Monitoring phases and phase transitions in phosphatidylethanolamine monolayers using active interfacial microrheology

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Issue Date
2015Author
Ghazvini, Saba
Ricke, Brandon
Zasadzinski, Joseph A.
Dhar, Prajnaparamita
Publisher
Royal Society of Chemistry
Type
Article
Article Version
Scholarly/refereed, publisher version
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Active interfacial microrheology is a sensitive tool to detect phase transitions and headgroup order in phospholipid monolayers. The re-orientation of a magnetic nickel nanorod is used to explore changes in the surface rheology of 1,2-dilauroyl-sn-glycero-3-phosphoethanolamine (DLPE) and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE), which differ by two CH2 groups in their alkyl chains. Phosphatidylethanolamines such as DLPE and DMPE are a major component of cell membranes in bacteria and in the nervous system. At room temperature, DLPE has a liquid expanded (LE) phase for surface pressure, Π < ∼38 mN m−1; DMPE has an LE phase for Π < ∼7 mN m−1. In their respective LE phases, DLPE and DMPE show no measurable change in surface viscosity with Π, consistent with a surface viscosity <10−9 N s m−1, the resolution of our technique. However, there is a measurable, discontinuous change in the surface viscosity at the LE to liquid condensed (LC) transition for both DLPE and DMPE. This discontinuous change is correlated with a significant increase in the surface compressibility modulus (or isothermal two-dimensional bulk modulus). In the LC phase of DMPE there is an exponential increase in surface viscosity with Π consistent with a two-dimensional free area model. The second-order LC to solid (S) transition in DMPE is marked by an abrupt onset of surface elasticity; there is no measurable elasticity in the LC phase. A measurable surface elasticity in the S phase suggests a change in the molecular ordering or interactions of the DMPE headgroups that is not reflected in isotherms or in grazing incidence X-ray diffraction. This onset of measurable elasticity is also seen in DLPE, even though no indication of a LC–S transition is visible in the isotherms.
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This is the published version. Copyright 2015 Royal Society of Chemistry
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Citation
Ghazvini, Saba, Brandon Ricke, Joseph A. Zasadzinski, and Prajnaparamita Dhar. "Monitoring Phases and Phase Transitions in Phosphatidylethanolamine Monolayers Using Active Interfacial Microrheology." Soft Matter 11.17 (2015): 3313-321. http://dx.doi.org/10.1039/C4SM02900C
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