arXiv subjects

Simon Titmuss

Publications and source records attributed to Simon Titmuss.

6 recordsLinked to original sources

arXiv2025

Neutron Reflectometry Reveals Conformational Changes in a Mechanosensitive Protein Induced by an Antimicrobial Peptide in Tethered Lipid Bilayers

Membrane proteins serve a wide range of vital roles in the functioning of living organisms. Compared to other classes of proteins, determining membrane protein structures remains a challenge, in large part due to the difficulty in establishing experimental conditions that can preserve the correct conformation and function of the protein in isolation from its native environment. We investigated the ion channel in lipid vesicles and in a planar lipid bilayer. By using a polymeric tether our planar membrane mimetic was not constrained by the underlying solid substrate, making it sufficiently flexible to allow for increases in bilayer curvature and changes in membrane tension. We used quartz crystal microbalance with dissipation (QCM-D), and polarised neutron reflectivity (PNR) to show the formation of MscL containing phospholipid bilayers, tethered with a high density PEG layer onto gold substrates from vesicle rupture. The MscL containing vesicles were separately characterised with small angle neutron scattering (SANS). MscL was expressed into vesicles using cell free protein expression. Analysing these vesicles with small angle neutron scattering, the radius of gyration of the protein was determined to be between 26-29~\AA{}, consistent with the crystal structure of individual MscL channels. The MscL composition of the formed bilayer was 14\%$v/v$, close to the initial composition of the vesicles, and a protein protrusion extending ca. 46~\AA{} into the solvent was determined by PNR. Addition of 1.6 and 3.2 $\mu$M pexiganan resulted in a decrease in the protrusion of MscL (from $\sim$46 to $\sim$38~\AA{}). To our knowledge, these findings represent the first direct experimental evidence of a structural change in the C-terminus containing protrusion of MscL, triggered by an antimicrobial peptide.

physics.bio-ph↗

arXiv2024

The interplay between topography and contact line pinning mechanisms on flat and superhydrophobic surfaces

Wettability of a surface depends on both surface chemistry and topography. To move a three-phase contact line, a de-pinning force needs to be applied, which is of practical importance in various applications. However, a unified understanding and description of the de-pinning force on both flat and superhydrophobic surfaces is still lacking. This study aims to bridge the existing gap in our understanding of the three-phase contact line pinning on flat and microstructured superhydrophobic surfaces. The findings indicate that a general model, based on two different pinning mechanisms, can describe the pinning force on both flat and microstructured surfaces. We compare the general model against experimental data from literature, as well as our experiments on flat and microstructured surfaces coated with a liquid-like layer of grafted polymer chains. While this theoretical framework can be useful for designing micro-engineered surfaces on which the contact line behaviour is important, it also provides a potential experimental strategy to distinguish the contribution of defects from that of molecular re-orientation to contact line pinning on a given solid material.

physics.flu-dyn↗

arXiv2023

Diffusive evaporation dynamics in polymer solutions is ubiquitous

Recent theory and experiments have shown how the buildup of a high-concentration polymer layer at a one-dimensional solvent-air interface can lead to an evaporation rate that scales with time as $t^{-1/2}$ and that is insensitive to the ambient humidity. Using phase field modelling we show that this scaling law constitutes a naturally emerging robust regime, Diffusion-Limited Evaporation (DLE). This regime dominates the dynamical state diagram of the system, which also contains regions of constant and arrested evaporation, confirming and extending understanding of recent experimental observations and theoretical predictions. We provide a theoretical argument to show that the scaling observed in the DLE regime occurs for a wide range of parameters, and our simulations predict that it can occur in two-dimensional geometries as well. Finally, we discuss possible extensions to more complex systems.

cond-mat.soft↗

arXiv2022

The evaporation of concentrated polymer solutions is insensitive to relative humidity

A recent theory suggests that the evaporation kinetics of macromolecular solutions is insensitive to the ambient relative humidity (RH) due to the formation of a `polarisation layer' of solutes at the air-solution interface. We confirm this insensitivity up to RH~80% in the evaporation of polyvinyl alcohol solutions from open-ended capillaries. To explain the observed drop in evaporation rate at higher RH, we need to invoke compressive stresses due to interfacial polymer gelation. Moreover, RH-insensitive evaporation sets in earlier than theory predicts, suggesting a further role for a gelled `skin'. We discuss the relevance of these observations for respiratory virus transmission via aerosols.

cond-mat.soft↗

arXiv2020

Soft matter science and the COVID-19 pandemic

Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes control their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. We survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.

cond-mat.soft↗

arXiv2018

Reduced adhesion between cells and substrate confers selective advantage in bacterial colonies

Microbial colonies cultured on agar Petri dishes have become a model system to study biological evolution in populations expanding in space. Processes such as clonal segregation and gene surfing have been shown to be affected by interactions between microbial cells and their environment. In this work we investigate the role of mechanical interactions such as cell-surface adhesion. We compare two strains of the bacterium E. coli: a wild-type strain and a "shaved" strain that adheres less to agar. We show that the shaved strain has a selective advantage over the wild type: although both strains grow with the same rate in liquid media, the shaved strain produces colonies that expand faster on agar. This allows the shaved strain outgrow the wild type when both strains compete for space. We hypothesise that, in contrast to a more common scenario in which selective advantage results from increased growth rate, the higher fitness of the shaved strain is caused by reduced adhesion and friction with the agar surface.

q-bio.PE↗