title: Thermal response of Langmuir-Blodgett films of dipalmitoylphosphatidylcholine studied by atomic force microscopy and force spectroscopy.
abstract: The topographic evolution of supported dipalmitoylphosphatidylcholine (DPPC) monolayers with temperature has been followed by atomic force microscopy in liquid environment, revealing the presence of only one phase transition event at approximately 46 degrees C. This finding is a direct experimental proof that the two phase transitions observed in the corresponding bilayers correspond to the individual phase transition of the two leaflets composing the bilayer. The transition temperature and its dependency on the measuring medium (liquid saline solution or air) is discussed in terms of changes in van der Waals, hydration, and hydrophobic/hydrophilic interactions, and it is directly compared with the transition temperatures observed in the related bilayers under the same experimental conditions. Force spectroscopy allows us to probe the nanomechanical properties of such monolayers as a function of temperature. These measurements show that the force needed to puncture the monolayers is highly dependent on the temperature and on the phospholipid phase, ranging from 120+/-4 pN at room temperature (liquid condensed phase) to 49+/-2 pN at 65 degrees C (liquid expanded phase), which represents a two orders-of-magnitude decrease respective to the forces needed to puncture DPPC bilayers. The topographic study of the monolayers in air around the transition temperature revealed the presence of boundary domains in the monolayer surface forming 120 degrees angles between them, thus suggesting that the cooling process from the liquid-expanded to the liquid-condensed phase follows a nucleation and growth mechanism.
authors.affiliation: Department of Physical Chemistry, Chemistry Faculty, University of Barcelona and Institut de Bioenginyeria de Catalunya, Barcelona, Spain.
authors.forename: Gerard
authors.initials: G
authors.lastname: Oncins
authors.forename: Laura
authors.initials: L
authors.lastname: Picas
authors.forename: Jordi
authors.initials: J
authors.lastname: Hernández-Borrell
authors.forename: Sergi
authors.initials: S
authors.lastname: Garcia-Manyes
authors.forename: Fausto
authors.initials: F
authors.lastname: Sanz
mesh_terms: D003198:Computer Simulation; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry*; D004548:Elasticity; D008051:Lipid Bilayers / Q000737:chemistry*; D008560:Membrane Fluidity; D018625:Microscopy, Atomic Force / Q000379:methods*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D013314:Stress, Mechanical; D013696:Temperature
chemical_list: D008051:Lipid Bilayers; D004134:Dimyristoylphosphatidylcholine