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Biomedical subjects

A C Maggs

Publications and source records attributed to A C Maggs.

12 recordsLinked to original sources
PubMed2002

Local simulation algorithms for Coulomb interactions.

Long ranged electrostatic interactions are time consuming to calculate in molecular dynamics and Monte Carlo simulations. We introduce an algorithmic framework for simulating charged particles which modifies the dynamics so as to allow equilibration using a local Hamiltonian. The method introduces an auxiliary field with constrained dynamics so that the equilibrium distribution is determined by the Coulomb interaction. We demonstrate the efficiency of the method by simulating a simple, charged lattice gas.

Journal Article↗
PubMed2001

Writhing photons and Berry phases in polarized multiple scattering.

We study theoretically the polarization state of light in multiple scattering media in the limit of low contrast in the refractive index. Linearly polarized photons are randomly rotated due to the Berry phase associated with the scattering path. For circularly polarized light independent speckle patterns are found for the two helical states. The statistics of the geometric phase is related to the writhe distribution of semiflexible polymers such as DNA.

Journal Article↗
PubMed2001

Viscoelasticity of solutions of motile polymers.

We explore the linear viscoelastic response of an entangled, isotropic solution of polar semiflexible polymers with active, motile centers which generate longitudinal motion. Because of the activity of these centers, the short-time modulus displays two novel power-law regimes: Initially G(t) proportional to t(-1/8), then the response is "Rouse-like" with G(t) proportional to t(-1/2). At longer times we find accelerated relaxation due to directed reptation, resulting in a reduced low frequency viscosity.

Actins↗
PubMed2001

Dynamic concentration of motors in microtubule arrays.

We present experimental and theoretical studies of the dynamics of molecular motors in microtubule arrays and asters. By solving a convection-diffusion equation we find that the density profile of motors in a two-dimensional aster is characterized by continuously varying exponents. Simulations are used to verify the assumptions of the continuum model. We observe the concentration profiles of kinesin moving in quasi-two-dimensional artificial asters by fluorescent microscopy and compare with our theoretical results.

Adenosine Triphosphate↗
PubMed2000

Twist and writhe dynamics of stiff polymers.

I consider the dynamics of a stiff filament, in particular the coupling of twist and bend via writhe. The time dependence of the writhe of a filament of length L is ([Wr(t) - Wr(0)]2) > approximately Lt(1/4). Simulations, on a simple model of a stiff polymer, are used to confirm scaling arguments. Fuller's theorem, and its relation with geometric phases, is reconsidered for open filaments.

Journal Article↗
PubMed1997

Self-organization of microtubules and motors.

Cellular structures are established and maintained through a dynamic interplay between assembly and regulatory processes. Self-organization of molecular components provides a variety of possible spatial structures: the regulatory machinery chooses the most appropriate to express a given cellular function. Here we study the extent and the characteristics of self-organization using microtubules and molecular motors as a model system. These components are known to participate in the formation of many cellular structures, such as the dynamic asters found in mitotic and meiotic spindles. Purified motors and microtubules have previously been observed to form asters in vitro. We have reproduced this result with a simple system consisting solely of multi-headed constructs of the motor protein kinesin and stabilized microtubules. We show that dynamic asters can also be obtained from a homogeneous solution of tubulin and motors. By varying the relative concentrations of the components, we obtain a variety of self-organized structures. Further, by studying this process in a constrained geometry of micro-fabricated glass chambers, we demonstrate that the same final structure can be reached through different assembly 'pathways.

Animals↗
PubMed1995

Diffusion and formation of microtubule asters: physical processes versus biochemical regulation.

Microtubule asters forming the mitotic spindle are assembled around two centrosomes through the process of dynamic instability in which microtubules alternate between growing and shrinking states. By modifying the dynamics of this assembly process, cell cycle enzymes, such as cdc2 cyclin kinases, regulate length distributions in the asters. It is believed that the same enzymes control the number of assembled microtubules by changing the "nucleating activity" of the centrosomes. Here we show that assembly of microtubule asters may be strongly altered by effects connected with diffusion of tubulin monomers. Theoretical analysis of a simple model describing assembly of microtubule asters clearly shows the existence of a region surrounding the centrosome depleted in GTP tubulin. The number of assembled microtubules may in some cases be limited by this depletion effect rather than by the number of available nucleation sites on the centrosome.

Animals↗
PubMed1995

Flexibility of actin filaments derived from thermal fluctuations. Effect of bound nucleotide, phalloidin, and muscle regulatory proteins.

Single actin filaments undergoing brownian movement in two dimensions were observed at 20 degrees C in fluorescence optical video microscopy. The persistence length (Lp) was derived from the analysis of either the cosine correlation function or the average transverse fluctuations of a series of recorded shapes of filaments assembled from rhodamine-action. Phalloidin-stabilized filaments had a persistence length of 18 +/- 1 micron, in agreement with recent observations. In the absence of phalloidin, rhodamine-labeled filaments could be observed under a variety of solution conditions once diluted in free unlabeled G-actin at the appropriate critical concentration. Such nonstabilized F-ADP-actin filaments had the same Lp of 9 +/- 0.5 microns, whether they had been assembled from ATP-G-actin or from ADP-G-actin, and independently of the tightly bound divalent metal ion. In the presence of BeF3-, which mimics the gamma-phosphate of ATP, F-ADP-BeF3-actin was appreciably more rigid, with Lp = 13.5 microns. Hence, newly formed F-ADP-Pi-actin filaments are more rigid than "old" F-ADP-actin filaments, a fact which has implications in actin-based motility processes. In the presence of skeletal tropomyosin and troponin, filaments were rigid (Lp = 20 +/- 1 micron) in the off state (-Ca2+), and flexible (Lp = 12 microns) in the on state (+Ca2+), consistent with the steric blocking model. In agreement with x-ray diffraction data, no appreciable difference was recorded between the off and on states using smooth muscle tropomyosin and caldesmon (Lp = 20 +/- 1 micron). In conclusion, this method allows accurate measurement of small (< or = 15%) changes in mechanical properties of actin filaments in correlation with their biological functions.

Actin Cytoskeleton↗
PubMed1994

Analysis of microtubule rigidity using hydrodynamic flow and thermal fluctuations.

We report the use of two independent new methods to measure the flexural rigidity of microtubules. Microtubules were grown off axonemal pieces adhering to a glass coverslip. In the first method, a hydrodynamic flow was applied to microtubules and the flexural rigidity was derived from the analysis of the bending shape of the microtubules at equilibrium in the flow. In the second method, the flexural rigidity was derived from the thermal fluctuations of the free end of axoneme-bound microtubules. With both methods, the flexural rigidity of standard GDP microtubules was estimated to be 0.85 +/- 0.2 x 10(-23) newtons x m2 which corresponded to a persistence length of 2 +/- 0.2 mm. Binding of ligands known to affect the biochemical properties of microtubules affected their rigidity. The structural analogs of inorganic phosphate AlF4- and [BeF3-, H2O], which bind to the site of the gamma-phosphate of GTP on GDP microtubule and reconstitute the GDP-Pi microtubule intermediate state of GTP hydrolysis, cause an approximately 3-fold increase in microtubule flexural rigidity and persistence length. Taxol and taxotere, antitumoral microtubule-stabilizing drugs, in contrast cause a decrease in flexural rigidity and appear to affect the three-dimensional superstructure of microtubules, which can no longer be considered as semi-flexible rods. The relationship between the mechanical properties of microtubules and their biological function is discussed.

Animals↗
PubMed1991

Computer simulations of self-assembled membranes.

Molecular dynamics simulations in three dimensions of particles that self-assemble to form two-dimensional, membrane-like objects are presented. Anisotropic, multibody forces, chosen so as to mimic real interactions between amphiphilic molecules, generate a finite rigidity and compressibility of the assembled membranes, as well as a finite line tension at their free edges. This model and its generalizations can be used to study a large class of phenomena taking place in fluctuating membranes. For instance, both fluid and solid-like phases, separated by a phase transition, are obtained and some of the large-scale properties of these membranes studied. In particular, thermal undulations of quasi-spherical fluid vesicles are analyzed, in a manner similar to recent experiments in lipid systems.

Computer Simulation↗
PubMed1990

Simulation of shape changes and adhesion phenomena in an elastic model of erythrocytes.

We present simulations of a model of a closed membrane that shares important features with erythrocytes: resistance to bending and shear, membrane asymmetry, and an osmotic pressure difference between the interior and exterior. By varying a few parameters we obtain several realistic (e.g., biconcave and cup-like) shapes whose fluctuations, analogous to flickering of erythrocytes, and mutual transformations are studied in thermal equilibrium. Our simulations form a basis for quantitative analysis of recent experiments done on erythrocytes and artificial bilayer vesicles. They also predict effects that could be observed in experiments such as an "'unbinding" phenomenon, i.e., a separation of adhering cells induced by thermal fluctuations.

Animals↗