Determination of the character of the interaction of bioactive ions with phospholipid membranes using nonlinear microscopy methods

The study of electrostatic and chemical interactions and transformations on the surface of lipid membranes is crucial for estimating the efficiency and toxicity of different drugs, as well as the mechanisms for their delivery. The description of these interactions at a molecular level requires the use of highly sensitive non-invasive methods. This work presents the experimental results demonstrating the capability of second harmonic generation microscopy to study electrochemical processes affecting the structure of a narrow (<1 nm) hydration layer of model cell membranes. On the example of bioactive potassium and calcium ions of different concentrations, a unique sensitivity of this method to a change in the surface potential of membranes has been demonstrated. An increase in the ionic strength of the solution above 15 mM in the case of calcium ions leads to a complete neutralization of the membrane surface, while potassium ions only partially screen the surface charge of a membrane. It has been shown that a change in the structure and composition of membranes leads to the emergence of steric effects in the interaction of head groups of lipids with water molecules in the hydration layer.