24,629 research outputs found
Driven Polymer Translocation Through a Narrow Pore
Motivated by experiments in which a polynucleotide is driven through a
proteinaceous pore by an electric field, we study the diffusive motion of a
polymer threaded through a narrow channel with which it may have strong
interactions. We show that there is a range of polymer lengths in which the
system is approximately translationally invariant, and we develop a
coarse-grained description of this regime. From this description, general
features of the distribution of times for the polymer to pass through the pore
may be deduced. We also introduce a more microscopic model. This model provides
a physically reasonable scenario in which, as in experiments, the polymer's
speed depends sensitively on its chemical composition, and even on its
orientation in the channel. Finally, we point out that the experimental
distribution of times for the polymer to pass through the pore is much broader
than expected from simple estimates, and speculate on why this might be.Comment: 16 pages, 8 figures, RevTex and harvard citation style, submitted to
Biophysical Journa
A Guide to Disability Statistics from the Survey of Income and Program Participation
This paper discusses the utility of the SIPP in disability analyses, including a summary of descriptive statistics on people with disabilities from multiple SIPP panels, including the most recent SIPP panel (2001). The findings provide insights into the various health, employment, income, and program participation outcomes that may be associated with different definitions of disability and illustrates the potential for using SIPP data in further disability analyses. Our descriptive findings highlight the differences in the demographic composition and outcomes across disability definitions, underscoring the importance of carefully selecting an appropriate disability conceptualization in generating disability statistics
Dynamically-Stabilized Pores in Bilayer Membranes
Zhelev and Needham have recently created large, quasi-stable pores in
artificial lipid bilayer vesicles [Biochim. Biophys. Acta 1147 (1993) 89].
Initially created by electroporation, the pores remain open for up to several
seconds before quickly snapping shut. This result is surprising in light of the
large line tension for holes in bilayer membranes and the rapid time scale for
closure of large pores. We show how pores can be dynamically stabilized via a
new feedback mechanism. We also explain quantitatively the observed sudden pore
closure as a tangent bifurcation. Finally we show how Zhelev and Needham's
experiment can be used to measure accurately the pore line tension, an
important material parameter. For their SOPC/CHOL mixture we obtain a line
tension of 2.6 10^{-6} erg/cm.Comment: 7 pages RevTeX, 3 included eps figures (published version).
Postscript also available at http://dept.physics.upenn.edu/~moro
Statistical mechanics of thin spherical shells
We explore how thermal fluctuations affect the mechanics of thin amorphous
spherical shells. In flat membranes with a shear modulus, thermal fluctuations
increase the bending rigidity and reduce the in-plane elastic moduli in a
scale-dependent fashion. This is still true for spherical shells. However, the
additional coupling between the shell curvature, the local in-plane stretching
modes and the local out-of-plane undulations, leads to novel phenomena. In
spherical shells thermal fluctuations produce a radius-dependent negative
effective surface tension, equivalent to applying an inward external pressure.
By adapting renormalization group calculations to allow for a spherical
background curvature, we show that while small spherical shells are stable,
sufficiently large shells are crushed by this thermally generated "pressure".
Such shells can be stabilized by an outward osmotic pressure, but the effective
shell size grows non-linearly with increasing outward pressure, with the same
universal power law exponent that characterizes the response of fluctuating
flat membranes to a uniform tension.Comment: 16 pages, 6 figure
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