Second-Order Nonlinear Optical Responses of Di-8-ANEPPS Embedded in DPPC and DOPC Lipid Bilayers: Effect of the Cholesterol Content

Charlotte Bouquiaux and Benoît Champagne

University of Namur, Theoretical Chemistry Lab, Unit of Theoretical and Structural Physical Chemistry, Namur Institute of Structured Matter, rue de Bruxelles, 61, B-5000 Namur (Belgium)

It is well known that the cholesterol has a strong influence on maintaining the correct fluidity and rigidity of the animals cell membranes, and so their functions. Therefore, having a tool to distinguish between membranes of various cholesterol content is helpful in the deeper understanding of lipid bilayers. In this work, we investigate two glycerophospholipid phosphatidylcholine (PC) lipids, namely dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC) with different levels of cholesterol by analysing the second-harmonic generation (SHG) nonlinear optical (NLO) response of a probe molecule, di-8-ANEPPS inserted into the membranes. This technique as the advantage to be specific to interfacial region, like lipid bilayer, and used in conjunction with an ANEPP-like molecule, allows us rapid acquisition at relatively low laser power. Six independent 400ns Molecular Dynamics simulations were performed on both lipids by varying the cholesterol mole fraction (from 0 to 0.66), giving twelve systems total. The effect of the cholesterol on the properties of the bilayer are studied, namely the thickness, the area per lipid, the hydrocarbon parameter and also the orientation of the diverse molecules within the membranes. All the analysis of the structural parameters of the bilayers studied converge toward one conclusion: as the mole fraction of cholesterol increases, the systems are more and more rigid. This is known as the condensing effect of cholesterol. The structural analyses are then confronted to the molecular NLO response, βzzz, computed at the TDDFT/M06-2X/6-311+G* level, and in particular the contribution to beta parallel to the bilayer normal, βzzz. This computational approach provides insights onto the link between the cholesterol content and the diagonal component βzzz of the first hyperpolarizability and so a first approach towards the unravelling of the changes due to the cholesterol content of lipid bilayers.