Molecular dynamics simulation of the formation, structure, and dynamics of small phospholipid vesicles

Here, we use coarse grained molecular dynamics (MD) simulations to study the spontaneous aggregation of dipalmitoylphosphatidylcholine (DPPC) lipids into small unilamellar vesicles. We show that the aggregation process occurs on a nanosecond time scale, with bicelles and cuplike vesicles formed at intermediate stages. Formation of hemifused vesicles is also observed at higher lipid concentration. With either 25% dipalmitoylphosphatidylethanolamine (DPPE) or lysoPC mixed into the system, the final stages of the aggregation process occur significantly faster. The structure of the spontaneously formed vesicles is analyzed in detail. Microsecond simulations of isolated vesicles reveal significant differences in the packing of the lipids between the inner and outer monolayers, and between PC, PE, and lysoPC. Due to the small size of the vesicles they remain almost perfectly spherical, undergoing very limited shape fluctuations or bilayer undulations. The lipid lateral diffusion rate is found to be faster in the outer than in the inner monolayer. The water permeability coefficient of the pure DPPC vesicles is of the order of 10(-)(3) cm s(-)(1), in agreement with experimental measurements.     

The mechanism of vesicle fusion as revealed by molecular dynamics simulations

We describe molecular dynamics simulations elucidating the molecular details of the process of fusion for small lipid vesicles. The simulations are based on a coarse grained (CG) lipid model that accurately represents the lamellar state of a variety of phospholipids and enables us to observe intermediate stages during fusion at near atomic detail. Simulations were conducted on a variety of systems containing common phospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), lysoPC, and mixtures of the above. The fusion intermediates found are in general agreement with the stalk-pore mechanism. Transient pores sometimes form adjacent to the stalk, however, resulting in the mixing of lipids from the outer and inner monolayers. The speed of stalk formation and the opening of the fusion pore can be modulated by altering the lipid composition in qualitative agreement with experimental observations.     

Application of mean field boundary potentials in simulations of lipid vesicles

A method is presented to enhance the efficiency of simulations of lipid vesicles. The method increases computational speed by eliminating water molecules that either surround the vesicle or reside in the interior of the vesicle, without altering the properties of the water at the membrane interface. Specifically, mean field force approximation (MFFA) boundary potentials are used to replace both the internal and external excess bulk solvent. In addition to reducing the cost of simulating preformed vesicles, the molding effect of the boundary potentials also enhances the formation and equilibration of vesicles from random solutions of lipid in water. Vesicles with diameters in the range from 20 to 60 nm were obtained on a nanosecond time scale, without any noticeable effect of the boundary potentials on their structure.     

Holographic chemical vapor sensor

A holographic interferometer senses vapor-induced optical path length changes in polymer or other chemically sensitive films. The interferometer is inherently sensitive to changes in chemical vapor content, self-compensates for drifts, and accommodates a large array of sensor elements. A sniff-locked-loop synchronous detection method takes advantage of the interferometer's rapid response to achieve vapor concentration sensitivity in the parts-per-billion (ppb, parts in 10(9)) range. We demonstrate, for example, 40 ppb sensitivity to ethyl alcohol using poly(N-vinyl pyrrolidone) with a measurement time of 5 s.     

Non-gray radiative transfer in plane-parallel media with reflecting boundaries

A generalized equation of radiative transfer in the two-group picket-fence model is analyzed for a plane parallel, emitting, absorbing and isotropically scattering medium containing uniform heat sources and having boundary surfaces which are diffuse emitters and diffuse reflectors and are maintained at uniform but arbitrary temperatures. The solution of the general problem is expressed by the superposition of simpler problems which are solved by the application of the normal-mode-expansion technique. Highly accurate numerical results are presented for the temperature distribution and the radiative heat flux in the medium.     

Ma?analytische Methoden

Ohne Zusammenfassung     

Aberration-correction results from a segmented microelectromechanical deformable mirror and a refractive lenslet array

Quadratic aberration is successfully corrected with a segmented microelectromechanical deformable mirror in conjunction with a refractive lenslet array. Use of the lenslet array greatly improves the effective fill factor of the correcting element. Experimental results show correction approaching the diffraction limit for an extreme spherical aberration.