Biomedical subjects

V A Raghunathan

Publications and source records attributed to V A Raghunathan.

17 recordsLinked to original sources

PubMed2006

Structure of DNA-CTAB-hexanol complexes.

We have probed structures of the complexes formed by DNA with the cationic surfactant cetyltrimethylammonium bromide in the presence of the cosurfactant hexanol, using small angle x-ray diffraction techniques. They are found to exhibit a hexagonal-->lamellar-->hexagonal transition with increasing hexanol content. Quantitative analysis of the diffraction data shows that the complexes formed at low hexanol concentrations have an intercalated hexagonal (HI) structure, whereas those formed at higher hexanol content have an inverted hexagonal (HII) structure.

Binding Sites↗

PubMed2005

Phase behavior of concentrated aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium hydroxy naphthoate (SHN).

We have characterized the phase behavior of mixtures of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN) over a wide range of surfactant concentrations using polarizing optical microscopy and X-ray diffraction. A variety of liquid crystalline phases, such as hexagonal, lamellar with and without curvature defects, and nematic, are observed in these mixtures. At high temperatures the curvature defects in the lamellar phase are annealed gradually on decreasing the water content. However, at lower temperatures these two lamellar structures are separated by an intermediate phase, where the bilayer defects appear to order into a lattice. The ternary phase diagram shows a high degree of symmetry about the line corresponding to equimolar CTAB/SHN composition, as in the case of mixtures of cationic and anionic surfactants.

Journal Article↗

PubMed2005

Structure of phospholipid-cholesterol membranes: an x-ray diffraction study.

We have studied the phase behavior of mixtures of cholesterol with dipalmitoyl phosphatidylcholine (DPPC), dimyristoyl phosphatidylcholine (DMPC), and dilauroyl phosphatidylethanolamine (DLPE), using x-ray diffraction techniques. Phosphatidylcholine (PC)-cholesterol mixtures are found to exhibit a modulated phase for cholesterol concentrations around 15 mol % at temperatures below the chain melting transition. Lowering the relative humidity from 98% to 75% increases the temperature range over which it exists. An electron density map of this phase in DPPC-cholesterol mixtures, calculated from the x-ray diffraction data, shows bilayers with a periodic height modulation, as in the ripple phase observed in many PCs in between the main- and pretransitions. However, these two phases differ in many aspects, such as the dependence of the modulation wavelength on the cholesterol content and thermodynamic stability at reduced humidities. This modulated phase is found to be absent in DLPE-cholesterol mixtures. At higher cholesterol contents the gel phase does not occur in any of these three systems, and the fluid lamellar phase is observed down to the lowest temperature studied (5 degrees C).

Cholesterol↗

PubMed2005

Comprehensive examination of mesophases formed by DMPC and DHPC mixtures.

Mixtures of long- and short-chain phospholipids, specifically 14:0 and 6:0 phosphatidylcholines (DMPC and DHPC), have been used successfully in NMR studies as magnetically alignable substrates for membrane-associated proteins. However, recent publications have shown that the phase behavior of these mixtures is much more complex than originally thought. Using polarized light microscopy and small-angle neutron scattering, phase diagrams of DMPC/DHPC mixtures at molar ratios of 2, 3.2, and 5 have been determined. Generally, at temperatures below the main-chain melting transition of DMPC (T(M) = 23 degrees C), an isotropic phase of disk-like micelles is found. At high temperatures (T > 50 degrees C), a lamellar phase consisting of either multilamellar vesicles (MLV) or extended lamellae is formed, which at low lipid concentrations (e.g., MLV) coexists with an excess of water. At intermediate temperatures and lipid concentrations, a chiral nematic phase made up of worm-like micelles was observed.

Journal Article↗

PubMed2004

Finite-size effects in biomimetic smectic films.

Thin stacks of lipid multibilayers supported on rigid silicon and mica substrates are found to exhibit finite-size effects. Using neutron diffraction we find that the repeat spacing (d) of stacks containing up to a few tens of bilayers depends on their thickness (D), with d increasing with decreasing D. Differences in d are larger in the low-temperature Lbeta' phase consisting of rigid bilayers than in the high-temperature Lalpha phase where the bilayers are more flexible. Various scenarios that may be responsible for this counterintuitive observation are discussed.

Biomimetic Materials↗

PubMed2004

Magnetically alignable phase of phospholipid "bicelle" mixtures is a chiral nematic made up of wormlike micelles.

We have studied the phase behavior of binary mixtures of long- and short-chain lipids, namely, dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC), using optical microscopy and small-angle neutron scattering. Samples with a total lipid content of 25 wt %, corresponding to ratios Q ([DMPC]/[DHPC]) of 5, 3.2, and 2, are found to exhibit an isotropic (I) --> chiral nematic (N) --> lamellar phase sequence on increasing temperature. The I-N transition coincides with the chain melting transition of DMPC at Q = 5 and 3.2, but the N phase forms at a higher temperature for Q = 2. All three samples form multilamellar vesicles in the lamellar phase. Our results show that disklike "bicellar" aggregates occur only in the lower temperature isotropic phase and not in the higher temperature magnetically alignable N phase, where they were previously believed to exist. The N phase is found to consist of long, flexible wormlike micelles, their entanglement resulting in the very high viscosity of this phase.

Dimyristoylphosphatidylcholine↗

PubMed2004

Relationship between the unbinding and main transition temperatures of phospholipid bilayers under pressure.

Using neutron diffraction and a specially constructed high pressure cell suitable for aligned multibilayer systems, we have studied, as a function of pressure, the much observed anomalous swelling regime in dimyristoyl- and dilauroyl-phosphatidylcholine bilayers, DMPC and DLPC, respectively. We have also reanalyzed data from a number of previously published experiments and have arrived at the following conclusions. (a). The power law behavior describing anomalous swelling is preserved in all PC bilayers up to a hydrostatic pressure of 240 MPa. (b). As a function of increasing pressure there is a concomitant decrease in the anomalous swelling of DMPC bilayers. (c). For PC lipids with hydrocarbon chains >or=13 carbons the theoretical unbinding transition temperature T small star, filled is coupled to the main gel-to-liquid crystalline transition temperature T(M). (d). DLPC is intrinsically different from the other lipids studied in that its T small star, filled is not coupled to T(M). (e). For DLPC bilayers we predict a hydrostatic pressure (>290 MPa) where unbinding may occur.

Binding Sites↗

PubMed2004

Reentrant phase transitions of DNA-surfactant complexes.

Complexes of double-stranded DNA with the cationic surfactant cetyltrimethylammonium bromide have been studied using small angle x-ray diffraction at varying concentrations of DNA and the cosurfactant hexanol. At low DNA concentrations, an intercalated hexagonal (H(c)(I))-->lamellar (L(c)(alpha))-->inverted hexagonal (H(c)(II)) transformation is found on increasing hexanol content. The H(c)(II) structure is converted into L(c)(alpha) on adding more DNA. Further increase in hexanol content leads to a phase separation in the surfactant solution, and a reentrant L(c)(alpha)-->H(c)(II)-->L(c)(alpha) transition is observed as DNA concentration is increased. Such structural transformations of DNA-surfactant complexes, driven by DNA concentration, have not been reported until now.

Cetrimonium↗

PubMed2004

Spontaneously formed monodisperse biomimetic unilamellar vesicles: the effect of charge, dilution, and time.

Using small-angle neutron scattering and dynamic light scattering, we have constructed partial structural phase diagrams of lipid mixtures composed of the phosphatidylcholines dimyristoyl and dihexanoyl doped with calcium ions (Ca2+) and/or the negatively charged lipid, dimyristoyl phosphatidylglycerol (DMPG). For dilute solutions (lipid concentration < or =1 wt %), spontaneously forming unilamellar vesicles (ULVs) were found, and their polydispersity was determined to be approximately 20%. The stability of the Ca2+- or DMPG-doped ULVs was monitored over a period of 4 days and their structural parameters (e.g., average outer radius, ) were found to be insensitive to the lipid concentration (Clp). However, doping the dimyristoyl/dihexanoyl system with both Ca2+ and DMPG resulted in ULVs whose was found to be Clp dependent. The of DMPG-doped ULVs remained unchanged over an extended period of time (at least 4 days), a good indication of their stability.

Calcium↗

PubMed2004

Finite-size effects do not reduce the repeat spacing of phospholipid multibilayer stacks on a rigid substrate.

Finite-size effects in stacks of phospholipid bilayers, in the fluid L alpha phase, are investigated using samples oriented on silicon substrates. Recently in this journal, such effects have been suggested as the probable cause of reduced lamellar repeat spacings in very thin samples made up of a few (<10) bilayers. Our systematic studies on samples of different thicknesses do not support this conclusion. At full hydration all samples are found to have the same repeat spacing, irrespective of their thickness. At lower hydrations, on the other hand, very thin samples, consisting of only a few bilayers, have a slightly larger spacing.

Biophysics↗

PubMed2003

Concentration-independent spontaneously forming biomimetric vesicles.

In this Letter we present small-angle neutron scattering data from a biomimetic system composed of the phospholipids dimyristoyl and dihexanoyl phosphorylcholine (DMPC and DHPC, respectively). Doping DMPC-DHPC multilamellar vesicles with either the negatively charged lipid dimyristoyl phosphorylglycerol (DMPG, net charge -1) or the divalent cation, calcium (Ca2+), leads to the spontaneous formation of energetically stabilized monodisperse unilamellar vesicles whose radii are concentration independent and in contrast with previous experimental observations.

Biomimetic Materials↗

PubMed2003

Structure of the ripple phase of phospholipid multibilayers.

We present electron density maps (EDMs) of the ripple phase formed by phosphorylcholine lipids such as dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), dihexadecyl phosphatidylcholine, and dilauroyl phosphatidylcholine (DLPC). With the exception of DLPC, the rippled bilayers have a sawtooth shape in all the systems, with one arm being almost twice as long as the other. For DMPC and POPC bilayers, EDMs have been obtained at different temperatures at a fixed relative humidity, and the overall shape of the ripples and the ratio of the lengths of the two arms are found to be insensitive to temperature. EDMs of all the systems with saturated hydrocarbon chains suggest the existence of a mean chain tilt along the ripple wave vector. In the literature it is generally assumed that the asymmetry of the rippled bilayers (absence of a mirror plane normal to the ripple wave vector) arises from a sawtoothlike height profile. However, in the case of DLPC, the height profile is found to be almost symmetric and the asymmetry results mainly from different bilayer thicknesses in the two arms of the ripple. We also present EDMs of the metastable ripple phase of dipalmitoyl phosphatidylcholine, formed on cooling from the L(alpha) phase.

Journal Article↗

PubMed2003

Cholesterol-induced modulated phase in phospholipid membranes.

We report the observation of a cholesterol-induced modulated phase (Pbeta) in dipalmitoyl phosphatidylcholine bilayers. It occurs below the main transition of the lipid at cholesterol concentrations of around 15 to 20 mol % and is distinct from the ripple (Pbeta') phase found in between the main and pretransitions at lower cholesterol concentrations. An electron density map of this phase, constructed from x-ray diffraction data from oriented multilayers, shows that the bilayers in this phase have a one-dimensional periodic height modulation with an amplitude of about 2.5 A. A partial phase diagram of the system deduced from diffraction data is in broad agreement with earlier studies.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2003

Discontinuous unbinding of lipid multibilayers.

We have observed a discontinuous unbinding transition of lipid bilayer stacks composed of phosphatidylethanolamine and phosphatidylglycerol using x-ray diffraction. The unbinding is reversible and coincides with the main (L(beta)-->L(alpha)) transition of the lipid mixture. Interbilayer interaction potentials deduced from the diffraction data reveal that the bilayers in the L(beta) phase are only weakly bound. The unbinding transition appears to be driven by an abrupt increase in steric repulsion resulting from increased thermal undulations of the bilayers upon entering the fluid L(alpha) phase.

Lipid Bilayers↗

PubMed2002

Enhancement of steric repulsion with temperature in oriented lipid multilayers.

We have studied the temperature dependence of the stacking periodicity, d, of oriented phospholipid multilayers using grazing angle neutron scattering techniques. d is found to increase substantially at higher temperatures, just before the bilayers peel off from the substrate. Although we do not observe thermal unbinding, our results are consistent with the notion that the unbinding transition is driven by steric repulsion arising from thermal fluctuations of the membranes, in contrast to those of a recent study by Vogel et al. [Phys. Rev. Lett. 84, 390 (2000)].

Dimyristoylphosphatidylcholine↗

PubMed2001

Role of tilt order in the asymmetric ripple phase of phospholipid bilayers.

We present the electron density map of the asymmetric ripple phase of dilauroylphosphatidylcholine. We find that the primary feature characterizing the "asymmetry" of the rippled bilayers is the difference in the bilayer thickness in the two arms, and not the asymmetry of the bilayer height profile as is generally assumed. This difference in the bilayer thickness can be attributed to a mean tilt of the hydrocarbon chains of the lipid molecules along the direction of the ripple wave vector. We propose a Landau theory for this phase which takes into account the anisotropic elastic properties of a bilayer with tilt order.

Lipid Bilayers↗

PubMed1995

Evidence for a two-dimensional molecular lattice in subgel phase DPPC bilayers.

Using a combination of X-ray diffraction data from oriented films and multilamellar liposomes of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) in the subgel phase, we have established the presence of a 2D molecular lattice containing two lipid molecules. The proposed 2D lattice is consistent with all the X-ray diffraction data on the subgel phase of DPPC available in the literature. In this phase, the DPPC molecules are ordered in the plane of the bilayer and are also found to be positionally correlated across a single bilayer but not with those in adjacent bilayers. We also present the possible molecular arrangements for the proposed lattice.

1,2-Dipalmitoylphosphatidylcholine↗