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Biomedical subjects

G Maret

Publications and source records attributed to G Maret.

At least 19 recordsLinked to original sources
PubMed2006

Segment distributions of end-tethered polymers in a good solvent.

We use confocal fluorescence microscopy to study the conformation of single DNA molecules end-tethered to a solid substrate. The segment distribution rho(z) measured for chains with contour lengths 15.4 microm <or= L <or= 59.4 microm as a function of the distance from the substrate can be scaled onto a master curve depending only on the scaled distance z/R(g), in quantitative agreement with theoretical predictions for end-tethered polymers in a good solvent. The scaling of the radius of gyration R(g) approximately L(0.57+/-0.05) shows the presence of excluded-volume interactions between the charged DNA segments. Independent measurements of R(g) from end-segment distributions are in good agreement with values obtained from the segment distributions and provide evidence that the radius of gyration of end-tethered chains in a good solvent is identical to that of the free chain.

Journal Article↗
PubMed2005

Pair interaction of dislocations in two-dimensional crystals.

The pair interaction between crystal dislocations is systematically explored by analyzing particle trajectories of two-dimensional colloidal crystals measured by video microscopy. The resulting pair energies are compared to Monte Carlo data and to predictions derived from the standard Hamiltonian of the elastic theory of dislocations. Good agreement is found with respect to the distance and temperature dependence of the interaction potential, but not regarding the angle dependence where discrete lattice effects become important. Our results on the whole confirm that the dislocation Hamiltonian allows a quantitative understanding of the formation and interaction energies of dislocations in two-dimensional crystals.

Journal Article↗
PubMed2005

Experimental realization of a model glass former in 2D.

We have studied binary two-dimensional (2D) mixtures of superparamagnetic colloidal particles interacting through magnetic dipole moments, which were induced by an external magnetic field B. By tuning B the effective system temperature could be widely adjusted. Time-dependent particle coordinates measured by video-microscopy provide radial pair-distribution functions, mean-square displacements as well as evidence for heterogeneous dynamics. Characteristic features of 3D glass formers are observed experimentally in 2D for the first time.

Journal Article↗
PubMed2005

Anisotropic mean-square displacements in two-dimensional colloidal crystals of tilted dipoles.

Superparamagnetic colloidal particles confined to a flat horizontal air-water interface in an external magnetic field, which is tilted relative to the interface, form anisotropic two-dimensional crystals resulting from their mutual dipole-dipole interactions. Using real-space experiments and harmonic lattice theory we explore the mean-square displacements of the particles in the directions parallel and perpendicular to the in-plane component of the external magnetic field as a function of the tilt angle. We find that the anisotropy of the mean-square displacement behaves nonmonotonically as a function of the tilt angle and does not correlate with the structural anisotropy of the crystal.

Journal Article↗
PubMed2005

Non-thermal microwave effects on protein dynamics? An X-ray diffraction study on tetragonal lysozyme crystals.

X-ray diffraction (XRD) was used to investigate the structural and dynamical effects of microwave fields on tetragonal single crystals of hen egg-white lysozyme at a resolution of 2.0 A. Using a modified slab-line waveguide allows on-line XRD to be carried out while the protein crystal is exposed to well defined microwave fields. High microwave power levels mainly lead to increased, but largely recoverable, lattice defects owing to the evaporation of crystal water. At lower microwave power levels, the presence of the microwave field results in localized reproducible changes in the mean-square displacements (B factors). At particular sites, it is found that the B factors even decrease with increasing microwave power. Most of these effects can be explained by a comparison of the data obtained under microwave irradiation with data obtained at elevated temperature which simulate heating owing to microwave absorption by unbound crystal water. The data show no indication of large microwave-driven displacements of structural subunits in the protein that would be expected if microwaves were to be absorbed resonantly by protein vibrations. Rather, the observed changes in the atomic mean-square displacements suggest that if microwaves couple non-thermally to globular proteins at hydration levels at which they still function, their effect on protein dynamics and structure is very small.

Crystallography, X-Ray↗
PubMed2004

Elastic behavior of a two-dimensional crystal near melting.

Using positional data from video microscopy, we determine the elastic moduli of two-dimensional colloidal crystals as a function of temperature. The moduli are extracted from the wave-vector-dependent normal-mode spring constants in the limit q-->0 and are compared to the renormalized Young's modulus of the Kosterlitz-Thouless-Halperin-Nelson-Young theory. An essential element of this theory is the universal prediction that Young's modulus must approach 16 pi at the melting temperature. This is indeed observed in our experiment.

Journal Article↗
PubMed2004

Anisotropic defect-mediated melting of two-dimensional colloidal crystals.

The melting transition of anisotropic two-dimensional (2D) crystals is studied in a model system of superparamagnetic colloids. The anisotropy of the induced dipole-dipole interaction is varied by tilting the external magnetic field off the normal to the particle plane. By analyzing the time-dependent Lindemann parameter as well as translational and orientational order we observe a 2D smecticlike phase. The Kosterlitz-Thouless-Halperin-Nelson-Young scenario of isotropic melting is modified: dislocation pairs and dislocations appear with different probabilities depending on their orientation with respect to the in-plane field.

Journal Article↗
PubMed2004

Harmonic lattice behavior of two-dimensional colloidal crystals.

Using positional data from videomicroscopy and applying the equipartition theorem for harmonic Hamiltonians, we determine the wave-vector-dependent normal mode spring constants of a two-dimensional colloidal model crystal and compare the measured band structure to predictions of the harmonic lattice theory. We find good agreement for both the transversal and the longitudinal modes. For q-->0, the measured spring constants are consistent with the elastic moduli of the crystal.

Journal Article↗
PubMed2004

Synthesis and characterization of colloidal fluorescent mesoporous silica particles.

Mesoporous silica particles with narrow size distribution were obtained by a seeded growth process. Depending on the size of seeds and on the time of addition of reactants, the size of particles can be varied between 300 and 1000 nm. In a second step the dye fluorescein isothiocyanate can be embedded. The structure of these new silica particles with low density was investigated by SEM, XRD, BET, and confocal microscopy.

Journal Article↗
PubMed2003

Three-particle correlations in simple liquids.

We use videomicroscopy to follow the phase-space trajectory of a two-dimensional colloidal model liquid and calculate three-point correlation functions from the measured particle configurations. Approaching the fluid-solid transition by increasing the strength of the pair-interaction potential, one observes the gradual formation of a crystal-like local order due to triplet correlations, while being still deep inside the fluid phase. Furthermore, we show that in a strongly interacting system the Born-Green equation can be satisfied only with the full triplet correlation function but not with three-body distribution functions obtained from superposing pair correlations (Kirkwood superposition approximation).

Journal Article↗
PubMed2003

Elastic properties of 2D colloidal crystals from video microscopy.

Elastic constants of two-dimensional (2D) colloidal crystals are determined by measuring strain fluctuations induced by Brownian motion of particles. Paramagnetic colloids confined to an air-water interface of a pendant drop are crystallized under the action of a magnetic field, which is applied perpendicular to the 2D layer. Using video microscopy and digital image processing we measure fluctuations of the microscopic strain obtained from random displacements of the colloidal particles from their mean (reference) positions. From these we calculate system-size dependent elastic constants, which are extrapolated using finite-size scaling to obtain their values in the thermodynamic limit. The data are found to agree rather well with zero-temperature calculations.

Journal Article↗
PubMed2000

Dynamic criteria for melting in two dimensions

The two-dimensional (2D) melting transition is analyzed on the basis of the long-time behavior of a modified Lindemann parameter in 2D gamma(L)(t) and the bond-angular correlation function g(6)(t). Using video microscopy complete positional data are obtained over five decades in time for an ensemble of superparamagnetic colloidal particles confined to an air-water interface. We find that each of the three phases (solid/hexatic/isotropic liquid) is uniquely characterized by the long-time behavior of gamma(L)(t), g(6)(t), and the non-Gaussian parameter of the relative neighbor-neighbor displacement.

Journal Article↗
PubMed1998

The susceptibility of pure tubulin to high magnetic fields: a magnetic birefringence and x-ray fiber diffraction study.

The orientational behavior of microtubules assembled in strong magnetic fields has been studied. It is shown that when microtubules are assembled in a magnetic field, they align with their long axis parallel to the magnetic field. The effect of several parameters known to affect the microtubule assembly are investigated with respect to their effect on the final degree of alignment. Aligned samples of hydrated microtubules suitable for low-resolution x-ray fiber diffraction experiments have been produced, and the results obtained from the fiber diffraction experiments have been compared with the magnetic birefringence experiments. Comparisons with earlier fiber diffraction work and small-angle x-ray solution scattering experiments have been made.

Animals↗
PubMed1991

Seventeen years of surgical treatment of thymoma: factors influencing survival.

We report on 17 years experience of the surgical treatment of thymoma in 65 patients, 11 with and 54 without myasthenia gravis. Patients were staged using the French "GETT" classification; 38 were in stage I (no invasive tumor), 6 in stage II, 13 in stage III and 8 in stage IV. In 45 patients, surgical excision was total while the remaining 20 underwent partial resection only. Postoperative radiotherapy was given in 12 cases, and 17 received a combination of radiotherapy and chemotherapy. One patient was lost to follow up, but no operative death occurred in the series. The mean survival for all patients was 70 +/- 7 months, and the 5- and 10-year survival was 91% +/- 4% and 69% +/- 8%, respectively. Follow-up for the 47 patients still alive and 4 patients deceased from unrelated causes ranged from 1.5 to 180 months (mean 142 +/- 10 months). Our data indicate that the prognosis of thymoma relates to radiological discovery (P less than 0.01), total surgical resection (P less than 0.01) and stage of tumor (P less than 0.01). It is not influenced by age, sex, tumor cell type or the presence or absence of myasthenia gravis.

Adult↗
PubMed1991

[Our experience of the surgical treatment of thirty-nine tracheobronchial carcinoid tumors].

Thirty-nine patients were operated for bronchial carcinoid tumour between 1967 and 1988 out of a total of 97 so-called benign bronchial tumours. Based on a clinical, radiological and endoscopic assessment of this series, the authors discuss the modalities of surgical treatment which depend on the site of the tumour. Treatment consisted of enucleation for the two peripheral carcinoid tumours and tracheobronchectomy in 6 cases (15%) and pulmonary resection in 31 cases (79%) for the central tumours. Lung resection was performed alone in 25 cases (64%): 1 pneumonectomy, 2 bilobectomies, 19 lobectomies and 3 segmental resections. The lung resection was associated with bronchial resection-anastomosis in 6 cases (15%). The immediate postoperative course was uneventful. There were two deaths: one from myocardial infarction after one year and one from hepatic metastases (malignant carcinoid) after 7 years.

Adolescent↗
PubMed1990

Toxication of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and analogs by monoamine oxidase. A structure-reactivity relationship study.

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) elicits motor deficits similar to those observed in Parkinson's disease. Before exerting its neurotoxic action, MPTP must be activated by brain monoamine oxidase (MAO) to the neurotoxic metabolite MPP+ (1-methyl-4-phenylpyridinium). MPTP derivatives differ in their reactivity as MAO substrates and in their neurotoxicity. A structure-reactivity relationship study based on literature data was undertaken in order to determine the key features in the structure of MPTP and analogs that are responsible for the reactivity towards MAO. Thirty-three MPTP derivatives (including MPTP itself) were included in the study. To explain the reactivity towards MAO of the 33 MPTP analogs, different statistical methods (principal component analysis, multiple linear regression analysis) as well as the CoMFA (Comparative Molecular Field Analysis) approach, a new tool in structure-activity correlations, were used. Linear regression analysis failed to yield any predictive model, but suggested some trends. In contrast, the CoMFA approach was successful in correlating structural features and MAO reactivity. Coefficient contour maps showed where differences in the steric field (van der Waals' interactions) are most highly associated with differences in MAO reactivity. Several positive (in the ortho- and meta-position of the phenyl group) and negative (in the para-position of the phenyl group; beyond the N-methyl group) interaction regions were identified. Some structural features of the MAO active site could be postulated. First, the N-methyl group has the ideal size and elicits ideal interactions within the MAO active pocket, while smaller or larger groups are less favorable; second, para-substituent on the phenyl ring produce steric hindrances and are unfavorable to reactivity; third, ortho- and meta-substituents may have stabilizing interactions within the active pocket and are favorable to the reactivity. Moreover the model derived by CoMFA allowed us to make successful predictions of reactivity towards MAO for several additional tetrahydropyridines.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗