Abstract
Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine, known as DPPC, at the air/water interface are extensively used as model systems of biomembranes and pulmonary surfactant. The properties of these monolayers have been mainly investigated by surface pressure–area isotherms coupled with different complementary techniques such as Brewster angle microscopy, for example. Several attempts using neutron reflectometry (NR) or ellipsometry have also appeared in the literature. Here, we report structural information obtained by using NR and ellipsometry on DPPC monolayers in the liquid condensed phase. On one side, NR can resolve the thickness of the aliphatic tails and the degree of hydration of the polar headgroups. On the other side, ellipsometry gives information on the refractive index and, therefore, on the physical state of the monolayer. The thickness and surface excess obtained by multiple-angle-of-incidence ellipsometry (MAIE) is compared with the results from NR measurements yielding a good agreement. Besides, a novel approach is reported to calculate the optical anisotropy of the DPPC monolayer that depends on the orientation of the aliphatic chains. The results from both NR and ellipsometry are also discussed in the context of the existing results for DPPC monolayers at the air/water interface. The differences observed are rationalized by the presence of buffer molecules interacting with phospholipids.
Highlights
Synthetic in vitro lipid mono- and bilayers, as well as uni- and multi-lamellar vesicles, can be considered as simple biomembrane models
Neutron reflectometry measurements were performed to study the structure of DPPC monolayers in the LC phase
We show how we can extract the surface excess and the optical anisotropy of the DPPC monolayer from further analysis of the results shown in Figure 4c and compare with the ones obtained by neutron reflectometry
Summary
Synthetic in vitro lipid mono- and bilayers, as well as uni- and multi-lamellar vesicles, can be considered as simple biomembrane models. For more than 20 years, 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) has been widely exploited to mimic plasma membranes or lung surfactant, mostly in the form of monolayers at the air/water interface. DPPC has being used alone or in combination with other lipids, to study the interaction of biomembrane with proteins [3,4], anticancer [5,6,7,8] and antifungal [9] compounds, and small molecules of biological relevance, such as cholesterol [10], hormones [11], and antibiotics [12]. Relatively low price, and stability at room temperature as well as when exposed to air, make DPPC a versatile model system for biomedical research
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