half-grabens associated with a synthetic
normal fault pattern. These results, together with the uplift and folding
style of the Cretaceous synextensional deposits recognized within the basin,
suggest that its structure could be explained by an inverted fault system
linked to the shortening of pre-existing Cretaceous normal fault systems.
Ages of the synorogenic deposits exposed unconformably over the frontal limb
of the Tierra Amarilla Anticlinorium confirm a Late Cretaceous age for the
Andean deformation and tectonic inversion of the basin