The
extensive collection of lipids found in cell membranes is justified
by the fact that each lipid contributes to their overall structure,
dynamics, and properties and so to the biological processes taking
place within them. It also showcases that, in order to deepen our
understanding of membranes, we need to have a tool to differentiate
lipid bilayers of varying composition. In this work, we investigate
a suite of single-component saturated glycerophospholipids varying
only in their headgroup structure by analyzing the second harmonic
generation (SHG) nonlinear optical (NLO) response of a probe, di-8-ANEPPS,
embedded into the membranes. The seven hydrophilic heads chosen (phosphatidylcholine
(PC), phosphatidylethanolamine (PE), diaglycerol (GL), phosphatidylserine
(PS), phosphatidylglycerol (PG), phosphatidylinositol (PI), and phosphatidyc
acid (PA)) represent all the major headgroups that are part of mammalian
plasma membranes and provide an assortment of neutral, zwiterrionic,
and charged species. First, molecular dynamics simulations revealed
that the lipidic arrangement is strongly sensitive to the nature of
the hydrophilic head and less to the variety in the hydrophobic region.
Membranes exhibiting drastically opposite structural properties can
be pointed out: 1,2-dihexadecanoyl-rac-glycerol (DPGL) is the thickest
and most ordered and aligned system, whereas 1,2-diacyl-sn-glycero-3-phospho-(1′-sn-glycerol)
(DPPG) is thinnest and least ordered and aligned system. The structural
analyses are then confronted with the molecular NLO responses, β,
computed at the time-dependent density functional theory (TDDFT) level.
As the orientation of the chromophore is impacted by the various degrees
of order within the lipid bilayers, the diagonal component of the
β tensor parallel to the bilayer normal, βZZZ, is as well. In the end, this computational approach provides insights
into the link between lipid building blocks and the NLO responses
of the embedded dye.