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Issue 17, 2015

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

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Abstract

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.

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

Supplementary files

Article information


Submitted
30 Dec 2014
Accepted
02 Mar 2015
First published
16 Mar 2015

Soft Matter, 2015,11, 3313-3321
Article type
Paper
Author version available

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

S. Ghazvini, B. Ricke, J. A. Zasadzinski and P. Dhar, Soft Matter, 2015, 11, 3313 DOI: 10.1039/C4SM02900C

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