Properties of metabolic graphs: biological organization or representation artifacts?

<p>Abstract</p> <p>Background</p> <p>Standard graphs, where each edge links two nodes, have been extensively used to represent the connectivity of metabolic networks. It is based on this representation that properties of metabolic networks, such as hierarchical and small-world structures, have been elucidated and null models have been proposed to derive biological organization hypotheses. However, these graphs provide a simplistic model of a metabolic network's connectivity map, since metabolic reactions often involve more than two reactants. In other words, this map is better represented as a hypergraph. Consequently, a question that naturally arises in this context is whether these properties truly reflect biological organization or are merely an artifact of the representation.</p> <p>Results</p> <p>In this paper, we address this question by reanalyzing topological properties of the metabolic network of <it>Escherichia coli </it>under a hypergraph representation, as well as standard graph abstractions. We find that when clustering is properly defined for hypergraphs and subsequently used to analyze metabolic networks, the scaling of clustering, and thus the hierarchical structure hypothesis in metabolic networks, become unsupported. Moreover, we find that incorporating the distribution of reaction sizes into the null model further weakens the support for the scaling patterns.</p> <p>Conclusions</p> <p>These results combined suggest that the reported scaling of the clustering coefficients in the metabolic graphs and its specific power coefficient may be an artifact of the graph representation, and may not be supported when biochemical reactions are atomically treated as hyperedges. This study highlights the implications of the way a biological system is represented and the null model employed on the elucidated properties, along with their support, of the system.</p

Étude des résonances de la réaction 27Al(p, γ)28 Si dans le domaine d'énergie Ep = 2100 - 3 100 keV

The excitation function of the 27Al(p, γ)28 Si, 27Al(p, p' γ)27Al and 27Al(p, α 1 γ)24Mg were investigated in the energy range Ep = 2 100-3 100 keV. Gamma ray spectra for twenty three resonances in the 27Al(p, γ) 28Si reaction were studied and decay schemes for the corresponding levels in the 28Si nucleus are proposed.La réaction 27Al(p, γ)28Si a été étudiée dans le domaine d'énergie Ep = 2 100-3 100 keV . Parallèlement les réactions 27Al(p, p' γ)27Al et 27Al(p, α 1 γ) 24Mg ont été étudiées par l'intermédiaire des rayonnements gamma issus de ces réactions. Les schémas de désexcitation de vingt-trois niveaux résonnants ont été déterminés

The Marey Graph Animation Tool Demo

Enabling the user of a graph drawing system to preserve the mental map between two different layouts of a graph is a major problem. In this paper we present Marey, a system that can smoothly transform one drawing of a graph into another without any restrictions to the class of graphs or type of layout algorithm.

Experimental Evidence on the Influence of Extended Defects on a Structural Phase Transition: Polymer Chains in a Monomer Matrix

The influence of the polymerization on the static aspects of the structural phase transition in a monomer-polymer pTS-diacetylene system is studied by neutron scattering. The disorder arises probably from the frustration of the incommensurate modulation present in the pure monomer; the influence of the polymer chains acting as extended defects appears the determining factor in the observed phenomena.Nous étudions les aspects statiques des transitions de phase dans un système mixte monomère-polymère par diffusion de neutrons. Le désordre provient probablement d'une frustration de la modulation incommensurable qui existe dans les cristaux de monomère pur. L'influence des défauts étendus que constituent les chaines de polymètres est le facteur prépondérant qui apparait dans les phénomènes observés

ÉTUDE DE CERTAINS NIVEAUX DE 219Rn, 215Po

Nous mesurons les fonctions de corrélation angulaire pour les transitions 219Rn [MATH] 215Po [MATH] 215Po 219Rn [MATH] 215Po [MATH] 215Po 223Ra [MATH] 219Rn [MATH] 219Rn afin de déterminer les moments angulaires des niveaux mis en cause.Angular correlations for the transitions 219Rn [MATH] 215Po [MATH] 215Po 219Rn [MATH] 215Po [MATH] 215Po 223Ra [MATH] 219Rn [MATH] 219Rn have been measured in order to assign angular moments for these levels