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Biomedical subjects

Adam E Cohen

Publications and source records attributed to Adam E Cohen.

6 recordsLinked to original sources
PubMed2006

Suppressing Brownian motion of individual biomolecules in solution.

Single biomolecules in free solution have long been of interest for detailed study by optical methods, but Brownian motion prevents the observation of one single molecule for extended periods. We have used an anti-Brownian electrokinetic (ABEL) trap to trap individual protein molecules in free solution, under ambient conditions, without requiring any attachment to beads or surfaces. We also demonstrate trapping and manipulation of single virus particles, lipid vesicles, and fluorescent semiconductor nanocrystals.

Fluorescent Dyes↗
PubMed2005

Control of nanoparticles with arbitrary two-dimensional force fields.

An anti-Brownian electrophoretic trap is used to create arbitrary two-dimensional force fields for individual nanoscale objects in solution. The trap couples fluorescence microscopy with digital particle tracking and real-time feedback to generate a position-dependent electrophoretic force on a single nanoparticle. The force may vary over nanometer distances and millisecond times and need not be the gradient of a potential. As illustrations of this technique, I study Brownian motion in harmonic, power-law, and double-well potentials.

Journal Article↗
PubMed2003

Force-extension curve of a polymer in a high-frequency electric field.

We study theoretically the conformation and force-extension curve of a semiflexible polymer in a spatially uniform ac electric field. The polymer backbone minimizes its energy by aligning along one of two orientations parallel to the field. In a strong ac field, hairpin kinks develop between regions of opposite alignment. These kinks are mathematically described as sine-Gordon solitons. We calculate the equation of state of the one-dimensional kink gas, which yields the force-extension curve of the polymer. A sufficiently strong ac field causes the polymer to extend spontaneously to almost its full contour length. The theory is applied to recent experiments on dielectrophoretic stretching of DNA.

DNA↗
PubMed2003

Resonant enhancement and dissipation in nonequilibrium van der Waals forces.

Dispersion forces between molecules that are in relative motion, coupled to baths at different temperatures, or in excited states, are calculated using a Green function Liouville space expansion that extends the celebrated McLachlan response theory to the nonlinear regime. Our dynamical theory is applicable to systems that may be in any initial nonequilibrium state and that are subject to an arbitrary time-dependent coupling. In contrast to equilibrium forces which are attractive, nonequilibrium forces may be attractive or repulsive, exhibit chemically specific resonances, are far stronger, and may be nonconservative (with either positive or negative dissipation).

Adenosine Triphosphate↗
PubMed2003

Kinks, rings, and rackets in filamentous structures.

Carbon nanotubes and biological filaments each spontaneously assemble into kinked helices, rings, and "tennis racket" shapes due to competition between elastic and interfacial effects. We show that the slender geometry is a more important determinant of the morphology than any molecular details. Our mesoscopic continuum theory is capable of quantifying observations of these structures and is suggestive of their occurrence in other filamentous assemblies as well.

Acrosome↗