Some current and past projects

 

DNA Molecules on Membranes Morphological changes of neutral and charged membranes induced by anchored molecules PhD student: Vesselin Nikolov link to PhD thesis             We studied changes in curvature and elastic properties of lipid membranes induced by anchoring of long hydrophilic polymers at low polymer surface concentrations (corresponding to the mushroom regime). The polymers used in our study are fluorescently labeled and biotinylated lambda-phage DNA molecules, which bind to biotinylated giant unilamellar vesicles via a biotin-avidin-biotin linkage (see the cartoon to the right). The effect of anchored polymers on the membrane spontaneous curvature was characterized by monitoring the changes in the fluctuation spectra and the morphology of giant unilamellar vesicles. By varying the amount of biotinylated lipid in the membrane, we control the surface concentration of anchors. At low anchor concentrations, the spontaneous curvature of the membrane increased linearly with the DNA concentration. At higher anchor concentrations, the vesicles undergo budding transitions. read more

 

Structure and thermodynamics of water                in collaboration with Prof. Dr. Markus Antonietti PhD student: Cornelia Sinn link to PhD thesis             Most processes involving biological cells take place in the presence of small molecules, in particular salts. These processes in most cases depend on the interaction of ions with membranes. Ion binding is specific and depends a lot on the anion type. This specificity of ion binding effects has often been referred to as the so called Hofmeister series. The Hofmeister series arranges various ions according to their ability to precipitate proteins. It was first reported by Lewith and Hofmeister in 1888. The various ions in the series have been defined as chaotropic or cosmotropic. However, it is not yet clearly understood if the effect of the ions can be ascribed to their direct affinity towards the proteins or to their influence on the water structure.             In this project, we focus on the effect of the different salts on the structure of water. The main challenge is to understand why some ions act as ‘water structure makers’ (or cosmotropic) and others as ‘water structure breakers’ (or chaotropic). Later on, we will attempt to apply this knowledge on the ion effect to more complicated systems such as polymers, proteins and lipid membranes. As a tool for this research we use the isothermal titration calorimetry, which has become very popular for studying heats of reaction observed upon binding of ions and molecules to membranes.

 

 

Temporal resolution of electro- poration, fusion and deformation of giant vesicles. Postdoc: Karin Riske PhD student: Said Aranda             Above a certain threshold, electric pulses induce perturbations in the lipid bilayer leading to electroporation and electrofusion, the latter provided that two vesicles are close enough and aligned. Furthermore, electric fields cause deformation of the vesicles. So far, studies on giant vesicle morphology subject to electric fields have been performed mainly in alternating fields. The vesicles elongate either parallel or perpendicular to the electric field, depending on the field frequency and medium conductivity. We use light microscopy and a fast digital camera to study shape deformations induced by square pulses (50 – 300 microsec duration). Surprisingly, the vesicles assume a pseudo-squared shape throughout the electric pulse. We study the time responses of the membrane and the induced vesicle deformation, and the characteristic relaxation time after the electric field is switched off. We are also able to capture the formation of some short-lived macropores and the moment of electrofusion in cases where two vesicles are close enough. We study both lipid and polymer vesicles and attempt to characterize the effect of the membrane shear surface viscosity.

Last update: June 09, 2009