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; DMPE has an LE phase for Π < ~ 7 mN/m. In their respective LE phases, DLPE and DMPE show no measurable change in surface viscosity with Π, consistent with a surface viscosity < 10−9 Ns/m, 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.

Interfacial rheology of coexisting solid and fluid monolayers

“…Over this surface pressure range, the continuous matrix is in the LE phase. There are few literature values of the surface viscosity of LE phases; for dimyristoylphosphatidylethanolamine, our recent measurements using a different rheometer suggests that the LE viscosity is < 5 × 10- 2 μPa-s-m (67). Measurements of domain fluctuations for LE phases of DPPC, cholesterol and diphytanoylphosphatidylcholine give an estimated surface viscosity of the LE phase of ∼ 4 × 10- 3 μPa-s-m (32).…”

“…Measurements of domain fluctuations for LE phases of DPPC, cholesterol and diphytanoylphosphatidylcholine give an estimated surface viscosity of the LE phase of ∼ 4 × 10- 3 μPa-s-m (32). For such low values of LE surface viscosity, Bo ≪ 1 (11, 67) for 100 μm diameter microbuttons on a water subphase. As a result, the apparent surface viscosity of the continuous LE phase from 0 to ∼12 mN/m is dominated by the drag from the water subphase and, as such, is independent of the LE phase composition or surface pressure.…”

“…However, Figs. 1,3 show that the continuous matrix changes from the LE to the TC phase, which increases the continuous matrix viscosity by orders of magnitude depending on the dihydrocholesterol fraction (9, 12, 67). From GIXD, isotherms, and confocal imaging, we can approximate the continuous matrixas a binary mixture of DPPC and dihydrocholesterol.…”

Viscoelastic properties of ECM-rich embryonic microenvironments

“…Nanorods 3µm long and 300 nm in diameter were synthesized by electrochemical deposition of nickel into alumina templates as described previously (Paxton et al, 2004;Ghazvini et al, 2015;Dhar et al, 2010). The magnetized nickel nanorods were dispersed in a 90% isopropyl alcohol, 10% water solution.…”

Emergence of a linear slope region of the isotherm in the first-order liquid-expanded–liquid-condensed phase transition in Langmuir monolayers

“…The isothermal compressibility, defined as the fractional change of surface area per unit change in surface pressure, characterizes the isotherm shape in the LE-LC phase-coexistence region more definitely. It normally shows a peak [17,18] or a plateau [13,19,20] corresponding to a curved line or a straight line in the isotherm, respectively (Fig. 1).…”