Alex Levine - Academia.edu (original) (raw)
Papers by Alex Levine
We report on the surface rheology of cross-linked F-actin networks associated with a lipid monola... more We report on the surface rheology of cross-linked F-actin networks associated with a lipid monolayer at the air-water interface of a Langmuir monolayer. The rheological measurements are made using a Couette cell. These data demonstrate that the network has a finite elastic modulus that grows as a function of the cross-linking concentration. We also note that under steady-state flow the system behaves as a power law fluid in which the effective viscosity decreases with imposed shear.
Proceedings of the National Academy of Sciences of the United States of America, Mar 27, 2017
The thermal fluctuations of membranes and nanoscale shells affect their mechanical characteristic... more The thermal fluctuations of membranes and nanoscale shells affect their mechanical characteristics. Whereas these fluctuations are well understood for flat membranes, curved shells show anomalous behavior due to the geometric coupling between in-plane elasticity and out-of-plane bending. Using conventional shallow shell theory in combination with equilibrium statistical physics we theoretically demonstrate that thermalized shells containing regions of negative Gaussian curvature naturally develop anomalously large fluctuations. Moreover, the existence of special curves, "singular lines," leads to a breakdown of linear membrane theory. As a result, these geometric curves effectively partition the cell into regions whose fluctuations are only weakly coupled. We validate these predictions using high-resolution microscopy of human red blood cells (RBCs) as a case study. Our observations show geometry-dependent localization of thermal fluctuations consistent with our theoretica...
Biomaterials, Jan 15, 2015
Site-specific chemical dimerization of fibroblast growth factor 2 (FGF2) with the optimal linker ... more Site-specific chemical dimerization of fibroblast growth factor 2 (FGF2) with the optimal linker length resulted in a FGF2 homodimer with improved granulation tissue formation and blood vessel formation at exceptionally low concentrations. Homodimers of FGF2 were synthesized through site-specific linkages to both ends of different molecular weight poly(ethylene glycols) (PEGs). The optimal linker length was determined by screening dimer-induced metabolic activity of human dermal fibroblasts and found to be that closest to the inter-cysteine distance, 70 Å, corresponding to 2 kDa PEG. A straightforward analysis of the kinetics of second ligand binding as a function of tether length showed that, as the polymerization index (the number of monomer repeat units in the polymer, N) of the tether decreases, the mean time for second ligand capture decreases as ∼N(3/2), leading to an enhancement of the number of doubly bound ligands in steady-state for a given (tethered) ligand concentration....
We report on the nonlinear mechanical properties of a statistically homogeneous, isotropic semifl... more We report on the nonlinear mechanical properties of a statistically homogeneous, isotropic semiflexible network cross-linked by polymers containing numerous small unfolding domains. This model captures the main mechanical features of F-actin networks cross-linked by filamin proteins, which contain twenty-four such Ig-domains that may unfold under applied strain. We show that under sufficiently high strain the network spontaneously organizes itself so that an appreciable fraction of the filamin cross-linkers are at the threshold of domain unfolding. We discuss via a simple model the cause of this network organization. We also discuss how observation of this critical state validates a mechanism proposed by Crocker et al. to explain the weak power law dependence of the measured strain modulus as observed in intracellular microrheology experiments.
Current Plant Science and Biotechnology in Agriculture, 1994
ABSTRACT
ABSTRACT We report on calculations regarding the effects of geometry on the dynamics of small und... more ABSTRACT We report on calculations regarding the effects of geometry on the dynamics of small undulations of an elastic membrane. From the work of Vaziri and Mahadevan (PNAS 105 7913, 2008) it is well understood that the Green's function (i.e. point indentation response) of the membrane changes character when the Gaussian curvature of the undeformed membrane changes sign. For positive Gaussian curvature, the effective linearized equation for the deformation is elliptic, but it is hyperbolic in the case of negative Gaussian curvature. We examine the dynamical generalization of this analysis by calculating the reflection and refraction of membrane undulatory waves when passing from a region of one geometry to another. We also examine the effect of lines of zero Gaussian curvature (separating regions of positive and negative Gaussian curvature) on the normal mode spectrum of a model discocyte and apply this analysis to understanding the membrane undulations of red blood cells, as measured by Park et al. (PNAS 107 6731, 2010).
International Handbook of Research in History, Philosophy and Science Teaching, 2013
Synthese, 2009
... Alex Levine ... Thomas Kuhn, in his 1962 Structure of Scientific Revolutions, initiated a sig... more ... Alex Levine ... Thomas Kuhn, in his 1962 Structure of Scientific Revolutions, initiated a signif-icant change in how philosophers of science think of scientific change, and of the scientific communities, scientific theories, and scientific explanations that experience such change. ...
Protein Expression and Purification, 1999
Physical Review Letters, 2012
Physical Review Letters, 1998
Philosophical Psychology, 1999
In keeping with other recent efforts, Fodor's CONCEPTS focuses on the metaphysics of... more In keeping with other recent efforts, Fodor's CONCEPTS focuses on the metaphysics of conceptual content, bracketing such epistemological questions as," How can we know the contents of our concepts?" Fodor's metaphysical account of concepts, called" informational atomism," stipulates that the contents of a subject's concepts are fixed by the nomological lockings between the subject and the world. After sketching Fodor's" what else?" argument in support of this view, we offer a number of related criticisms. All point to the same ...
Journal of Biological Chemistry, 2001
We report on the surface rheology of cross-linked F-actin networks associated with a lipid monola... more We report on the surface rheology of cross-linked F-actin networks associated with a lipid monolayer at the air-water interface of a Langmuir monolayer. The rheological measurements are made using a Couette cell. These data demonstrate that the network has a finite elastic modulus that grows as a function of the cross-linking concentration. We also note that under steady-state flow the system behaves as a power law fluid in which the effective viscosity decreases with imposed shear.
Proceedings of the National Academy of Sciences of the United States of America, Mar 27, 2017
The thermal fluctuations of membranes and nanoscale shells affect their mechanical characteristic... more The thermal fluctuations of membranes and nanoscale shells affect their mechanical characteristics. Whereas these fluctuations are well understood for flat membranes, curved shells show anomalous behavior due to the geometric coupling between in-plane elasticity and out-of-plane bending. Using conventional shallow shell theory in combination with equilibrium statistical physics we theoretically demonstrate that thermalized shells containing regions of negative Gaussian curvature naturally develop anomalously large fluctuations. Moreover, the existence of special curves, "singular lines," leads to a breakdown of linear membrane theory. As a result, these geometric curves effectively partition the cell into regions whose fluctuations are only weakly coupled. We validate these predictions using high-resolution microscopy of human red blood cells (RBCs) as a case study. Our observations show geometry-dependent localization of thermal fluctuations consistent with our theoretica...
Biomaterials, Jan 15, 2015
Site-specific chemical dimerization of fibroblast growth factor 2 (FGF2) with the optimal linker ... more Site-specific chemical dimerization of fibroblast growth factor 2 (FGF2) with the optimal linker length resulted in a FGF2 homodimer with improved granulation tissue formation and blood vessel formation at exceptionally low concentrations. Homodimers of FGF2 were synthesized through site-specific linkages to both ends of different molecular weight poly(ethylene glycols) (PEGs). The optimal linker length was determined by screening dimer-induced metabolic activity of human dermal fibroblasts and found to be that closest to the inter-cysteine distance, 70 Å, corresponding to 2 kDa PEG. A straightforward analysis of the kinetics of second ligand binding as a function of tether length showed that, as the polymerization index (the number of monomer repeat units in the polymer, N) of the tether decreases, the mean time for second ligand capture decreases as ∼N(3/2), leading to an enhancement of the number of doubly bound ligands in steady-state for a given (tethered) ligand concentration....
We report on the nonlinear mechanical properties of a statistically homogeneous, isotropic semifl... more We report on the nonlinear mechanical properties of a statistically homogeneous, isotropic semiflexible network cross-linked by polymers containing numerous small unfolding domains. This model captures the main mechanical features of F-actin networks cross-linked by filamin proteins, which contain twenty-four such Ig-domains that may unfold under applied strain. We show that under sufficiently high strain the network spontaneously organizes itself so that an appreciable fraction of the filamin cross-linkers are at the threshold of domain unfolding. We discuss via a simple model the cause of this network organization. We also discuss how observation of this critical state validates a mechanism proposed by Crocker et al. to explain the weak power law dependence of the measured strain modulus as observed in intracellular microrheology experiments.
Current Plant Science and Biotechnology in Agriculture, 1994
ABSTRACT
ABSTRACT We report on calculations regarding the effects of geometry on the dynamics of small und... more ABSTRACT We report on calculations regarding the effects of geometry on the dynamics of small undulations of an elastic membrane. From the work of Vaziri and Mahadevan (PNAS 105 7913, 2008) it is well understood that the Green's function (i.e. point indentation response) of the membrane changes character when the Gaussian curvature of the undeformed membrane changes sign. For positive Gaussian curvature, the effective linearized equation for the deformation is elliptic, but it is hyperbolic in the case of negative Gaussian curvature. We examine the dynamical generalization of this analysis by calculating the reflection and refraction of membrane undulatory waves when passing from a region of one geometry to another. We also examine the effect of lines of zero Gaussian curvature (separating regions of positive and negative Gaussian curvature) on the normal mode spectrum of a model discocyte and apply this analysis to understanding the membrane undulations of red blood cells, as measured by Park et al. (PNAS 107 6731, 2010).
International Handbook of Research in History, Philosophy and Science Teaching, 2013
Synthese, 2009
... Alex Levine ... Thomas Kuhn, in his 1962 Structure of Scientific Revolutions, initiated a sig... more ... Alex Levine ... Thomas Kuhn, in his 1962 Structure of Scientific Revolutions, initiated a signif-icant change in how philosophers of science think of scientific change, and of the scientific communities, scientific theories, and scientific explanations that experience such change. ...
Protein Expression and Purification, 1999
Physical Review Letters, 2012
Physical Review Letters, 1998
Philosophical Psychology, 1999
In keeping with other recent efforts, Fodor's CONCEPTS focuses on the metaphysics of... more In keeping with other recent efforts, Fodor's CONCEPTS focuses on the metaphysics of conceptual content, bracketing such epistemological questions as," How can we know the contents of our concepts?" Fodor's metaphysical account of concepts, called" informational atomism," stipulates that the contents of a subject's concepts are fixed by the nomological lockings between the subject and the world. After sketching Fodor's" what else?" argument in support of this view, we offer a number of related criticisms. All point to the same ...
Journal of Biological Chemistry, 2001