82 research outputs found
Self-Diffusion of a Polymer Chain in a Melt
Self-diffusion of a polymer chain in a melt is studied by Monte Carlo
simulations of the bond fluctuation model, where only the excluded volume
interaction is taken into account. Polymer chains, each of which consists of
segments, are located on an simple cubic lattice
under periodic boundary conditions, where each segment occupies unit cells. The results for
and 512 at the volume fraction are reported, where
for and L=192 for . The -dependence of the
self-diffusion constant is examined. Here, is estimated from the mean
square displacements of the center of mass of a single polymer chain at the
times larger than the longest relaxation time. From the data for , 384
and 512, the apparent exponent , which describes the apparent power
law dependence of on as , is estimated as
. The ratio seems to be a
constant for and 512, where and
denote the longest relaxation time and the mean square end-to-end distance,
respectively.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp
Relaxation of a Single Knotted Ring Polymer
The relaxation of a single knotted ring polymer is studied by Brownian
dynamics simulations. The relaxation rate lambda_q for the wave number q is
estimated by the least square fit of the equilibrium time-displaced correlation
function to a double exponential decay at long times. The relaxation rate
distribution of a single ring polymer with the trefoil knot appears to behave
as lambda_q=A(1/N^)x for q=1 and lambda_q=A'(q/N)^x' for q=2 and 3, where
x=2.61, x'=2.02 and A>A'. The wave number q of the slowest relaxation rate for
each N is given by q=2 for small values of N, while it is given by q=1 for
large values of N. This crossover corresponds to the change of the structure of
the ring polymer caused by the localization of the knotted part to a part of
the ring polymer.Comment: 13 pages, 5 figures, uses jpsj2.cl
Dynamic Changes of Sp6 Transgene Expression in Dental Epithelial Cells During Long-term Culture
To investigate the function of specificity protein 6 (SP6) transcription factor by gain-of-function procedure, we established cytomegalovirus (CMV) promoter-driven Sp6 stable transformants, C9 cells, using dental epithelialderived cells. Initially, C9 cells produced a significant amount of SP6 protein. However, SP6 expression was reduced in these cells upon long-term culture. We could detect Sp6 transcripts in C9 cells by RT-PCR throughout the passages, although the CMV promoter is known to be epigenetically silenced. We recently found that SP6 was a short-lived protein that was degraded by a ubiquitin-independent proteasome pathway, although it is yet unclear how Sp6 expression was regulated during culture. Thus, we studied the possibility of epigenetic regulation of Sp6 expression. Comparative analysis of endogenous and exogenous Sp6 mRNA expressions demonstrated the specific down-regulation of exogenous Sp6 mRNA levels during culture passages. A DNA methyltransferase inhibitor, 5-Aza-2\u27-deoxycytidine (5AC), and a histone deacetylase inhibitor, valproic acid (VPA), enhanced or induced SP6 protein expression up to passage 28 without enhancing the mRNA level. The dramatic up-regulation of exogenous Sp6 mRNA was uniquely observed only at passage 50 by 5AC or VPA treatment. These findings indicate that multiple epigenetic regulatory mechanisms operate to fine-tune Sp6 expression during long-term culture
Isolation and Characterization of Mouse Specificity Protein 6 Promoter
Specificity protein 6 (SP6) is a member of the SP/Krüppel-like transcription factor family and plays key roles in tooth development. To study its biological roles, it is important to understand the spatiotemporal regulation of Sp6 gene expression. For this purpose, we first identified two separate 5\u27 ends of the Sp6 cDNA by 5\u27 RACE analysis using mouse mandibular RNA. Next, we isolated mouse genomic DNA fragments covering the Sp6 gene including two putative mouse Sp6 promoter regions and generated a series of luciferase reporter constructs. We confirmed the activity of both promoters by a luciferase assay and found strong second promoter activity in dental epithelial cells. Unexpectedly, we also detected potential third promoter activity in the intron 2 of the Sp6 gene. Last, we also found that bone morphogenetic protein and wingless signals could enhance Sp6 promoter activity in dental epithelial cells, suggesting the regulatory roles of two cytokines in Sp6 gene expression during tooth development. Our findings may shed new light on the regulatory mechanisms of Sp6 gene expression and provide a possible linkage between cytokine regulation of Sp6 expression and inductive epithelial and mesenchymal interactions
Screening by symmetry of long-range hydrodynamic interactions of polymers confined in sheets
Hydrodynamic forces may significantly affect the motion of polymers. In
sheet-like cavities, such as the cell's cytoplasm and microfluidic channels,
the hydrodynamic forces are long-range. It is therefore expected that that
hydrodynamic interactions will dominate the motion of polymers in sheets and
will be manifested by Zimm-like scaling. Quite the opposite, we note here that
although the hydrodynamic forces are long-range their overall effect on the
motion of polymers vanishes due to the symmetry of the two-dimensional flow. As
a result, the predicted scaling of experimental observables such as the
diffusion coefficient or the rotational diffusion time is Rouse-like, in accord
with recent experiments. The effective screening validates the use of the
non-interacting blobs picture for polymers confined in a sheet.Comment: http://www.weizmann.ac.il/complex/tlusty/papers/Macromolecules2006.pdf
http://pubs.acs.org/doi/abs/10.1021/ma060251
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