Investigation of the Second-Order Nonlinear Optical Responses of an Embedded di-8-ANEPPS Probe: Influence of the Lipid Polar Headgroup

C. Bouquiaux1, F. Castet2, B. Champagne1

1Theoretical Chemistry Lab, Unit of Theoretical and Structural Physical Chemistry, Namur Institute of Structured Matter, University of Namur, rue de Bruxelles, 61, B-5000 Namur (Belgium)
2Institut des Sciences Moléculaires, UMR 5255 CNRS, University of Bordeaux, cours de la Libération 351, F-33405 Talence Cedex (France)

Biological membranes are thin selectively permeable amphiphilic films that play a key and active role in the life of the cell. Those different functions are carried out by different lipids, explaining why eukaryotic cells invest substantial resources in generating thousands of different lipids. The huge diversity of lipids is due to the combination of various building blocks: (i) the fatty acids that vary in length, in the number and position of the unsaturation(s), and (ii) the diverse headgroup structures available that vary in size, polarity, charge, H-bond-donating, and -accepting capacities, which strongly impact interfacial structure and dynamics. Given all the possible combinations of headgroups and hydrocarbon chains, several thousand structurally distinct lipid molecules could theoretically exist, each potentially contributing to the properties of cellular membranes and the biological processes taking place within them. Therefore, having a tool to distinguish between membranes of various composition is helpful in the deeper understanding of lipid bilayers. In this work we investigate a collection of glycerophospholipid saturated bilayers varying only in their head group structure (namely PC, PE, GL, PS, PG, PI, and PA, see Figure 1 for their definition and structure) via Molecular Dynamics simulations. The effect changing the headgroup structure on the properties of the bilayer are studied, namely the thickness, the area per lipid, the hydrocarbon parameter, the orientation of the diverse molecules within the membranes, and also the hydrogen bonds network. To complement the study, we also analyse 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. The structural analyses are then confronted to the molecular NLO response, β , computed at the TDDFT/M06-2X/6-311+G* level, and in particular the contribution to β parallel to the bilayer normal,βZZZ. This computational approach provides insights onto the link between the headgroup structure and the diagonal component βZZZ of the first hyperpolarizability and so a first approach towards unravelling the changes due to the variation of one of the building blocks of lipid bilayers.