Theory of the ripple phase coexistance

The macroscopic theory of the competition between two different modifications ($\Lambda$- and $\Lambda$/2-phases) of P$_{\beta'}$ (ripple) phase in lipid bilayers is presented. It is shown that the increase of the membrane curvature should lead to the phase transition from the $\Lambda$/2-phase to the $\Lambda$-phase; the critical value of the curvature $R^{-1}_{\rm c}$ is obtained as a function of the geometrical parameters of the $\Lambda$/2-phase and the free energy difference $\Delta F^0$ between both phases in the case of planar membranes. The observed [3] splitting of disclinations in the $\Lambda$/2-phase into well-separated half-integer parts connected by linear defects (of the lenght $\ell_d \ll \lambda$) is shown to be related with the smallness of $\Delta F^0$. the relation between $R_{\rm c},~\ell_{\rm d}$ and geometrical and thermodynamic parameters of the $\Lambda$/2-phase alone is obtained

Low pH induces an interdigitated gel to bilayer gel phase transition in dihexadecylphosphatidylcholine membrane.

We have investigated the influence of pH on the structures and phase behaviors of multilamellar vesicles of the ether-linked dihexadecylphosphatidylcholine (DHPC-MLV). This phospholipid is known to be in the interdigitated gel (L(beta)I) phase in excess water at 20 degrees C at neutral pH. The results of X-ray diffraction experiments indicate that a phase transition from L(beta)I phase to the bilayer gel phase occurred in DHPC-MLV in 0.5 M KCl around pH 3.9 with a decrease in pH, and that at low pH values, less than pH 2.2, DHPC-MLVs were in L(beta') phase. The results of fluorescence and light scattering method indicate that the gel to liquid-crystalline phase transition temperature (T(m)) of DHPC-MLV increased with a decrease in pH. On the basis of a thermodynamic analysis, we conclude that the main mechanism of the low-pH induced L(beta)I to bilayer gel phase transition in DHPC-MLV and the increase in its T(m) is connected with the decrease in the repulsive interaction between the headgroups of these phospholipids. As pH decreases, the phosphate groups of the headgroups begin to be protonated, and as a result, the apparent positive surface charges appear. However, surface dipoles decrease and the interaction free energy of the hydrophilic segments with water increases. The latter effect dominates the pure electrostatic repulsion between the charged headgroups, and thereby, the total repulsive interaction in the interface decreases

Theory of electrostatic effects in soft biological interfaces using atomic force microscopy.

We calculated the electrostatic force between a planar interface, such as a planar-supported lipid bilayer membrane, and the tip of a stylus on which another lipid bilayer or some other biomacromolecular system might be deposited. We considered styli with rounded tips as well as conical tips. To take into account the effect of dynamical hydrogen-bonded structures in the aqueous phase, we used a theory of nonlocal electrostatics. We used the Derjaguin approximation and identified the systems for which its use is valid. We pointed out where our approach differs from previous calculations and to what extent the latter are inadequate. We found that 1) the nonlocal interactions have significant effects over distances of 10-15 A from the polar zone and that, at the surface of this zone, the effect on the calculated force can be some orders of magnitude; 2) the lipid dipoles and charges are located a distance L from the hydrophobic layer in the aqueous medium and this can have consequences that may not be appreciated if it is ignored; 3) dipoles, located in the aqueous region, can give rise to forces even though the polar layer is unchanged, and if this is ignored the interpretation of force data can be erroneous if an attempt is made to rationalize an observed force with a knowledge of an uncharged surface; 4) the shape of the stylus tip can be very important, and a failure to take this into account can result in incorrect conclusions, a point made by other workers; and 5) when L is nonzero, the presence of charges and dipoles can yield a force that can be nonmonotonic as a function of ionic concentration

The Single GUV Method for Probing Biomembrane Structure and Function

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Lob der Verschwendung: Redundanz in der Regionalentwicklung; ein sozioökonomisches Plädoyer

Dem Autor geht es darum, die Bedeutung einer "verschwenderischen" Produktion von Entwicklungsoptionen für Regionen herauszuarbeiten bzw. "die im Offenhalten unterschiedlicher Entwicklungspfade angelegte regionale Anpassungsfähigkeit im Konzept der Redundanz theoretisch zu erfassen." Nach einigen grundsätzlichen konzeptionellen und methodischen Vorbemerkungen zum Verhältnis von Anpassungsfähigkeit und Redundanz gibt der Autor im zweiten Abschnitt einen breiten Überblick über die unterschiedlichen natur-, geistes- und sozialwissenschaftlichen Redundanz-Konzepte. Zweck dieses Überblicks ist, "an theoretischen Vertrautheiten zu rütteln und konzeptionelle Ansatzpunkte zu identifizieren, über die sich empirisches Material Umschichten läßt." Der dritte Abschnitt schätzt die Relevanz von Redundanz für Regionalentwicklung und Regionalpolitik ab. Er zeigt, daß Strukturredundanz für die Anpassungsfähigkeit von Regionen in doppelter Hinsicht bedeutsam ist. Zum einen gibt Funktionsredundanz auf der Ebene des einzelnen Betriebs Anpassungsspielräume auf der Ebene des zentralen ökonomischen Akteurs der Region vor. Zum zweiten erschwert Beziehungsredundanz auf der regionalen bzw. zwischenbetrieblichen Ebene eine Perfektionierung von Unzulänglichkeiten durch Überanpassung von Regionen an eine spezifische Umwelt. Abschließend werden einige zentrale regionalpolitische Angelpunkte zur Erhöhung der regionalen Anpassungsfähigkeit aufgezeigt. (ICD