Phase diagram and tie-line determination for the ternary mixture DOPC/eSM/cholesterol.

We propose a novel, to our knowledge, method for the determination of tie lines in a phase diagram of ternary lipid mixtures. The method was applied to a system consisting of dioleoylphosphatidylcholine (DOPC), egg sphingomyelin (eSM), and cholesterol (Chol). The approach is based on electrofusion of single- or two-component homogeneous giant vesicles in the fluid phase and analyses of the domain areas of the fused vesicle. The electrofusion approach enables us to create three-component vesicles with precisely controlled composition, in contrast to conventional methods for giant vesicle formation. The tie lines determined in the two-liquid-phase coexistence region are found to be not parallel, suggesting that the dominant mechanism of lipid phase separation in this region changes with the membrane composition. We provide a phase diagram of the DOPC/eSM/Chol mixture and predict the location of the critical point. Finally, we evaluate the Gibbs free energy of transfer of individual lipid components from one phase to the other.

Nonisomorphic nucleation pathways arising from morphological transitions of liquid channels.

Motivated by unexpected morphologies of the emerging liquid phase (channels, bulges, droplets) at the edge of thin, melting alkane terraces, we propose a new heterogeneous nucleation pathway. The competition between bulk and interfacial energies and the boundary conditions determine the growth and shape of the liquid phase at the edge of the solid alkane terraces. Calculations and experiments reveal a "precritical" shape transition (channel-to-bulges) of the liquid before reaching its critical volume along a putative shape-conserving path. Bulk liquid emerges from the new shape, and, depending on the degree of supersaturation, the new pathway may have two, one, or zero energy barriers. The findings are broadly relevant for many heterogeneous nucleation processes because the novel pathway is induced by common, widespread surface topologies (scratches, steps, etc.).

Operation modes of the molecular motor kinesin.

The velocity and the adenosine triphosphate (ATP) hydrolysis rate of the molecular motor kinesin are studied using a general network representation for the motor, which incorporates both the energetics of ATP hydrolysis and the experimentally observed separation of time scales between chemical and mechanical transitions. Both the motor velocity and its hydrolysis rate can be expressed as superpositions of excess fluxes for the directed cycles (or dicycles) of the network. The sign of these dicycle excess fluxes depends only on two thermodynamic control parameters as provided by the load force F and the chemical energy Deltamicro released during the hydrolysis of a single ATP molecule. In contrast, both the motor velocity and its hydrolysis rate depend, in general, on the load force F as well as on the three concentrations of ATP, adenosine diphosphate (ADP), and inorganic phosphate (P), separately. Thus, in order to represent the different operation modes of the motor in the (F,Deltamicro) plane, one has to specify two concentrations such as the product concentrations [ADP] and [P]. As a result, we find four different operation modes corresponding to the four possible combinations of ATP hydrolysis or synthesis with forward or backward mechanical steps. Our operation diagram implies in particular that backward steps are coupled to ATP hydrolysis for sufficiently large ATP concentrations, but to ATP synthesis for sufficiently large ADP and/or P concentrations.

The morphology of lipid membranes.

Current research into the morphology of lipid membranes focuses on three areas: first, the transformations and fluctuations of the shape of freely suspended vesicles; second, the morphology of membranes that experience mutual interactions or external forces arising, for example, from a macroscopic surface or from optical tweezers; and third, the behavior of heterogeneous membranes with respect to bilayer asymmetry; intramembrane domains and anchored polymers. Our understanding of the morphology of lipid membranes has progressed substantially through the development of experimental techniques and theories.

Sociomedical aspects of malaria control in Colombia.

A household interview survey combined with a serological survey on the incidence of malaria attacks and prevalence of antibodies has been carried out in rural and urban areas of the pacific coast of Colombia. Additional information on people's knowledge, attitudes and behaviour towards malaria was collected by means of participant observation and informal interviews. The results show that people incorporate modern and traditional elements into their concepts of the disease and treatment strategies. The deficiencies of the official control programmes are shown from the people's point of view. Some human factors which influence malaria transmission are discussed and an estimate of the accuracy of self-diagnosis presented.

Adhesion-induced phase behavior of multicomponent membranes.

Biomimetic membranes that contain several molecular components are studied theoretically. In contact with another surface, such as a solid substrate or another membrane, some of these intramembrane components are attracted by the second surface and, thus, act as local stickers. The cooperative behavior of these systems is characterized by the interplay of (i) attractive binding energies, (ii) entropic contributions arising from the shape fluctuations of the membranes, and (iii) the entropy of mixing of the stickers. A systematic study of this interplay, which starts from the corresponding partition functions, reveals that there are several distinct mechanisms for adhesion-induced phase separation within the membranes. The first of these mechanisms is effective for flexible stickers with attractive cis interactions (within the same membrane) and arises from the renormalization of these interactions by the confined membrane fluctuations. A second, purely entropic mechanism is found for rigid stickers without attractive cis interactions and arises from a fluctuation-induced line tension. Finally, a third mechanism is present if the membrane contains both stickers and repellers, i.e., nonadhesive molecules that protrude from the membrane surface. This third mechanism is based on an effective potential barrier and becomes less effective if the shape fluctuations of the membrane become more pronounced.