Thermal response of Langmuir-Blodgett films of dipalmitoylphophatidylcholine(DPPC) studied by Atomic Force Microscopy and Force Spectroscopy
Gerard Oncins 1, Laura Picas 1, Jordi Hernandez-Borrell 1, Sergi Garcia-Manyes 1 and Fausto Sanz 1*
1 Universitat de Barcelona
* To whom correspondence should be addressed. E-mail: fsanz{at}ub.edu.
Submitted on April 26, 2007
Revised on May 25, 2007
Accepted on 15 June 2007
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Abstract |
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The topographic evolution of supported DPPC monolayers with temperature has been followed by AFM in liquid environment, revealing the presence of only one phase transition event around 46°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°C (liquid expanded phase), which represents a two orders of magnitude decrease respect 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° angles between them, thus suggesting that the cooling process from the liquid expanded to the liquid condensed phase follows a nucleation and growth mechanism.
Key Words:
AFM, Membrane, force spectroscopy, phase transition., phospholipid
