title: Monitoring phases and phase transitions in phosphatidylethanolamine monolayers using active interfacial microrheology.
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.
authors.affiliation: Department of Chemical Engineering, University of Kansas, Lawrence, KS 66045, USA. prajnadhar@ku.edu.
authors.forename: Saba
authors.initials: S
authors.lastname: Ghazvini
authors.forename: Brandon
authors.initials: B
authors.lastname: Ricke
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Zasadzinski
authors.forename: Prajnaparamita
authors.initials: P
authors.lastname: Dhar
mesh_terms: D004548:Elasticity; D044085:Microfluidics; D043942:Nanotubes / Q000737:chemistry; D044367:Phase Transition; D010714:Phosphatidylethanolamines / Q000737:chemistry*; D044366:Transition Temperature; D014783:Viscosity
chemical_list: D010714:Phosphatidylethanolamines; C483858:phosphatidylethanolamine