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A Provata

Publications and source records attributed to A Provata.

11 recordsLinked to original sources
PubMed2002

Oscillatory reactive dynamics on surfaces: a lattice limit cycle model.

Complex reactive dynamics on low-dimensional lattices is studied using mean-field models and Monte Carlo simulations. A lattice-compatible reactive scheme that gives rise to limit cycle behavior is constructed, involving a quadrimolecular reaction step and bimolecular adsorption and desorption steps. The resulting lattice limit cycle model is dissipative and, in the mean-field limit, exhibits sustained oscillations of the species concentrations for a wide range of parameter values. Lattice Monte Carlo simulations of the lattice limit cycle model show locally the emergence of sustained oscillations of the species concentrations. Random fluctuations of the concentrations, clustering between homologous species, and competition between the various clusters/species cause the in-phase oscillations of neighboring sites. Distant regions oscillate out of phase and spatial correlations decay exponentially with the distance. The amplitude and period of the local oscillations depend on the system parameters.

Journal Article↗
PubMed2001

Fractal properties of the lattice Lotka-Volterra model.

The lattice Lotka-Volterra (LLV) model is studied using mean-field analysis and Monte Carlo simulations. While the mean-field phase portrait consists of a center surrounded by an infinity of closed trajectories, when the process is restricted to a two-dimensional (2D) square lattice, local inhomogeneities/fluctuations appear. Spontaneous local clustering is observed on lattice and homogeneous initial distributions turn into clustered structures. Reactions take place only at the interfaces between different species and the borders adopt locally fractal structure. Intercluster surface reactions are responsible for the formation of local fluctuations of the species concentrations. The box-counting fractal dimension of the LLV dynamics on a 2D support is found to depend on the reaction constants while the upper bound of fractality determines the size of the local oscillators. Lacunarity analysis is used to determine the degree of clustering of homologous species. Besides the spontaneous clustering that takes place on a regular 2D lattice, the effects of fractal supports on the dynamics of the LLV are studied. For supports of dimensionality D(s)<2 the lattice can, for certain domains of the reaction constants, adopt a poisoned state where only one of the species survives. By appropriately selecting the fractal dimension of the substrate, it is possible to direct the system into a poisoned or oscillatory steady state at will.

Journal Article↗
PubMed2001

Reactive dynamics on two-dimensional supports: Monte Carlo simulations and mean-field theory.

Monte Carlo simulations and mean-field models are used for the study of nonequilibrium reactions taking place on the surface of a catalyst. The model represents the catalytic reduction of NO with H2 on a Pt surface. Both Monte Carlo simulations and mean-field results predict the existence of a critical surface in the parameter space where the catalyst remains active for long times. Outside this critical region the catalyst remains active for finite times only. A discrete version of the mean-field model is proposed that takes into account the discrete, two-dimensional nature of the catalyst. For homogeneous initial conditions this improved model provides better quantitative agreement with the Monte Carlo results.

Journal Article↗
PubMed2001

An evolutionary model for the origin of non-randomness, long-range order and fractality in the genome.

We present a model for genome evolution, comprising biologically plausible events such as transpositions inside the genome and insertions of exogenous sequences. This model attempts to formulate a minimal proposition accounting for key statistical properties of genomes, avoiding, as far as possible, unsupportable hypotheses for the remote evolutionary past. The statistical properties that are observed in genomic sequences and are reproduced by the proposed model are: (i) deviations from randomness at different length scales, measured by suitable algorithms, (ii) a special form of size distribution (power law distribution) characterising different levels of genome organisation in the non-coding, and (iii) extensive resemblance in the alternation of coding and non-coding regions at several length scales (self-similarity) in long genomic sequences of higher eukaryotes.

DNA↗
PubMed1999

The differential effect of pentoxifylline on cytokine production by alveolar macrophages and its clinical implications.

The aim of this study was to investigate the effects of pentoxifylline (PTX) on the production of TNF-alpha, IL-1 beta, IL-6 and GM-CSF by lipopolysaccharide (LPS)-stimulated alveolar macrophages (AM). AM and peripheral blood monocytes (PBM) from 10 patients were cultured for 24 h in the presence of LPS (10 micrograms ml-1) and PTX at concentrations of 2.0 mM, 1.0 mM, 0.5 mM, 0.1 mM and 0.01 mM. TNF-alpha and GM-CSF were measured from the culture supernatants of both the AM and PBM from all 10 patients and IL-1 beta and IL-6 from the culture supernatants of the AM from five patients. The TNF-alpha production by AM was significantly suppressed in the presence of PTX at concentrations of 2.0 and 1.0 mM, while production of IL-1 beta, IL-6 and GM-CSF remained unaffected. In PBM cultures, PTX significantly suppressed the production of TNF-alpha and GM-CSF, at all tested concentrations. The present study provides evidence that PTX selectively suppresses the production of TNF-alpha by LPS-stimulated AM and may have a role in the treatment of lung diseases where TNF-alpha is involved. The mode of administration of PTX should take into account the suppressive effect of this drug on GM-CSF production by PBM.

Adult↗
PubMed1997

The "clustered structure" of the purines/pyrimidines distribution in DNA distinguishes systematically between coding and non-coding sequences.

A method allowing to measure the inhomogeneous distribution of purines/pyrimidines in nucleotide sequences is developed. We show that this measure relates to the coding or non-coding character of the considered sequence. Coding sequences present a near to the random Pu or Py distribution. This property is shared by both protein-coding DNA and functional RNA-coding DNA. Non-coding sequences present a highly clustered inhomogeneity. We propose the hypothesis, corroborated with appropriate computer simulations, that this is due to the action of various transposition events accumulated for long time periods.

Animals↗
PubMed1994

Randomized comparison of etoposide-cisplatin vs. etoposide-carboplatin and irradiation in small-cell lung cancer. A Hellenic Co-operative Oncology Group study.

PURPOSE: To compare the efficacy and toxicity of etoposide and cisplatin (EP) with etoposide and carboplatin (EC) in combination with irradiation in small-cell lung cancer (SCLC). METHODS: Previously untreated patients (pts) with SCLC and measurable or evaluable disease were randomized to receive either cisplatin 50 mg/m2 on days 1-2 or carboplatin 300 mg/m2 on day 1, both combined with etoposide 300 mg/m2 on days 1-3 every 21 days for 6 treatment cycles. The vast majority of responding limited disease (LD) pts and complete responders (CR) with extensive disease (ED), also received thoracic irradiation (TI) and prophylactic cranial irradiation (PCI) concurrently with the third cycle. RESULTS: Of the 147 patients registered, 143 were eligible; median performance status (PS, WHO) was 1, and tumour stage was LD in 41 pts of each treatment group. The mean delay between cycles was 8 days in the EP group and 9 in the EC group increasing in both arms with the number of treatment courses. The drug dose administered per unit time as a proportion of the protocol dose was 74% and 80% for the two groups respectively. Leukopenia, neutropenic infections, nausea, vomiting, neurotoxicity and hyperergic reactions were more frequent and/or severe in the EP group. The CR rates were 57% and 58% for EP and EC respectively. Median survival for all pts was 12.5 and 11.8 months, respectively. CONCLUSION: Both treatments proved to be effective, with no differences in response and survival between the two treatment arms. The EC regimen was associated with significantly less toxicity.

Adult↗