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).

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.

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.

Phase-matching and Nonlinear Optical Processes in Silicon Waveguides.

The efficiency of four-wave-mixing arising from Raman and non-resonant nonlinear susceptibilities in silicon waveguides is studied in the 1.3 - 1.8microm regime.

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.

Anti-Stokes Raman conversion in silicon waveguides.

The first observation of parametric down-conversion in silicon is reported. Conversion from 1542.3nm to 1328.

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.

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.

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.

Observation of stimulated Raman amplification in silicon waveguides.

We report the first observation of Stimulated Raman Scattering (SRS) in silicon waveguides. Amplification of the Stokes signal, at 1542.3 nm, of up to 0.

Chain length dependence of apomyoglobin folding: structural evolution from misfolded sheets to native helices.

Very little is known about how protein structure evolves during the polypeptide chain elongation that accompanies cotranslational protein folding. This in vitro model study is aimed at probing how conformational space evolves for purified N-terminal polypeptides of increasing length. These peptides are derived from the sequence of an all-alpha-helical single domain protein, Sperm whale apomyoglobin (apoMb).

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.

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.

Structural characterization of manganese(II)-nucleotide complexes bound to yeast 3-phosphoglycerate kinase: 13C relaxation measurements using [U-13C]ATP and [U-13C]ADP.

A complete characterization of the conformations of Mn.ADP and Mn.ATP bound to the active site of yeast 3-P-glycerate kinase is presented.

The variable domain glycosylation in a monoclonal antibody specific to GnRH modulates antigen binding.

Functionally important glycosylation has been identified in the antigen binding domain of an anti-GnRH monoclonal antibody. Presence of mannose and sialic acid residues is revealed from con A immunoblots and positive staining with a sialic acid detection kit, respectively. Desialylation of the antibody reduces GnRH binding, suggesting the role of terminal sialic acid residues in modulating antigen binding.

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.

Crystallization and molecular packing analysis of barstar crystals.

Barstar, the natural inhibitor of barnase crystallizes in many different crystal forms under almost identical conditions. Although barstar is a monomeric protein, it crystallizes with four molecules in the asymmetric unit in two crystal forms, rhombohedral (space group R3; a = b = 118.0 angstrum; c = 75.

Structural changes in profilin accompany its binding to phosphatidylinositol, 4,5-bisphosphate.

The effect on the structure of profilin of phosphatidylinositol 4,5-bisphosphate (PIP2) binding was probed by fluorescence and circular dichroism (CD) spectroscopy. Fluorescence of Trp3 and Trp31 of profilin at 292 nm showed a linear decrease in solution emission at 340 nm as PIP2/profilin was increased from 0 to 80:1, apparently due to a static quenching mechanism involving formation of a nonfluorescent PIP2/profilin complex. CD spectra revealed an increase of up to 3.