A Novel Physical Investigation of Salt Interactions with Lipid Membranes

The transport of molecules and compounds across cell membranes is vital for life and a process that has been the topic of intricate research for some time. However, it has been a challenge due to cell membranes dynamic and complex nature. It has been known for years that an effective way to undergo this research was to investigate the interactions of lipid membranes, these serve as an appropriate mimic to the cell membranes, and by looking at molecule – lipid interactions it can greatly help to predict the mechanisms of transport and furthermore the effects of these interactions. The following work explores the area of salt interactions with lipid membranes. It is well known that various salts have significant importance on bodily functions and physiological impacts yet their interactions with membranes seem obscure. In order to attempt to elucidate this topic of research, an investigation was carried out which focuses on sodium and calcium interactions with monolayers and bilayers. This was conducted using a Quartz Crystal Microbalance, Langmuir-Schaefer deposition and data retrieved Neutron reflectometry experiments. Whereby the aim of these experiments was to provide a coherent and thorough practical study to be considered along the vast simulation studies present in this area of research. The QCM data revealed some lipid loss overall. Although some of this may have been due to the initial lipid deposition as there is evidence suggesting some aggregates forming. This loss of lipid occurred more erratically for NaCl than it did for CaCl₂. Unfortunately, due to faults in the QCM system and software, data from higher concentrations of CaCl₂ was unable to be retrieved, therefore a full comparison could not be made between sodium and calcium QCM runs. For the Langmuir isotherms which focused on monolayers, an overall decrease in area per molecule was observed. Similar to the QCM, this implied disruption of the lipid. Although a more in-depth look into the data also suggests a possibility of monolayer saturation. The data also provides an indication towards Ion bridging occurring between lipids, however this only seen with the CaCl2 isotherm. Neutron reflectometry data was collected from a single bilayer of hDPPC in an CaCl2 subphase, this data was then fit with Rascal software. The SLD profiles revealed that the salt solutions applied had little effect on the bilayer and the interaction was minimal. When comparing the neutron data to the QCM or Langmuir it was proven difficult as any loss of lipid as implied prior would have been beyond the detection of a neutron reflectometry experiment due to the resolution being too high. Neutron reflectometry data was also collected from SURF on floating bilayers, this was also fit using Rascal. Again, the floating bilayer experienced little change with the addition of CaCl2 overall. However, a more in-depth look was conducted investigating the fluid and gel phases. It was here that the results revealed affects to the parameter values which further implied ion bridging and the lipid disruption much like the Langmuir isotherm data. An overview of the results revealed a concurrence that the addition of NaCl and CaCl2 salt solutions to DPPC lipid membranes had little effect, however it is shown that the methods provided evidence leading to suggested disruptions in the lipid, the QCM demonstrated this through mass losses, whilst the isotherms and neutron reflectometry experiments displayed overlaps in data. Therefore, it can be confidently said that the experiments met the aims of the study, which was to provide practical evidence of salt interactions with lipid membranes. This is promising for future work as shows potential for practical avenues to be explored in this topic that can be combined with simulation studies.