212 research outputs found
Control of the Onset of Filamentation in Condensed Media
Propagation of intense, ultrashort laser pulses through condensed media like
crystals of BaF and sapphire results in the formation of filaments. We
demonstrate that the onset of filamentation may be controlled by rotating the
plane of polarization of incident light. We directly visualize filamentation in
BaF_2 via six-photon absorption-induced fluorescence and, concomitantly, by
probing the spectral and spatial properties of white light that is generated.Comment: To appear in Phys. Rev.
Amplified Spontaneous Emission enhanced Forward Stimulated Raman Scattering in dye solutions
We study forward stimulated Raman emission from weakly fluorescent dye
4'-diethylamino-N-methyl-4-stilbazolium tosylate (DEST) in 1,2,dichloroethane
solution excited by a 28 ps, 532 nm Nd: YAG laser. Neat 1, 2, dichloroethane
emits the first Stokes line at 631 nm with a spectral width of 1.6 nm
corresponding to a Raman shift of 2956 per cm. We observe reduction of spectral
width with the addition of DEST in 1, 2, dichloroethane solution. The single
pass conversion efficiency for forward Raman emission is as high as 20 percent
in a 1 cm path length sample. The pulse duration of forward stimulated Raman
emission measured by a third order autocorrelation technique is 10 ps in neat
1, 2, dichloroethane, whereas it is nearly 3 ps for 0.04 mM of DEST solution.Comment: 10 pages, 5 figure
Plasmodium-infected red blood cells exhibit enhanced rolling independent of host cells and alter flow of uninfected red cells
The pathogenicity of Plasmodium falciparum results from its unique ability to adhere to endothelium and uninfected erythrocytes. It is, therefore, important to understand the events leading to flowing blood cells undergoing such adhesion. Largely based on the leukocyte adhesion model, it is postulated that the slowing down (rolling) of Plasmodium-infected red blood cells (PRBCs) is initiated by interactions between certain host adhesion molecules and the parasite proteins. In this article we present data demonstrating that PRBCs do not require the presence of host adhesion molecules to slow down and roll. In a synchronized culture, the proportion of slow-flowing cells increased with parasite development and was highest at the trophozoite stage. We also observed that the uninfected red cells (URBCs), originating from a parasite culture containing PRBCs, were also inherently slower compared to malaria-unexposed normal red blood cells (NRBCs). NRBCs became slower upon incubation with supernatant taken from a parasite culture. However, such an effect was transient and the NRBCs reverted to their normal flow speed within 12 h upon withdrawal of culture supernatant. Based on our observations, we suggest that the higher propensity of PRBCs and URBCs to slow down is due to inherent structural anisotropy and altered membrane rigidity. Thus the initial events leading to the slowing down of malaria-infected blood cells appear to be different from those occurring during leukocyte adhesion
Let's not forget plants.
'Many physicists see the interface with biology as an exciting place to be'. Athene Donald provides a personal perspective on working at the interface between the physical and biological sciences.This is the accepted manuscript. The final version is available from IOP at http://iopscience.iop.org/1478-3975/11/5/053008
Euler buckling in red blood cells: An optically driven biological micromotor
We investigate the physics of an optically-driven micromotor of biological
origin. A single, live red blood cell, when placed in an optical trap folds
into a rod-like shape. If the trapping laser beam is circularly polarized, the
folded RBC rotates. A model based on the concept of buckling instabilities
captures the folding phenomenon; the rotation of the cell is simply understood
using the Poincar\`e sphere. Our model predicts that (i) at a critical
intensity of the trapping beam the RBC shape undergoes large fluctuations and
(ii) the torque is proportional to the intensity of the laser beam. These
predictions have been tested experimentally. We suggest a possible mechanism
for emergence of birefringent properties in the RBC in the folded state
Human Rad52 binding renders ssDNA unfolded: image and contour length analyses by Atomic Force Microscopy
Atomic force microscopy imaging has been used to study the changes associated with human Rad52 (HsRad52) protein in solution, in dried state as well as following ssDNA (linear and circular) binding. In the dried state, the free protein exists predominantly as a characteristic panoply of novel trifoliate forms. However, in solution, the level of trifoliates diminishes significantly. Height analyses of either form reveal two categories: smaller (~ 3-5 nm) and larger ((~ 10-12 nm) particles, perhaps related to sub-heptameric and heptameric forms respectively. Interestingly, binding of the protein to linear ssDNA smoothly extends and unfolds the naked DNA. Contour length measurements performed on several individual circular ssDNA/nucleoprotein complexes reveal marked (about threefold) extension of naked ssDNA, following HsRad52 binding. We speculate that the alignment of HsRad52 on ssDNA into a smoothly extended and unfolded strand from that of highly compact morphology of naked ssDNA, may have bearing on the recombination function of HsRad52 protein
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