Polymer adsorption on a fractal substrate: numerical study

We study the adsorption of flexible polymer macromolecules on a percolation cluster, formed by a regular two-dimensional disordered lattice at critical concentration p_c of attractive sites. The percolation cluster is characterized by a fractal dimension d_s^{p_c}=91/49. The conformational properties of polymer chains grafted to such a fractal substrate are studied by means of the pruned-enriched Rosenbluth method (PERM). We find estimates for the surface crossover exponent governing the scaling of the adsorption energy in the vicinity of the transition point, \phi_s^{p_c}=0.425\pm0.009, and for the adsorption transition temperature, T_A^{p_c}=2.64\pm0.02. As expected, the adsorption is diminished when the fractal dimension of the substrate is smaller than that of a plain Euclidean surface. The universal size and shape characteristics of a typical spatial conformation which attains a polymer chain in the adsorbed state are analyzed as well.Comment: 11 pages, 16 figure

Shape anisotropy of polymers in disordered environment

We study the influence of structural obstacles in a disordered environment on the size and shape characteristics of long flexible polymer macromolecules. We use the model of self-avoiding random walks on diluted regular lattices at the percolation threshold in space dimensions d=2, 3. Applying the Pruned-Enriched Rosenbluth Method (PERM), we numerically estimate rotationally invariant universal quantities such as the averaged asphericity A_d and prolateness S of polymer chain configurations. Our results quantitatively reveal the extent of anisotropy of macromolecules due to the presence of structural defects.Comment: 8 page

Multifractality of self-avoiding walks on percolation clusters

We consider self-avoiding walks (SAWs) on the backbone of percolation clusters in space dimensions d=2, 3, 4. Applying numerical simulations, we show that the whole multifractal spectrum of singularities emerges in exploring the peculiarities of the model. We obtain estimates for the set of critical exponents, that govern scaling laws of higher moments of the distribution of percolation cluster sites visited by SAWs, in a good correspondence with an appropriately summed field-theoretical \varepsilon=6-d-expansion (H.-K. Janssen and O. Stenull, Phys. Rev. E 75, 020801(R) (2007)).Comment: 4 page

Molekularbiologische Charakterisierung des Gens für die pflanzliche Isovaleryl-CoA-Dehydrogenase als Auxin-bindendes Protein ABP 44 aus Pisum sativum L

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Percolation thresholds and fractal dimensions for square and cubic lattices with long-range correlated defects

We study long-range power-law correlated disorder on square and cubic lattices. In particular, we present high-precision results for the percolation thresholds and the fractal dimension of the largest clusters as function of the correlation strength. The correlations are generated using a discrete version of the Fourier filtering method. We consider two different metrics to set the length scales over which the correlations decay, showing that the percolation thresholds are highly sensitive to such system details. By contrast, we verify that the fractal dimension $d_{\rm f}$ is a universal quantity and unaffected by the choice of metric. We also show that for weak correlations, its value coincides with that for the uncorrelated system. In two dimensions we observe a clear increase of the fractal dimension with increasing correlation strength, approaching $d_{\rm f}\rightarrow 2$. The onset of this change does not seem to be determined by the extended Harris criterion.Comment: 12 pages, 8 figure

Cloning of a Gene for an Acyl-CoA Dehydrogenase from Pisum sativum L. and Purification and Characterization of Its Product as an Isovaleryl-CoA Dehydrogenase

Isovaleryl-CoA dehydrogenase (IVD, EC 1.3.99.10) catalyzes the third step in the catabolism of leucine in mammals. Deficiency of this enzyme leads to the clinical disorder isovaleric acidemia. IVD has been purified and characterized from human and rat liver, and the x-ray crystallographic structure of purified recombinant human IVD has been reported. Nothing is known about IVD activity in plants, although cDNA clones from Arabidopsis thaliana and partial sequences from Gossypium hirsutum and Oryza sativa have been identified as putative IVDs based on sequence homology and immuno cross-reactivity. In this report we describe the identification and characterization of an IVD from pea, purification of the enzyme using a novel and rapid auxin affinity chromatography matrix, and cloning of the corresponding gene. At the amino acid level, pea IVD is 60% similar to human and rat IVD. The specific activity and abundance of plant IVD was found to be significantly lower than for its human counterpart and exhibits developmental regulation. Substrate specificity of the plant enzyme is similar to the human IVD, and it cross-reacts to anti-human IVD antibodies. Molecular modeling of the pea enzyme based on the structure of human IVD indicates a high degree of structural similarity among these enzymes. Glu-244, shown to function as the catalytic base in human IVD along with most of the amino acids that make up the acyl CoA binding pocket, is conserved in pea IVD. The genomic structure of the plant IVD gene consists of 13 exons and 12 introns, spanning approximately 4 kilobases, and the predicted RNA splicing sites exhibit the extended consensus sequence described for other plant genes

Fractals Meet Fractals: Self-Avoiding Random Walks on Percolation Clusters

The scaling behavior of linear polymers in disordered media, modelled by self-avoiding walks (SAWs) on the backbone of percolation clusters in two, three and four dimensions is studied by numerical simulations. We apply the pruned-enriched Rosenbluth chain-growth method (PERM). Our numerical results yield estimates of critical exponents, governing the scaling laws of disorder averages of the configurational properties of SAWs, and clearly indicate a multifractal spectrum which emerges when two fractals meet each other.Comment: 5 pages, to appear in Proceedings of "Computer Simulations in Condensed Matter Physics XXII", eds. D.P. Landau, S.P. Lewis, and H.-B. Schumltler, The Procedia: Physics Procedia (in print

Theta-polymers in crowded media under the stretching force

We study the peculiarities of stretching of globular polymer macromolecules in a disordered (crowded) environment, using the model of self-attracting self-avoiding walks on site-diluted percolative lattices in space dimensions d=3. Applying the pruned-enriched Rosenbluth chain-growth method (PERM), we construct the phase diagram of collapsed-extended state coexistence when varying temperature and stretching force. The change in shape characteristics of globular polymers under stretching is analyzed as well.Comment: 10 page

Conformational Properties of Polymers Near a Fractal Surface

AbstractThe conformational properties of flexible polymer macromolecules grafted to an attractive partially penetrable surface with fractal dimension dpcs=91/49 are studied. Employing computer simulations based on the prunedenriched Rosenbluth chain-growth method, estimates for the surface crossover exponent and adsorption transition temperature are found. Our results quantitatively reveal the slowing down of the adsorption process caused by the fractal self-similar structure of the underlying substrate