The effect of membrane thickness on the membrane permeabilizing activity of the cyclic lipopeptide tolaasin II

Tolaasin II is an amphiphilic, membrane-active, cyclic lipopeptide produced by Pseudomonas tolaasii and is responsible for brown blotch disease in mushroom. To better understand the mode of action and membrane selectivity of tolaasin II and related lipopeptides, its permeabilizing effect on liposomes of different membrane thickness was characterized. An equi-activity analysis served to distinguish between the effects of membrane partitioning and the intrinsic activity of the membrane-bound peptide. It was found that thicker membranes require higher local peptide concentrations to become leaky. More specifically, the mole ratio of membrane-bound peptide per lipid needed to induce 50% leakage of calcein within 1 h, R-e (50), increased monotonically with membrane thickness from 0.0016 for the 14:1 to 0.0070 for the 20:1 lipid-chains. Moreover, fast but limited leakage kinetics in the low-lipid regime were observed implying a mode of action based on membrane asymmetry stress in this time and concentration window. While the assembly of the peptide to oligomeric pores of defined length along the bilayer z-axis can in principle explain inhibition by increasing membrane thickness, it cannot account for the observed limited leakage. Therefore, reduced intrinsic membrane-permeabilizing activity with increasing membrane thickness is attributed here to the increased mechanical strength and order of thicker membranes

Surface chemistry of InP quantum dots, amine-halide co- passivation, and binding of Z-type ligands

Understanding and controlling the surface chemistry of colloidal quantum dots (QDs) are essential steps toward improving their opto-electronic properties and tailoring the material for specific applications. For oleylamine–chloride co-passivated InP QDs synthesized using di-ethylaminophosphine (DEAP), knowledge of possible exchange reactions and their effect on the QD properties is still very limited. In this work, we address this issue by a combination of experimental and computational studies. First, we prove that InP QDs are passivated by a combination of oleylamine (OlNH2) and chloride, bound as L-type and X-type ligands, respectively. By exposure to organic acids such as carboxylic acids or thiols, this L–X combination can be replaced with oleylammonium chloride in an acid–base-mediated ligand exchange reaction that results in the binding of carboxylates or thiolates as X-type ligands. The latter tend to quench the band-edge emission by forming strongly localized mid-gap states on the sulfur atoms of the thiolates. Furthermore, we observe that the binding of ZnCl2 to the InP QD surface, a process enabled by the prior complexation of this Z-type ligand with OlNH2, considerably increases the band-edge emission. However, as the resulting photoluminescence efficiency remains modest, we conclude that InP QDs synthesized using DEAP feature a diverse set of surface states, for which passivation depends at least on the elimination of undercoordinated surface phosphorous and the choice of the X-type ligand

Expanding the toolbox : use of biosensing technology to elucidate the mode of action of cyclic lipodepsipeptides

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Form and function : structural variations of natural cyclic lipodepsipeptides

Cyclic lipodepsipeptides (CLPs) are non-ribosomal bacterial peptides, which exhibit antagonistic activity against several bacterial and fungal species. CLPs increasingly attract attention in the field of crop protection, where they serve as valuable alternatives to chemical pesticides. A full understanding of their conformation and membrane interactions is essential to elucidate the exact working mechanism of these molecules. [1 – 4] CLPs consist of an oligopeptide chain containing both D- and L-amino acids which forms a cyclic structure by means of an ester (depsi) bond. Depending on the bacterial producer strain, there is a very large range of different CLPs that are produced. Therefore, the CLPs are classified into several groups according to oligopeptide chain length, size of the cyclic fragment and sequence similarity. CLPs adopt a well-defined amphipathic conformation, as determined by liquid-state NMR and X-ray crystallography [2, 3]. Based on a detailed analysis of individual peptide sequences, similar structural features can be found across different CLP groups. We hypothesize the existence of “super groups” with fixed structural motifs, founded on clear differences and similarities in sequence composition and length. This proposal will be discussed in the presentation by confronting the NMR-derived three-dimensional conformations of several CLPs belonging to different groups, including viscosins, orfamides, amphisins and xantholysins. These structural motifs potentially have predictive capabilities: they will allow to predict the three-dimensional structure of new CLPs and new CLP groups, or predict the effect of certain structural modification introduced by solid-phase peptide synthesis. [5] Furthermore, they can potentially be linked to a differential working mechanism across the different super groups. References 1. Geudens N., M.N. Nasir, et al., Biochimica Biophysica Acta, 2017, 1859, 331-339 2. Geudens N., M. De Vleeschouwer, et al., ChemBioChem, 2014, 15, 2736-2746 3. Sinnaeve, D., C. Michaux, et al., Tetrahedron, 2009, 65(21): 4173-4181 4. Sinnaeve, D., P. M. Hendrickx, et al., Chemistry - A European Journal, 2009, 15(46): 12653-12662 5. De Vleeschouwer, M., D. Sinnaeve, et al., Chemistry - A European Journal, 2014, 20, 7766-777