97 research outputs found
Polymer Segmental Cross-Correlations from Dielectric Relaxation Spectra of Block Copolymers
Dielectric relaxation spectra of block polymers containing sequential type-A
dipoles are considered. Spectra of a specific set of block copolymers can be
combined to isolate the dynamic cross-correlation between the motions of two
distinct parts of the same polymer chain. Unlike past treatments of this
problem, no model is assumed for the underlying polymer dynamics.Comment: 7 pages, zero figure
Dynamics of proteins: Light scattering study of dilute and dense colloidal suspensions of eye lens homogenates
We report a dynamic light scattering study on protein suspensions of bovine
lens homogenates at conditions (pH and ionic strength) similar to the
physiological ones. Light scattering data were collected at two temperatures,
20 oC and 37 oC, over a wide range of concentrations from the very dilute limit
up to the dense regime approaching to the physiological lens concentration. A
comparison with experimental data from intact bovine lenses was advanced
revealing differences between dispersions and lenses at similar concentrations.
In the dilute regime two scattering entities were detected and identified with
the long-time, self-diffusion modes of alpha-crystallins and their aggregates,
which naturally exist in lens nucleus. Self-diffusion coefficients are
temperature insensitive, whereas the collective diffusion coefficient depends
strongly on temperature revealing a reduction of the net repulsive
interparticle forces with lowering temperature. While there are no rigorous
theoretical approaches on particle diffusion properties for multi-component,
non-ideal hard-sphere, polydispersed systems, as the suspensions studied here,
a discussion of the volume fraction dependence of the long-time, self-diffusion
coefficient in the context of existing theoretical approaches was undertaken.
This study is purported to provide some insight into the complex light
scattering pattern of intact lenses and the interactions between the
constituent proteins that are responsible for lens transparency. This would
lead to understand basic mechanisms of specific protein interactions that lead
to lens opacification (cataract) under pathological conditions.Comment: To appear in J. Chem. Phy
Diffusion in Model Networks as Studied by NMR and Fluorescence Correlation Spectroscopy
We have studied the diffusion of small solvent molecules (octane) and larger hydrophobic dye probes in octane-swollen poly(dimethyl siloxane) linear-chain solutions and end-linked model networks, using pulsed-gradient nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS), respectively, focusing on diffusion in the bulk polymer up to the equilibrium degree of swelling of the networks, that is, 4.8 at most. The combination of these results allows for new conclusions on the feasibility of different theories describing probe diffusion in concentrated polymer systems. While octane diffusion shows no cross-link dependence, the larger dyes are increasingly restricted by fixed chemical meshes. The simple Fujita free-volume theory proved most feasible to describe probe diffusion in linear long-chain solutions with realistic parameters, while better fits were obtained assuming a stretched exponential dependence on concentration. Importantly, we have analyzed the cross-link specific effect on probe diffusion independently of any specific model by comparing the best-fit interpolation of the solution data with the diffusion in the networks. The most reasonable description is obtained by assuming that the cross-link effect is additive in the effective friction coefficient of the probes. The concentration dependences as well as the data compared at the equilibrium degrees of swelling indicate that swelling heterogeneities and diffusant shape have a substantial influence on small-molecule diffusion in networks.
A Coarse-Grained Biophysical Model of E. coli and Its Application to Perturbation of the rRNA Operon Copy Number
We propose a biophysical model of Escherichia coli that predicts growth rate
and an effective cellular composition from an effective, coarse-grained
representation of its genome. We assume that E. coli is in a state of balanced
exponential steadystate growth, growing in a temporally and spatially constant
environment, rich in resources. We apply this model to a series of past
measurements, where the growth rate and rRNA-to-protein ratio have been
measured for seven E. coli strains with an rRNA operon copy number ranging from
one to seven (the wild-type copy number). These experiments show that growth
rate markedly decreases for strains with fewer than six copies. Using the
model, we were able to reproduce these measurements. We show that the model
that best fits these data suggests that the volume fraction of macromolecules
inside E. coli is not fixed when the rRNA operon copy number is varied.
Moreover, the model predicts that increasing the copy number beyond seven
results in a cytoplasm densely packed with ribosomes and proteins. Assuming
that under such overcrowded conditions prolonged diffusion times tend to weaken
binding affinities, the model predicts that growth rate will not increase
substantially beyond the wild-type growth rate, as indicated by other
experiments. Our model therefore suggests that changing the rRNA operon copy
number of wild-type E. coli cells growing in a constant rich environment does
not substantially increase their growth rate. Other observations regarding
strains with an altered rRNA operon copy number, such as nucleoid compaction
and the rRNA operon feedback response, appear to be qualitatively consistent
with this model. In addition, we discuss possible design principles suggested
by the model and propose further experiments to test its validity
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