Smaller Scaling Exponent Research Articles

Emerging β-Sheet Rich Conformations in Supercompact Huntingtin Exon-1 Mutant Structures.

There exists strong correlation between the extended polyglutamines (polyQ) within exon-1 of Huntingtin protein (Htt) and age onset of Huntington's disease (HD); however, the underlying molecular mechanism is still poorly understood. Here we apply extensive molecular dynamics simulations to study the folding of Htt-exon-1 across five different polyQ-lengths. We find an increase in secondary structure motifs at longer Q-lengths, including β-sheet content that seems to contribute to the formation of increasingly compact structures. More strikingly, these longer Q-lengths adopt supercompact structures as evidenced by a surprisingly small power-law scaling exponent (0.22) between the radius-of-gyration and Q-length that is substantially below expected values for compact globule structures (∼0.33) and unstructured proteins (∼0.50). Hydrogen bond analyses further revealed that the supercompact behavior of polyQ is mainly due to the "glue-like" behavior of glutamine's side chains with significantly more side chain-side chain H-bonds than regular proteins in the Protein Data Bank (PDB). The orientation of the glutamine side chains also tend to be "buried" inside, explaining why polyQ domains are insoluble on their own.

Anomalous quantum criticality in an itinerant ferromagnet.

The dynamics of continuous phase transitions is governed by the dynamic scaling exponent relating the correlation length and correlation time. For transitions at finite temperature, thermodynamic critical properties are independent of the dynamic scaling exponent. In contrast, at quantum phase transitions where the transition temperature becomes zero, static and dynamic properties are inherently entangled by virtue of the uncertainty principle. Consequently, thermodynamic scaling equations explicitly contain the dynamic exponent. Here we report on thermodynamic measurements (as a function of temperature and magnetic field) for the itinerant ferromagnet Sr1-xCaxRuO3 where the transition temperature becomes zero for x=0.7. We find dynamic scaling of the magnetization and specific heat with highly unusual quantum critical dynamics. We observe a small dynamic scaling exponent of 1.76 strongly deviating from current models of ferromagnetic quantum criticality and likely being governed by strong disorder in conjunction with strong electron-electron coupling.

How Does a Hyperbranched Chain Pass through a Nanopore?

Starting from seesaw-type linear macromonomer azide∼∼alkyne∼∼azide, where “∼∼” represents polystyrene with a controllable length, we successfully obtained two series of narrowly distributed “defect-free” hyperbranched polystyrenes with an identical subchain length but different overall molar masses or with a similar overall molar mass but different subchain lengths. Our ultrafiltration study reveals that the critical flow rate (qc,b) to pull these branched chains through a small cylindrical pore under an elongational flow field depends on both polymerization degrees of the entire chain and the subchain (Nt and Nb) as qc,b ∼ NtγNbφ, where γ and φ are 1.0 and −0.4, different from those predicted values γ = 1/3 and 1/4, and φ = 1/15 and −1/4 for the weak and strong confinements, respectively. Besides the previously improper assumption of each blob as a hard sphere, such discrepancies are also attributed to (1) a smaller scaling exponent between the size and molar mass of the chain squeezed inside the pore be...

What Determines Motor Neuron Number? Slow Scaling of Facial Motor Neuron Numbers With Body Mass in Marsupials and Primates

How does the number of motor neurons in the brain correlate with the muscle mass to be controlled in the body? Numbers of motor neurons are known to be adjusted during development by cell death, but the change in the percentage of surviving motor neurons in response to experimental changes in target muscle mass is relatively small. Here we address the quantitative matching between final numbers of motor neurons in the facial nucleus and body mass (which we use as a proxy for the muscle mass). In 22 marsupial species, we found that the number of facial motor neurons is strongly correlated with body mass, and scales across species as a power function of body mass with a very small exponent of 0.184, which is close to the exponent found in primates from previously published data. With such an exponent, doubling the body mass is accompanied by a modest increase of only 14% in numbers of facial motor neurons, while halving body mass results in a decrease of only 12%. These numbers are remarkably similar to the 15-20% increase or 8% decrease in the number of spinal cord motor neurons that results from experimental or natural doubling or reducing by half the target muscle field of birds and amphibians. The scaling rule presented here might thus account for the quantitative matching of motor neurons to their target muscle mass in evolution. With this small scaling exponent, our data also raise the possibility that larger animals will have larger motor units.

The long range correlation of East Asia’s atmosphere

The paper investigates the height and temperature fields reanalysis data of NCEP/NCAR by means of detrended fluctuation analysis method, and gives their scaling exponent distribution of East Asia region. The empirical investigation indicates that both of height field and temperature filed have the property of long range correlation, which matches with the features of space distribution as a whole. As for the grid data of the same layer, the long range correlation varies obviously when the latitude changes. The low-latitude areas near the equator have a bigger scaling exponent and a better long range correlation, whereas the mid-latitude and high latitude areas have a smaller scaling exponent and a worse long range correlation. The average scaling exponent increases with the layer growth of height field data, and the scaling exponent varies obviously when the latitude changes. The scaling exponent of Qinghai-Tibet Plateau is obviously greater than other areas at the same latitude. For the data of temperature, the average scaling exponent first decreases with the layer growth then increases, and the average scaling exponent has a definite latitude trait. All in all, the scaling exponent of height field’s long range correlation is greater than that of temperature field. The investigation for different seasons displays that the long range correlation exists in height field and temperature field as well. And we find that the scaling exponent of winter time is greater than that of other seasons. This may provide a theory for the summer flood season forecast which is based on the winter information.

Flight Speeds among Bird Species: Allometric and Phylogenetic Effects

Flight speed is expected to increase with mass and wing loading among flying animals and aircraft for fundamental aerodynamic reasons. Assuming geometrical and dynamical similarity, cruising flight speed is predicted to vary as (body mass)1/6 and (wing loading)1/2 among bird species. To test these scaling rules and the general importance of mass and wing loading for bird flight speeds, we used tracking radar to measure flapping flight speeds of individuals or flocks of migrating birds visually identified to species as well as their altitude and winds at the altitudes where the birds were flying. Equivalent airspeeds (airspeeds corrected to sea level air density, U e) of 138 species, ranging 0.01–10 kg in mass, were analysed in relation to biometry and phylogeny. Scaling exponents in relation to mass and wing loading were significantly smaller than predicted (about 0.12 and 0.32, respectively, with similar results for analyses based on species and independent phylogenetic contrasts). These low scaling exponents may be the result of evolutionary restrictions on bird flight-speed range, counteracting too slow flight speeds among species with low wing loading and too fast speeds among species with high wing loading. This compression of speed range is partly attained through geometric differences, with aspect ratio showing a positive relationship with body mass and wing loading, but additional factors are required to fully explain the small scaling exponent of U e in relation to wing loading. Furthermore, mass and wing loading accounted for only a limited proportion of the variation in U e. Phylogeny was a powerful factor, in combination with wing loading, to account for the variation in U e. These results demonstrate that functional flight adaptations and constraints associated with different evolutionary lineages have an important influence on cruising flapping flight speed that goes beyond the general aerodynamic scaling effects of mass and wing loading.

Scaling of postcontractile phosphocreatine recovery in fish white muscle: effect of intracellular diffusion

In some fish, hypertrophic growth of white muscle leads to very large fibers. The associated low-fiber surface area-to-volume ratio (SA/V) and potentially long intracellular diffusion distances may influence the rate of aerobic processes. We examined the effect of intracellular metabolite diffusion on mass-specific scaling of aerobic capacity and an aerobic process, phosphocreatine (PCr) recovery, in isolated white muscle from black sea bass (Centropristis striata). Muscle fiber diameter increased during growth and was >250 mum in adult fish. Mitochondrial volume density and cytochrome-c oxidase activity had similar small scaling exponents with increasing body mass (-0.06 and -0.10, respectively). However, the mitochondria were more clustered at the sarcolemmal membrane in large fibers, which may offset the low SA/V, but leads to greater intracellular diffusion distances between mitochondrial clusters and ATPases. Despite large differences in intracellular diffusion distances, the postcontractile rate of PCr recovery was largely size independent, with a small scaling exponent for the maximal rate (-0.07) similar to that found for the indicators of aerobic capacity. Consistent with this finding, a mathematical reaction-diffusion analysis indicated that the resynthesis of PCr (and other metabolites) was too slow to be substantially limited by diffusion. These results suggest that the recovery rate in these fibers is primarily limited by low mitochondrial density. Additionally, the change in mitochondrial distribution with increasing fiber size suggests that low SA/V and limited O(2) flux are more influential design constraints in fish white muscle, and perhaps other fast-twitch vertebrate muscles, than is intracellular metabolite diffusive flux.

Monte Carlo Simulation of the Topology and Conformational Behavior of Hyperbranched Molecules: Pd-Diimine-Catalyzed Polyethylene

A kinetic Monte Carlo model was developed to simulate the polymerization of ethylene with palladium-α-diimine catalyst wherein hyperbranched molecules are formed through a chain-walking mechanism. The total degree of branching and the distribution of short branches obtained with the model agree well with reported 13 C NMR experimental results. Different chain topologies were generated by varying the probability of chain walking, P n , which controls the competition between chain-walking and monomer insertion. Molecular Monte Carlo simulations were subsequently conducted to study the conformations of isolated molecules (created by the kinetic Monte Carlo scheme) to relate molecular shape and topology, Our results provide evidence that the topology varies from predominantly linear with many short branches at low P w to a densely branched,globular structure at high P w . In contrast to experimental observations, our results for the molecular weight (N) dependence of the radius of gyration (R g N') indicate that the branching topology has an effect on this relation, i.e., high-P w molecules have a smaller scaling exponent v. The simulated N-dependence of the second virial coefficient exhibits a similar behavior. We also discnss the unusual conformational behavior of highly branched polymers obtained when P w → 1.