Mammalian brain plasma membranes: Isolation, enzymatic, and chemical characterization

Two variants of a simplified procedure for the isolation of plasma membrane fractions from monkey and rat brains, are described. The preparations show marked enrichments in the marker enzymes, (Na+-K+) adenosine triphosphatase, acetylcholinesterase, 5′-nucleotidase and adenylate cyclase. Lipid analysis and a protein electrophoretic pattern are presented. An enzymatic check has been made to assess for contamination by other cellular organelles. The amino acid composition of brain membrane proteins show a resemblance to the reported composition of erythrocyte ghost proteins but differ from myelin proteins.

Polyvanadate-stimulated NADH oxidation by plasma membranes-The need for a mixture of deca and meta forms of vanadate

Polyvanadate solutions obtained by extracting vanadium pentoxide with dilute alkali over a period of several hours contained increasing amounts of decavanadate as characterized by NMR and ir spectra. Those solutions having a metavanadate:decavanadate ratio in the range of 1-5 showed maximum stimulation of NADH oxidation by rat liver plasma membranes. Reduction of decavanadate, but not metavanadate, was obtained only in the presence of the plasma membrane enzyme system. High simulation of activity of NADH oxidation was obtained with a mixture of the two forms of vanadate and this further increased on lowering the pH. Addition of increasing concentrations of decavanadate to metavanadate and vice versa increased the stimulatory activity, reaching a maximum when the metavanadate:decavanadate ratio was in the range of 1-5. Increased stimulatory activity can also be obtained by reaching these ratios by conversion of decavanadate to metavanadate by alkaline phosphate degradation, and of metavanadate to decavanadate by acidification. These studies show for the first time that both deca and meta forms of vanadate present in polyvanadate solutions are needed for maximum activity of NADH oxidation.

Novel organic-inorganic composite polymer-electrolyte membranes for DMFCs

Organic-inorganic composite membranes comprising Nation with inorganic materials such as silica, mesoporous zirconium phosphate (MZP) and mesoporous titanium phosphate (MTP) are fabricated and evaluated as proton-exchange-membrane electrolytes for direct methanol fuel cells (DMFCs). For Nation-silica composite membrane, silica is impregnated into Nation matrix as a sol by a novel water hydrolysis process precluding the external use of an acid. Instead, the acidic nature of Nation facilitates in situ polymerization reaction with Nation leading to a uniform composite membrane. The rapid hydrolysis and polymerization reaction while preparing zirconia and titania sols leads to uncontrolled thickness and volume reduction in the composite membranes, and hence is not conducive for casting membranes. Nafion-MZP and Nafion-MTP composite membranes are prepared by mixing pre-formed porous MZP and MTP with Nation matrix. MZP and MTP are synthesised by co-assembly of a tri-block co-polymer, namely pluronic-F127, as a structure-directing agent, and a mixture of zirconium butoxide/titanium isopropoxide and phosphorous trichloride as inorganic precursors. Methanol release kinetics is studied by volume-localized NMR spectroscopy (employing ``point resolved spectroscopy'', PRESS), the results clearly demonstrating that the incorporation of inorganic fillers in Nation retards the methanol release kinetics under osmotic drag. Appreciable proton conductivity with reduced methanol permeability across the composite membranes leads to improved performance of DMFCs in relation to commercially available Nafion-117 membrane.

Mechanical response of the tympanal membranes of the tree cricket Oecanthus henryi

Crickets have two tympanal membranes on the tibiae of each foreleg. Among several field cricket species of the genus Gryllus (Gryllinae), the posterior tympanal membrane (PTM) is significantly larger than the anterior membrane (ATM). Laser Doppler vibrometric measurements have shown that the smaller ATM does not respond as much as the PTM to sound. Hence the PTM has been suggested to be the principal tympanal acoustic input to the auditory organ. In tree crickets (Oecanthinae), the ATM is slightly larger than the PTM. Both membranes are structurally complex, presenting a series of transverse folds on their surface, which are more pronounced on the ATM than on the PTM. The mechanical response of both membranes to acoustic stimulation was investigated using microscanning laser Doppler vibrometry. Only a small portion of the membrane surface deflects in response to sound. Both membranes exhibit similar frequency responses, and move out of phase with each other, producing compressions and rarefactions of the tracheal volume backing the tympanum. Therefore, unlike field crickets, tree crickets may have four instead of two functional tympanal membranes. This is interesting in the context of the outstanding question of the role of spiracular inputs in the auditory system of tree crickets.

Nafion and modified-Nafion membranes for polymer electrolyte fuel cells: An overview

Polymer electrolyte fuel cells (PEFCs) employ membrane electrolytes for proton transport during the cell reaction. The membrane forms a key component of the PEFC and its performance is controlled by several physical parameters, viz. water up-take, ion-exchange capacity, proton conductivity and humidity. The article presents an overview on Nafion membranes highlighting their merits and demerits with efforts on modified-Nafion membranes.

Improved procedures for static and dynamic analyses of wrinkled membranes

An analysis of large deformations of flexible membrane structures within the tension field theory is considered. A modification-of the finite element procedure by Roddeman et al. (Roddeman, D. G., Drukker J., Oomens, C. W J., Janssen, J. D., 1987, ASME J. Appl. Mech. 54, pp. 884-892) is proposed to study the wrinkling behavior of a membrane element. The state of stress in the element is determined through a modified deformation gradient corresponding to a fictive nonwrinkled surface. The new model uses a continuously modified deformation gradient to capture the location orientation of wrinkles more precisely. It is argued that the fictive nonwrinkled surface may be looked upon as an everywhere-taut surface in the limit as the minor (tensile) principal stresses over the wrinkled portions go to zero. Accordingly, the modified deformation gradient is thought of as the limit of a sequence of everywhere-differentiable tensors. Under dynamic excitations, the governing equations are weakly projected to arrive at a system of nonlinear ordinary differential equations that is solved using different integration schemes. It is concluded that, implicit integrators work much better than explicit ones in the present context.

Thermotropic and hydration studies of membranes formed from gemini pseudoglyceryl lipids possessing polymethylene spacers

Membrane formation from gemini pseudoglyceryl lipids bearing n-C14H29 and n-C16H33 chains has been reported. These lipid aggregates have been characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), high sensitivity differential scanning calorimetry (DSC), and Paldan fluorescence studies. The length of the spacer between the cationic ammonium headgroups has been varied from -(CH2)(3)- (propandiyl) to -(CH2)(12)- (dodecandiyl) in these lipids. All gemini lipids were found to generate stable suspensions in aqueous media. Electron microscopic studies revealed the smaller size of the gemini lipid aggregates as compared to their monomeric lipid counterparts. DLS measurements showed that the gemini lipid suspensions with a -(CH2)(8)- spacer length were bigger in size than that of other analogues. DSC studies suggest the unusual behavior of the gemini lipids bearing -(CH2)3- propanediyl spacer based lipids. These observations were consistent irrespective of the hydrocarbon chain lengths of the lipids. Paldan fluorescence based hydration studies showed that the hexadecyl chain based gemini lipid aggregates bearing a -(CH2)(12)- spacer were the most hydrated in their gel states among all the gemini lipid series investigated herein.

Understanding Membranes through the Molecular Design of Lipids

Lipids are amphiphilic molecules that are composed of hydrophilic and hydrophobic regions. A typical membranous aggregate (vesicles, water-filled lipid nanospheres) is formed upon the self-organization of lipids in water from a diverse collection of amphiphiles producing a dynamic supramolecular structure that shows phase behavior and ordering as required for specific biological functions. The determination of various physical properties of lipid aggregates is the key to determining structure-function relationships. Over the years, we have designed and synthesized a wide variety of lipid molecular systems for the investigation of their membrane-forming properties and have used them for purposes such as gene delivery and enzyme activation. In this feature article, we focus on our work on various types of lipids including ion-paired amphiphiles, cholesterol-based lipids, aromatic lipids, macrocyclic lipids containing disulfide tethers; cationic dimeric lipids, and so forth. The emphasis is oil experimental design and bottom-line conclusions.

Export of proteins across membranes: The helix reversion hypothesis

A model is presented which explains the biological role of the leader peptide in protein export. Along the lines of this model, the conformational changes of a protein with environment serves as a general mechanism for translocation. The leader peptide in the cytoplasm takes a hairpin like conformation which reverts to an extended helix upon integration into the membrane. The essential features of this model are in accord with recent results of protein export.

New computational approaches for wrinkled and slack membranes

The static response of thin, wrinkled membranes is studied using both a tension field approximation based on plane stress conditions and a 3D nonlinear elasticityformulation, discretized through 8-noded Cosserat point elements. While the tension field approach only obtains the wrinkled/slack regions and at best a measure of the extent of wrinkliness, the 3D elasticity solution provides, in principle, the deformed shape of a wrinkled/slack membrane. However, since membranes barely resist compression, the discretized and linearized system equations via both the approaches are ill-conditioned and solutions could thus be sensitive to discretizations errors as well as other sources of noises/imperfections. We propose a regularized, pseudo-dynamical recursion scheme that provides a sequence of updates, which are almost insensitive to theregularizing term as well as the time step size used for integrating the pseudo-dynamical form. This is borne out through several numerical examples wherein the relative performance of the proposed recursion scheme vis-a-vis a regularized Newton strategy is compared. The pseudo-time marching strategy, when implemented using 3D Cosserat point elements, also provides a computationally cheaper, numerically accurate and simpler alternative to that using geometrically exact shell theories for computing large deformations of membranes in the presence of wrinkles. (C) 2010 Elsevier Ltd. All rights reserved.

On the admittance of lipid bilayer membranes: Part I. Membrane-permeable ions

The modular formalism of Rangarajan [J. Electroanal. Chem., 55 (1974) 297] has been applied to the admittance of lipid bilayer membranes. The method leads to equations which clearly show the interrelations between the various partial processes involved in ion transport, and which allow examination of model assumptions without the need for a complete rederivation of the membrane admittance. Explicit expressions are given for both the continuum and single jump models. The former includes the ionic displacement component, important mostly at high frequencies.

On the admittance of lipid bilayer membranes: Part I. Membrane-permeable ions

The modular formalism of Rangarajan [J. Electroanal. Chem., 55 (1974) 297] has been applied to the admittance of lipid bilayer membranes. The method leads to equations which clearly show the interrelations between the various partial processes involved in ion transport, and which allow examination of model assumptions without the need for a complete rederivation of the membrane admittance. Explicit expressions are given for both the continuum and single jump models. The former includes the ionic displacement component, important mostly at high frequencies.

Reactive hot pressing of ZrB2-ZrCx ultra-high temperature ceramic composites with the addition of SiC particulate

Starting with non-stoichiometric Zr-B4C powder mixture ZrB2-ZrC matrix composites with SiC particulate addition have been made. It was found that variable amounts (5-25 vol%) of SiC could be incorporated and reactively hot pressed (RHPed) to relative densities of 97-99% at 1400-1500 degrees C. This technique has the potential to fabricate ZrB2-based matrices at low temperatures with a variety of reinforcements whose composition and volume fraction are not limited by stoichiometric considerations. The hardness of the composites is in the range of 17-22 GPa. (C) 2010 Elsevier Ltd. All rights reserved.