Lamellar-hexagonal transition

The shifts of the lamellar-hexagonal transition during titration are greater and in the opposite direction relative to changes of Tm- Thus, for didodecyl PE, the lamellar-hexagonal transition decreases by 41°C during phosphate protonation (pK 1.9) and by 50°C during amine protonation (pK 9.3), whereas for Tm these shifts are 6°C and 15° C, respectively, in the opposite direction (Fig. 3f) (54). 

The above example is only one out of a series of similar experiments with various lipid system showing the lamellar-hexagonal transition. In combination they allow a number of important conclusions regarding the mechanism and kinetics of this processes ... 

Sot J, Aranda FJ, Collado MI, Goni FM, Alonso A. Different effects of long- and short-chain ceramides on the gel-fluid and lamellar-hexagonal transitions of phospholipids a calorimetric, NMR, and x-ray diffraction study. Biophys J 88(2005) 3368-3380. 

Using the so-called "block copolymers (a block of Na A-monomers at one end is covalently bonded to a block of Nb B-monomers) one can also realize the analogy of order-disorder phenomena in metallic alloys with polymers one observes transitions from the disordered melt to mesophases with various types of long range order (lamellar, hexagonal, cubic, etc ). We shall not consider these phenomena here further, however... 

An A-B diblock copolymer is a polymer consisting of a sequence of A-type monomers chemically joined to a sequence of B-type monomers. Even a small amount of incompatibility (difference in interactions) between monomers A and monomers B can induce phase transitions. However, A-homopolymer and B-homopolymer are chemically joined in a diblock therefore a system of diblocks cannot undergo a macroscopic phase separation. Instead a number of order-disorder phase transitions take place in the system between the isotropic phase and spatially ordered phases in which A-rich and B-rich domains, of the size of a diblock copolymer, are periodically arranged in lamellar, hexagonal, body-centered cubic (bcc), and the double gyroid structures. The covalent bond joining the blocks rests at the interface between A-rich and B-rich domains. 

Seddon, J. M. (1990). Structure of the inverted hexagonal (HII) phase, and non-lamellar phase transitions of lipids, Biochim. Biophys. Acta, 1031, 1-69. 

Fig. 11 Phase transitions in mesoporous solids a-d, lamellar-hexagonal e-f, hexagonal-cubic. The circular objects around the surfactant assemblies are the metal-oxo species (from Neeraj and...

The question of intermediate structures around a phase transition has caught particular attention in the case of the lamellar-to-inverted hexagonal transition of ethanolamine phospholipids, since this involves a major topological change (see Fig. 6). Inverted micellar structures were proposed as intermediates on the basis of P-NMR and electron microscopic results and also rationalized in a theoretical mechanism On the other hand, first results of time-resolved X-ray diffraction... 

AOT Acrylamide/JV,J T -methy- lenebisacrylamide Lamellar, hexagonal, cubic No phase transition Not characterized - [49]... 

It is well known that membranes of various kinds of phospholipids undergo phase transitions, such as a gel-to-liquid crystalline transition and a lamellar-to-hexagonal transition. These phase transitions have been used for the design of stimuli-sensitive liposomes. The, temperature sensitization of liposomes has been attempted using thermo-sensitive polymers that exhibit a lower critical solution temperature (CST). Attachment of these polymers to liposomes could give temperature-sensitive functionalities to the liposomes. ... 

Figure 4.6 Lamellar-to-inverted hexagonal transition of DEPE-ceramide mixtures. Ceramide was A/-hexadecanoyL sphingosine (Cerl6). Data from J. Sot (unpublished)...

Membrane proteins have a profound influence on the thermotropic properties of lipid bilayers. Moreover, the effects of intrinsic (or integral) proteins are very different from those of extrinsic (or peripheral) proteins, reflecting their different modes of interaction, respectively, from the hydrophobic and hydrophyllic moieties of lipids. Some proteins can also influence, even at very low proteindipid ratios, the lamellar to inverted hexagonal transitions of certain lipids. 

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