Thermodynamics of monolayers formed by mixtures of phosphatidylcholine/phosphatidylserine |
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Authors: | Luna Carlos Stroka Kimberly M Bermudez Harry Aranda-Espinoza Helim |
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Affiliation: | a Fischell Department of Bioengineering, University of Maryland at College Park, College Park, MD 20740, United States;b Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, MA 01003, United States |
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Abstract: | In this work we obtain the thermodynamic properties of mixed (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) PC and (1-stearoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (sodium salt)) PS monolayers. Measurements of compressibility (isotherms, bulk modulus, and excess area per molecule) and surface potential show that the properties of monolayers at the air-water interface depend on the concentration of ions (Na(+) and K(+)) and the proportion of PS in the mixture. The dependence on PS arises because the molecule is originally bound to a Na(+) counterion; by increasing the concentration of ions the entropy changes, creating a favorable system for the bound counterions of PS to join the bulk, leaving a negatively charged molecule. This change leads to an increase in electrostatic repulsions which is reflected by the increase in area per molecule versus surface pressure and a higher surface potential. The results lead to the conclusion that this mixture of phospholipids follows a non ideal behavior and can help to understand the thermodynamic behavior of membranes made of binary mixtures of a zwitterionic and an anionic phospholipid with a bound counterion. |
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Keywords: | Phosphatidylserine Phosphatidylcholine Monolayer Isotherms Counterions Non-ideality |
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