Arginine (R)-rich peptides constitute the most relevant class of cell-penetrating peptides and other membrane-active peptides that can translocate across the cell membrane or generate defects in lipid bilayers such as water-filled pores. The mode of action of R-rich peptides remains a topic of controversy, mainly because a quantitative and energetic understanding of arginine effects on membrane stability is lacking. Here, we explore the ability of several oligo-arginines R$$_n$$ n and of an arginine side chain mimic R$$_\mathrm {Side}$$ Side to induce pore formation in lipid bilayers employing MD simulations, free-energy calculations, breakthrough force spectroscopy and leakage assays. Our experiments reveal that R$$_\mathrm {Side}$$ Side but not R$$_n$$ n reduces the line tension of a membrane with anionic lipids. While R$$_n$$ n peptides form a layer on top of a partly negatively charged lipid bilayer, R$$_\mathrm {Side}$$ Side leads to its disintegration. Complementary, our simulations show R$$_\mathrm {Side}$$ Side causes membrane thinning and area per lipid increase beside lowering the pore nucleation free energy. Model polyarginine R$$_8$$ 8 similarly promoted pore formation in simulations, but without overall bilayer destabilization. We conclude that while the guanidine moiety is intrinsically membrane-disruptive, poly-arginines favor pore formation in negatively charged membranes via a different mechanism. Pore formation by R-rich peptides seems to be counteracted by lipids with PC headgroups. We found that long R$$_n$$ n and R$$_\mathrm {Side}$$ Side but not short R$$_n$$ n reduce the free energy of nucleating a pore. In short R$$_n$$ n , the substantial effect of the charged termini prevent their membrane activity, rationalizing why only longer $$\mathrm {R}_{n}$$ R n are membrane-active.

Membrane-coated colloidal probes combine the benefits of solid-supported membranes with a more complex three-dimensional geometry. This combination makes them a powerful model system that enables the visualization of dynamic biological processes with high throughput and minimal reliance on fluorescent labels. Here, we want to review recent applications of colloidal probes for the study of membrane fusion. After discussing the advantages and disadvantages of some classical vesicle-based fusion assays, we introduce an assay using optical detection of fusion between membrane-coated glass microspheres in a quasi two-dimensional assembly. Then, we discuss free energy considerations of membrane fusion between supported bilayers, and show how colloidal probes can be combined with atomic force microscopy or optical tweezers to access the fusion process with even greater detail.

The fusion of lipid membranes is a key process in biology. It enables cells and organelles to exchange molecules with their surroundings, which otherwise could not cross the membrane barrier. To study such complex processes we use simplified artificial model systems, i.e., an optical fusion assay based on membrane-coated glass spheres. We present a technique to analyze membrane-membrane interactions in a large ensemble of particles. Detailed information on the geometry of the fusion stalk of fully fused membranes is obtained by studying the diffusional lipid dynamics with fluorescence recovery after photobleaching experiments. A small contact zone is a strong obstruction for the particle exchange across the fusion spot. With the aid of computer simulations, fluorescence-recovery-after-photobleaching recovery times of both fused and single-membrane-coated beads allow us to estimate the size of the contact zones between two membrane-coated beads. Minimizing delamination and bending energy leads to minimal angles close to those geometrically allowed.

Europium(III) and terbium(III) complexes have been prepared using the ligands 2‐(4,5‐dihydro‐1,3‐oxazol‐2‐yl)phenol (HL1) and 2‐(4,5‐dihydro‐1,3‐thiazol‐2‐yl)phenol (HL2) in yields ranging between 74 and 100 %. Metal‐to‐ligand ratios of 1:3 and 1:4 can be achieved, giving rise to compounds with the formulae [Ln2(L)6] and NR4[Ln(L)4], Ln = Eu, Tb. Recrystallisation of the complexes from DMSO resulted in the formation of octanuclear complexes [Na2(Ln(L1)3)2(CO3)(DMSO)2]2 held tightly together by carbonate ions that have been formed from CO2 from the atmosphere. Five structures have been determined, showing a bidentate binding mode of the ligand through the phenolate oxygen and the nitrogen atom of the five‐membered ring. Most terbium compounds show bright luminescence upon excitation with near‐UV radiation, with quantum yields of 16 % to 79 %. Strong emission is observed for NBu4[Eu(L1)4] and NEt4[Eu(L2)4] with quantum yields of 43 % and 20 %, respectively.

Effects of arginine derivatives and oligopeptides on the physical properties of model membranesDissertation for the award of the degree "Doctor rerum naturalium" of the Georg-August-Universität Göttingen within the doctoral program Physics of Biological and Complex Systems