Dr Ivan Kassal (School of Mathematics and Physics) and Dr Evelyne Deplazes (School of Chemistry and Molecular Biosciences) will be presenting. 

All welcome. 

Dr Ivan Kassal 

Title: Entropy and disorder enable charge separation in organic solar cells

Abstract: Although organic heterojunctions can separate charges with near-unity efficiency and on a sub-picosecond timescale, the precise mechanisms of charge separation remain unclear. In a model with localised charges, the Coulombic binding between the electron and the hole can exceed 15kT, which would make it impossible for the charges to separate in a reasonable time. Although several mechanisms have been proposed to explain the separation--including charge-carrier delocalisation and excesses energy of “hot" states--the description remains incomplete. We consider the entropic contribution to charge separation in the presence of disorder and find that even modest amounts of disorder have a decisive effect, reducing the charge-separation barrier to about kT or eliminating it altogether. Therefore, in most cases the charges are not bound at all and can separate spontaneously. This conclusion holds despite our worst-case assumption of localised, thermally relaxed carriers, and is only strengthened if mechanisms like delocalisation are also present. In other words, efficient charge generation is not hampered by the thermodynamics of the Coulombic binding, but by the kinetic competition between charge separation and recombination.


Dr Evelyne Deplazes 

Title: Membrane-binding properties of gating-modifier and pore-blocking venom peptides: membrane interaction is not a prerequisite for modification of channel gating

Abstract: Many venom peptides are potent and selective inhibitors of voltage-gated ion channels, including channels that are validated targets for the development of new drugs and biodegradable insecticides. However, to fully exploit the potential of venom peptides for these applications an understanding of their mechanism of action is required. In the case of voltage-gated ion channels, venom peptides act either as pore blockers that plug the extracellular face of the channel pore or gating modifiers that bind to one or more of the membrane-embedded voltage sensor domains. It has been debated whether membrane partitioning is an intrinsic feature of gating modifier toxins. In this study we used unrestrained molecular dynamics simulations combined with surface plasmon resonance and fluorescence experiments for the first direct comparison of the lipid binding properties of two gating modifiers (μ-TRTX-Hd1a and ProTx-I) and two pore-blockers (ShK and KIIIA). Of the peptides tested only ProTx-I showed concentration-dependent binding to phospholipid model membranes. Our results provide further evidence that the ability to insert into the lipid bilayer is not a requirement to be a gating modifier. In addition, we have characterised the lipid interaction surface of ProTx1 to neutral and anionic phospholipid membranes. The results indicate that the peptide resides predominately at the water-lipid interface and that a set of specific residues govern the peptide-membrane interactions. Furthermore, comparison of the structure and surface composition of a number of gating modifiers shows that the presence of hydrophobic face surrounded by charged residues is not sufficient to predict the membrane-binding properties of a peptide, as has been suggested previously. This has motivated more recent work, in which umbrella sampling techniques are used to develop method for predicting the binding affinity of venom peptides to model membranes.