Luminescent fractal reactions: Exciton fusion on percolation clusters

Experimental measurements of exciton annihilation in isotopic mixed crystals of naphthalene-h8 in naphthalene-d8 exhibit fractal reaction kinetics on samples well below and slightly above the percolation threshold. The effective spectral dimensions are in very good agreement with the superuniversality conjecture and 2-dim. random walk simulations. Crystals well above the percolation threshold exhibit the expected classical behavior.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24624/1/0000034.pd

PHYSICAL ASPECTS OF REVERSIBLE INACTIVATION OF ENDOTOXIN *

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74729/1/j.1749-6632.1966.tb52394.x.pd

Segregation measures for diffusion-controlled A+B reactions

New measures of segregation are defined and calculated for the A + B-->0 reaction on one-dimensional and fractal lattices (Sierpinski carpet and gasket). We differentiate between local and global segregation parameters. Results and comparisons are given for steady-state simulations that strictly conserve the equality of A and B particle densities. This work demonstrates that global kinetic laws are related to both local and global segregation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27683/1/0000066.pd

Low dimensional reaction kinetics and self-organization

Diffusion-limited reaction kinetics becomes anomalous not only for fractals, with their anomalous diffusion, but also for low-dimensional (one and two) and disperse media, where the random walk is compact. We focus on annihilation, recombination and trapping reactions under non-equilibrium steady state (steady source) or batch (big bang) conditions. The typical reactions are: A + A --> Products, A + B --> Products and A + C --> Products. We are interested in the global rate laws, and their relation to particle-particle distributions (e.g., pair-correlation and nearest-neighbor distribution functions) and in local rate laws (if definable). Anomalous reaction kinetics (more than classical kinetics) is particularly sensitive to initial conditions, source term structure, conservation laws (e.g., equal densities for A and B), excluded volume effects, and medium size, dimensionality and anisotropy. Analytical formalisms, scaling arguments, computer (and supercomputer) simulations and experiments (on chemical and physical reactions) all play an important role in the newly emerging picture.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29468/1/0000554.pd

Productive Replication of Human Papillomavirus 31 Requires DNA Repair Factor Nbs1

Activation of the ATM (ataxia telangiectasia-mutated kinase)-dependent DNA damage response (DDR) is necessary for productive replication of human papillomavirus 31 (HPV31). We previously found that DNA repair and homologous recombination (HR) factors localize to sites of HPV replication, suggesting that ATM activity is required to recruit factors to viral genomes that can productively replicate viral DNA in a recombination-dependent manner. The Mre11-Rad50-Nbs1 (MRN) complex is an essential component of the DDR that is necessary for ATM-mediated HR repair and localizes to HPV DNA foci. In this study, we demonstrate that the HPV E7 protein is sufficient to increase levels of the MRN complex and also interacts with MRN components. We have found that Nbs1 depletion blocks productive viral replication and results in decreased localization of Mre11, Rad50, and the principal HR factor Rad51 to HPV DNA foci upon differentiation. Nbs1 contributes to the DDR by acting as an upstream activator of ATM in response to double-strand DNA breaks (DSBs) and as a downstream effector of ATM activity in the intra-S-phase checkpoint. We have found that phosphorylation of ATM and its downstream target Chk2, as well as SMC1 (structural maintenance of chromosome 1), is maintained upon Nbs1 knockdown in differentiating cells. Given that ATM and Chk2 are required for productive replication, our results suggest that Nbs1 contributes to viral replication outside its role as an ATM activator, potentially through ensuring localization of DNA repair factors to viral genomes that are necessary for efficient productive replication

Scaling properties of diffusion-limited reactions on fractal and euclidean geometries

We review our scaling results for the diffusion-limited reactions A + A → 0 and A+B→0 on Euclidean and fractal geometries. These scaling results embody the anomalies that are observed in these reactions in low dimensions; we collect these observations under a single phenomenological umbrella. Although we are not able to fix all the exponents in our scaling expressions from first principles, we establish bounds that bracket the observed numerical results.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45166/1/10955_2005_Article_BF01049612.pd

The Role of Early Life Experience and Species Differences in Alcohol Intake in Microtine Rodents

Social relationships have important effects on alcohol drinking. There are conflicting reports, however, about whether early-life family structure plays an important role in moderating alcohol use in humans. We have previously modeled social facilitation of alcohol drinking in peers in socially monogamous prairie voles. We have also modeled the effects of family structure on the development of adult social and emotional behaviors. Here we assessed whether alcohol intake would differ in prairie voles reared by both parents compared to those reared by a single mother. We also assessed whether meadow voles, a closely related species that do not form lasting reproductive partnerships, would differ in alcohol drinking or in the effect of social influence on drinking. Prairie voles were reared either bi-parentally (BP) or by a single mother (SM). BP- and SM-reared adult prairie voles and BP-reared adult meadow voles were given limited access to a choice between alcohol (10%) and water over four days and assessed for drinking behavior in social and non-social drinking environments. While alcohol preference was not different between species, meadow voles drank significantly lower doses than prairie voles. Meadow voles also had significantly higher blood ethanol concentrations than prairie voles after receiving the same dose, suggesting differences in ethanol metabolism. Both species, regardless of rearing condition, consumed more alcohol in the social drinking condition than the non-social condition. Early life family structure did not significantly affect any measure. Greater drinking in the social condition indicates that alcohol intake is influenced similarly in both species by the presence of a peer. While the ability of prairie voles to model humans may be limited, the lack of differences in alcohol drinking in BP- and SM-reared prairie voles lends biological support to human studies demonstrating no effect of single-parenting on alcohol abuse

Rate processes on fractals: Theory, simulations, and experiments

Heterogeneous kinetics are shown to differ drastically from homogeneous kinetics. For the elementary reaction A + A → products we show that the diffusion-limited reaction rate is proportional to t − h[A] 2 or to [A] x , where h=1- d s /2, X=1+ 2/d s =(h-2)(h-1 ), and d s is the effective spectral dimension. We note that for d = d s =1, h =1/2 and X = 3 , for percolating clusters d s = 4/3, h = 1/3 and X = 5/2 , while for “dust” d s h > 1/2 and ∞ > X > 3. Scaling arguments, supercomputer simulations and experiments give a consistent picture. The interplay of energetic and geometric heterogeneity results in fractal-like kinetics and is relevant to excitation fusion experiments in porous membranes, films, and polymeric glasses. However, in isotopic mixed crystals, the geometric fractal nature (percolation clusters) dominates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45150/1/10955_2005_Article_BF01010846.pd

Single random walker on disordered lattices

Random walks on square lattice percolating clusters were followed for up to 2×105 steps. The mean number of distinct sites visited 〈 (S N ⊃> gives a spectral dimension of d s = 1.30±0.03 consistent with superuniversality ( d s =4J3) but closer to the alternative d s = 182/139, based on the low dimensionality correction. Simulations are also given for walkers on an energetically disordered lattice, with a jump probability that depends on the local energy mismatch and the temperature. An apparent fractal behavior is observed for a low enough reduced temperature. Above this temperature, the walker exhibits a “crossover” from fractal-to-Euclidean behavior. Walks on two- and three-dimensional lattices are similar, except that those in three dimensions are more efficient.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45148/1/10955_2005_Article_BF01012923.pd