Influence of micellar size on the structure of surfactant-DNA complexes.

We have studied the structure of complexes of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) with DNA as a function of surfactant to DNA base molar ratio (R) and salt concentration. Small-angle x-ray scattering data show the formation of nematic gels at lower and higher salt concentrations, irrespective of the value of R. Two crystalline phases are observed over intermediate salt concentrations; a square (S) phase for R>3 and a hexagonal (H_{S}) phase for lower R. Electron density maps of these phases show intercalated structures, with DTAB micelles sandwiched between long DNA strands. The composition of these complexes, estimated using elemental analysis, indicates that the micelles are not very long, and they occupy only about half of the interstitial volume between the DNA strands. This phase behavior is strikingly different from that of complexes of DNA with longer chain surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB), which show only a hexagonal (H) phase over similar ranges of R and salt concentration, the H_{S} structure observed in the present study being a sqrt[3]×sqrt[3] superlattice of the H structure. Madelung energies of the S and H structures, calculated from the electrostatic interaction between their cylindrical constituents, suggest that the former is preferred in DTAB-DNA complexes due to the smaller micellar radius of DTAB. The propensity of DTAB to form short micelles seems also to favor the H_{S} phase at lower R. These results illustrate the important role of micellar size in determining the structure of these two-dimensional macro-ion crystals.

Osmotic-pressure-induced phase transition of a surfactant-DNA complex.

We have studied the effect of osmotic pressure on complexes formed by DNA with the cationic surfactant cetyltrimethylammonium tosylate using small-angle x-ray scattering. Earlier studies have shown that these complexes exhibit three different phases depending on the DNA and surfactant concentrations in the solution. The hexagonal superlattice phase (H_{I,s}^{c}) is found to be corralled into the hexagonal phase (H_{I}^{c}) above a threshold osmotic pressure. We have also estimated the DNA to surfactant micelle stoichiometry of the complexes in the three phases using elemental analysis. Our results provide further support for the structures of these complexes proposed earlier based on small-angle x-ray scattering data.

Single-Protein Tracking to Study Protein Interactions During Integrin-Based Migration.

Cell migration is a complex biophysical process which involves the coordination of molecular assemblies including integrin-dependent adhesions, signaling networks and force-generating cytoskeletal structures incorporating both actin polymerization and myosin activity. During the last decades, proteomic studies have generated impressive protein-protein interaction maps, although the subcellular location, duration, strength, sequence, and nature of these interactions are still concealed. In this chapter we describe how recent developments in superresolution microscopy (SRM) and single-protein tracking (SPT) start to unravel protein interactions and actions in subcellular molecular assemblies driving cell migration.

Structure and stoichiometry of CTAB-DNA complexes.

We have studied the structure of cetyltrimethylammonium bromide-DNA complexes using small angle x-ray diffraction and elemental analysis. These complexes exhibit a two-dimensional hexagonal phase. The diffraction data have been analyzed using electron density models based on two different structures of these complexes proposed in the literature, which differ in the micelle to DNA stoichiometry. The structure with a 1:2 micelle-DNA stoichiometry is found to be more consistent with the diffraction data. Furthermore, this structure is also supported by the stoichiometry deduced from elemental analysis. Madelung energies of the two structures, calculated from the electrostatic interaction between their cylindrical constituents, give insight into their relative stability.

Compressive force induces reversible chromatin condensation and cell geometry-dependent transcriptional response.

Fibroblasts exhibit heterogeneous cell geometries in tissues and integrate both mechanical and biochemical signals in their local microenvironment to regulate genomic programs via chromatin remodelling. While in connective tissues fibroblasts experience tensile and compressive forces (CFs), the role of compressive forces in regulating cell behavior and, in particular, the impact of cell geometry in modulating transcriptional response to such extrinsic mechanical forces is unclear. Here we show that CF on geometrically well-defined mouse fibroblast cells reduces actomyosin contractility and shuttles histone deacetylase 3 (HDAC3) into the nucleus. HDAC3 then triggers an increase in the heterochromatin content by initiating removal of acetylation marks on the histone tails. This suggests that, in response to CF, fibroblasts condense their chromatin and enter into a transcriptionally less active and quiescent states as also revealed by transcriptome analysis. On removal of CF, the alteration in chromatin condensation was reversed. We also present a quantitative model linking CF-dependent changes in actomyosin contractility leading to chromatin condensation. Further, transcriptome analysis also revealed that the transcriptional response of cells to CF was geometry dependent. Collectively, our results suggest that CFs induce chromatin condensation and geometry-dependent differential transcriptional response in fibroblasts that allows maintenance of tissue homeostasis.

Nuclear Positioning and Its Translational Dynamics Are Regulated by Cell Geometry.

The collective activity of several molecular motors and other active processes generate large forces for directional motion within the cell, which is vital for a multitude of cellular functions such as migration, division, contraction, transport, and positioning of various organelles. These processes also generate a background of fluctuating forces, which influence intracellular dynamics and thereby create unique biophysical signatures, which are altered in many diseases. In this study, we have used the nucleus as a probe particle to understand the microrheological properties of altered intracellular environments by using micropatterning to confine cells in two structurally and functionally extreme geometries. We find that nuclear positional dynamics is sensitive to the cytoskeletal organization by studying the effect of actin polymerization and nuclear rigidity on the diffusive behavior of the nucleus. Taken together, our results suggest that mapping nuclear positional dynamics provides important insights into biophysical properties of the active cytoplasmic medium. These biophysical signatures have the potential to be used as an ultrasensitive single-cell assay for early disease diagnostics.

Tuning DNA-amphiphile condensate architecture with strongly binding counterions.

Electrostatic self-assembly of colloidal and nanoparticles has attracted a lot of attention in recent years, since it offers the possibility of producing novel crystalline structures that have the potential to be used as advanced materials for photonic and other applications. The stoichiometry of these crystals is not constrained by charge neutrality of the two types of particles due to the presence of counterions, and hence a variety of three-dimensional structures have been observed depending on the relative sizes of the particles and their charge. Here we report structural polymorphism of two-dimensional crystals of oppositely charged linear macroions, namely DNA and self-assembled cylindrical micelles of cationic amphiphiles. Our system differs from those studied earlier in terms of the presence of a strongly binding counterion that competes with DNA to bind to the micelle. The presence of these counterions leads to novel structures of these crystals, such as a square lattice and a √3 x √3 superlattice of an underlying hexagonal lattice, determined from a detailed analysis of the small-angle diffraction data. These lower-dimensional equilibrium systems can play an important role in developing a deeper theoretical understanding of the stability of crystals of oppositely charged particles. Further, it should be possible to use the same design principles to fabricate structures on a longer length-scale by an appropriate choice of the two macroions.

Synthesis and pharmacological properties of some 4-amino-5-substituted thiazole-2(3H)-thiones and thiazolo(4,5-d)pyrimidin-7(6H)-one-2(3H)-thiones.

4-Amino-5-substituted thiazole-2(3H)-thiones and thiazolo(4,5-d)-pyrimidin-7(6H)-one-2(3H)-thiones have been synthesized and screened for antimicrobial and pharmacological activities. Significant analgesic, antiinflammatory, anticonvulsant and antimicrobial properties have been found in some of the compounds synthesized. Analgesic and antiinflammatory activities are reported for the first time in thiazole-2(3H)-thiones.