In this master’s thesis, we investigate how the composition of a lipid layer influences the distribution of charged particles near the membrane surface. Two complementary modelling approaches are used: a coarse grained membrane model (with explicit lipid heads and tails) and a simplified charge based model (without tails, focusing on charged domains). Using Using systems of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphate, sodium salt (DOPA), we analyze the effects of ion valence and concentration, as well as the presence of larger positively charged model globular proteins.
The results show that counterions accumulate in a narrow region directly above the membrane. Divalent ions, due to their stronger electrostatic attraction, localize more tightly and more prominently than monovalent ions. A higher salt concentration leads to a greater accumulation of cations at the membrane, whereas anions are displaced away from the membrane due to their like charge repulsion with the negatively charged lipid groups. In phase separated membranes, pronounced lateral differences arise: Na⁺ concentration is higher above negatively charged DOPA rich regions than above zwitterionic DPPC regions. The most significant finding is that, under phase separation of lipids, positively charged model globular proteins accumulate above negatively charged membrane domains, with the highest density observed at the domain boundaries, where the local electric field is strongest. This effect weakens at higher salt concentrations due to electrostatic screening.
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