Biomedical subjects

Thomas E Angelini

Publications and source records attributed to Thomas E Angelini.

7 recordsLinked to original sources

PubMed2006

The cell as a material.

To elucidate the dynamic and functional role of a cell within the tissue it belongs to, it is essential to understand its material properties. The cell is a viscoelastic material with highly unusual properties. Measurements of the mechanical behavior of cells are beginning to probe the contribution of constituent components to cell mechanics. Reconstituted cytoskeletal protein networks have been shown to mimic many aspects of the mechanical properties of cells, providing new insight into the origin of cellular behavior. These networks are highly nonlinear, with an elastic modulus that depends sensitively on applied stress. Theories can account for some of the measured properties, but a complete model remains elusive.

Animals↗

PubMed2006

Counterions between charged polymers exhibit liquid-like organization and dynamics.

Current understanding of electrostatics in water is based on mean-field theories like the Poisson-Boltzmann formalism and its approximations, which are routinely used in colloid science and computational biology. This approach, however, breaks down for highly charged systems, which exhibit counterintuitive phenomena such as overcharging and like-charge attraction. Models of counterion correlations have been proposed as possible explanations, but no experimental comparisons are available. Here, collective dynamics of counterions that mediate like-charge attraction between F-actin filaments have been directly observed in aqueous solution using high-resolution inelastic x-ray scattering down to molecular length-scales. We find a previously undescribed acoustic-like phonon mode associated with correlated counterions. The excitation spectra at high wave-vector Q reveal unexpected dynamics due to ions interacting with their "cages" of nearest neighbors. We examine this behavior in the context of intrinsic charge density variations on F-actin. The measured speed of sound and collective relaxation rates in this liquid agree surprisingly well with simple model calculations.

Actins↗

PubMed2005

Structure and stability of self-assembled actin-lysozyme complexes in salty water.

Interactions between actin, an anionic polyelectrolyte, and lysozyme, a cationic globular protein, have been examined using a combination of synchrotron small-angle x-ray scattering and molecular dynamics simulations. Lysozyme initially bridges pairs of actin filaments, which relax into hexagonally coordinated columnar complexes comprised of actin held together by incommensurate one-dimensional close-packed arrays of lysozyme macroions. These complexes are found to be stable even in the presence of significant concentrations of monovalent salt, which is quantitatively explained from a redistribution of salt between the condensed and the aqueous phases.

Actins↗

PubMed2004

Roles of anionic and cationic template components in biomineralization of CdS nanorods using self-assembled DNA-membrane complexes.

Complexes of anionic DNA and cationic liposomes self-assemble into a multilamellar structure where two-dimensional lipid sheets confine a periodic one-dimensional lattice of parallel DNA chains, between which Cd(2+) ions can condense, and be subsequently reacted with H(2)S to form CdS nanorods. In this work, we identify the synergistic roles of the anionic and cationic components within the DNA-membrane template; DNA is highly anionic and condenses the Cd(2+) ions, while the cationic membrane modulates the concentration of condensed Cd(2+) ions to control the final CdS nanorod dimensions. Due to the strong electrostatic interactions between the DNA sugar-phosphate backbone and the Cd(2+) ions, crystallographic control of CdS nanostructures is possible using these simple DNA-membrane templates, which we demonstrate using nanobeam electron diffraction experiments on individual templated CdS nanorods.

Animals↗

PubMed2004

Self-assembled virus-membrane complexes.

Anionic polyelectrolytes and cationic lipid membranes can self-assemble into lamellar structures ranging from alternating layers of membranes and polyelectrolytes to 'missing layer' superlattice structures. We show that these structural differences can be understood in terms of the surface-charge-density mismatch between the polyelectrolyte and membrane components by examining complexes between cationic membranes and highly charged M13 viruses, a system that allowed us to vary the polyelectrolyte diameter independently of the charge density. Such virus-membrane complexes have pore sizes that are about ten times larger in area than DNA-membrane complexes, and can be used to package and organize large functional molecules; correlated arrays of Ru(bpy)(3)(2+) macroionic dyes have been directly observed within the virus-membrane complexes using an electron-density reconstruction. These observations elucidate fundamental design rules for rational control of self-assembled polyelectrolyte-membrane structures, which have applications ranging from non-viral gene therapy to biomolecular templates for nanofabrication.

Bacteriophage M13↗

PubMed2003

Molecular imprinting of biomineralized CdS nanostructures: crystallographic control using self-assembled DNA-membrane templates.

A wide range of biomineralization and templating methods exist for organizing inorganic materials at a wide range of length-scales. Here, we show that crystallographic control of the inorganic nanostructures is possible using synthetic biomolecular templates comprised of anionic DNA and cationic membranes, which self-assemble into a multilamellar structure where a periodic one-dimensional (1D) lattice of parallel DNA chains is confined between stacked two-dimensional (2D) lipid sheets. We have organized Cd2+ ions within the interhelical pores between DNA strands and subsequently reacted them with H2S to form CdS nanorods of controllable widths and crystallographic orientation. The strong electrostatic interactions align the templated CdS (002) polar planes parallel to the negatively charged sugar-phosphate DNA backbone, which indicates that molecular details of the DNA molecule are imprinted onto the inorganic crystal structure. The resultant nanorods have (002) planes tilted by 60 degrees with respect to the rod axis, in contrast to all known II-VI semiconductor nanorods.

Biomimetic Materials↗

PubMed2003

Like-charge attraction between polyelectrolytes induced by counterion charge density waves.

Electrostatics in aqueous media is commonly understood in terms of screened Coulomb interactions, where like-charged objects, such as polyelectrolytes, always repel. These intuitive expectations are based on mean field theories, such as the Poisson-Boltzmann formalism, which are routinely used in colloid science and computational biology [Israelachvili, J. (1992) Intermolecular and Surface Forces (Academic, London), 2nd ed.]. Like-charge attractions, however, have been observed in a variety of systems [Gelbart, W. M., Bruinsma, R. F., Pincus, P. A. & Parsegian, V. A. (2000) Phys. Today 53, 38-44]. Intense theoretical scrutiny over the last 30 years suggests that counterions play a central role, but no consensus exists for the precise mechanism. We have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well defined biological polyelectrolyte) by using synchrotron x-ray diffraction and discovered an unanticipated symmetry-breaking collective counterion mechanism for generating attractions. Surprisingly, the counterions do not form a lattice that simply follows actin's helical symmetry; rather, the counterions organize into "frozen" ripples parallel to the actin filaments and form 1D charge density waves. Moreover, this 1D counterion charge density wave couples to twist distortions of the oppositely charged actin filaments. This general cooperative molecular mechanism is analogous to the formation of polarons in ionic solids and mediates attractions by facilitating a "zipper-like" charge alignment between the counterions and the polyelectrolyte charge distribution. We believe these results can fundamentally impinge on our general understanding of electrostatics in aqueous media and are relevant to a wide range of colloidal and biomedical processes.

Actins↗