Thermodynamics of membrane lipid hydration
H. Wennerstr�m and E. Sparr
Division of Physical Chemistry 1, Chemical Center,
P.O.Box 124, SE-22100 Lund, Sweden
Abstract: Zwitterionic or polar lipids that form lamellar phases
swell in the presence of water to take up between 10 and 30 water molecules
per lipid. The degree of the water uptake depends both on the state
of the alkyl chains, liquid or solid, and on the nature of the polar
group. The swelling behavior has been extensively characterized on the
free-energy level through measuring the relation between the chemical
potential of the water and the degree of swelling. In spite of the extensive
studies of this type, consensus is still lacking concerning the molecular
mechanism causing the swelling. The two main ideas that explain the
existence of the effectively repulsive force between two opposing bilayers
are (i) a water structure effect and (ii) thermal excitations of the
lipid molecules. The first is from a thermodynamic perspective caused
by a negative partial molar enthalpy of the water, whereas for the second,
the repulsion is caused by positive entropy.
A further insight into the swelling behavior is obtained by simultaneously
measuring the partial molar free energy and the partial molar enthalpy
using a twin double calorimeter. Such measurements show for binary lipidwater
and for ternary lipid cholesterol water systems that the
first four water molecules enter the bilayer driven by a favorable enthalpic
interaction, whereas for higher water contents, the partial free energy
and the partial enthalpy have opposite signs. In spite of the fact that
the partial free energy varies with water content in a similar way,
the partial enthalpies depend strongly on the nature of the lipid sample.
Thus, it is obvious that a molecular interpretation of the enthalpy
values has to involve the lipid degrees of freedom. This is a strong
indication that the interpretation of the free energies should also
include the same degrees of freedom. This gives experimental evidence
in favor of an interpretation of the molecular mechanism causing the
swelling that involves changes in the thermal excitation of the lipid
degrees of freedom.
*Plenary lecture presented at the 17th IUPAC Conference
on Chemical Thermodynamics (ICCT), Rostock, Germany, 28 July - 2 August
2002. Other presentations are published in this issue,
pp.859 -936.
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