New approach for the dynamical simulation of CBED patterns in heavily strained specimens.

A new method for the dynamical simulation of convergent beam electron diffraction (CBED) patterns is proposed. In this method, the three-dimensional stationary Schrödinger equation is replaced by a two-dimensional time-dependent equation, in which the direction of propagation of the electron beam, variable z, stands as a time. We demonstrate that this approach is particularly well-suited for the calculation of the diffracted intensities in the case of a z-dependent crystal potential. The corresponding software has been developed and implemented for simulating CBED patterns of various specimens, from perfect crystals to heavily strained cross-sectional specimens. Evidence is given for the remarkable agreement between simulated and experimental patterns.

BUILD: a program generator for modelling experimental biological data.

BUILD is a program generator acting at source code level. The generated code corresponds to a whole application in order to model a biological process of interest using an iterative adjustment of experimental data. The program is designed to be executed in command line mode for processing of multiple data files with an individual execution control for each file. The results are completed by modular statistical and graphical functions. This approach has been shown to reduce the time and the amount of work needed for program development, debugging and maintenance. To date, BUILD has been successfully used in mathematical analysis of phenomenological approaches, but other fields of activity, such as educational software, are also conceivable.

Continuous fluorescence microphotolysis of anthracene-labeled phospholipids in membranes. Theoretical approach of the simultaneous determination of their photodimerization and lateral diffusion rates.

Anthracene is a fluorescent and photoactivatable (dimerization) group which can be used for investigating the lateral distribution and dynamics of lipids in membranes. In fluorescence recovery after photobleaching or in microphotolysis experiments, and when using this fluorophore, the bleaching (or microphotolysis) step in the illuminated part of the membrane is in fact the sum of two antagonistic processes: fluorescence decay, which is due to dimerization of anthracene residues, and fluorescence recovery, which is due to a diffusion mediated exchange of bleached and unbleached particles between the illuminated and diffusion area in the membrane. Here, we propose a new mathematical algorithm that enables such a second-order reaction-diffusion process to be analyzed. After coupling a fluorescence recovery step to a microphotolysis step, this algorithm allows us to calculate the lateral diffusion coefficient D and the photodimerization constant K of anthracene-labeled lipids in membranes, two parameters which contribute to the understanding of the fluidity of the lipid phase in membranes. This algorithm also provides us with a complete description of the anthracene-labeled molecules distribution in the illuminated and diffusion area, at any time of the experiment. The fluorescence recovery after microphotolysis procedure we propose was tested with an anthracene-labeled phosphatidylcholine inserted in egg-phosphatidylcholine multilayers, in monolayers adsorbed onto alkylated glass surfaces and in the plasma membrane of Chinese hamster ovary cells. It is shown that this procedure can also be used to evaluate the important parameters of probe mobile fraction and to determine the relative size of the illuminated and diffusion areas. This will enable membranes to be explored in terms of microdomains and/or macrodomains.

Adsorption of the cationic antitumoral drug celiptium to phosphatidylglycerol in membrane model systems. Effect on membrane electrical properties.

The binding of the cationic antitumoral drug Celiptium to the anionic phospholipid phosphatidylglycerol was studied by measuring surface potentials and surface pressures in monolayers, and by determination of electrophoretic mobility on liposomes. Surface potential and zeta potential data were interpreted in terms of the Gouy-Chapman-Stern theory of the diffuse electrical double layer. A unique drug-to-lipid adsorption constant KaD, could not be calculated. KaD was observed to increase rapidly from 10(4) M-1 to 10(6) M-1 with an increase in drug concentration from 5 x 10(-7) M to 7 x 10(-6) M. This was accompanied by a marked decrease (in absolute value) in the corresponding electrophoretic mobilities which, from negative at low drug concentrations, became positive at drug concentrations of 10(-5) M and above. This indicates that the drug-to-lipid binding cannot be accounted for by a simple Langmuir adsorption isotherm, but corresponds to a more complex process, probably of a cooperative nature. Comparison of delta V and zeta potential data shows that adsorption of Celiptium to phosphatidylglycerol not only lowers the electrical surface potential, psi 0 (in absolute value) but also markedly reduces the polarization potential, delta Vp. These observations suggest that Celiptium destabilizes the electrical properties of cell plasma membranes.

Fluorescence recovery after photobleaching (FRAP) experiments under conditions of uniform disk illumination. Critical comparison of analytical solutions, and a new mathematical method for calculation of diffusion coefficient D.

A simple fluorescence recovery after photobleaching (FRAP) apparatus using a fluorescence microscope with a conventional mercury arc lamp, working under conditions of "uniform disk illumination" is described. This set-up was designed essentially for the use of anthracene as fluorescent probe, which is bleached (photodimerization reaction) by illumination in the near ultraviolet range (360 nm). It is shown that the lateral diffusion coefficients D can be readily calculated from fluorescence recovery curves using a finite differentiate method in combination with statistical analysis of the data. In contrast to the analytical solutions so far described, this numerical approach is particularly versatile. With a minimization algorithm, D and the probe mobile fraction can be readily calculated for any recovery time under various experimental conditions. These include different probe concentration profiles in the illuminated area after the bleaching step, and situations of infinite or noninfinite reservoir in the diffusion area outside the illuminated area.