Food Lipids: Chemistry, Nutrition, and Biotechnology

such as PI or phosphatidylserine (PS), the phospholipid imbibed water without limit
[44,45].
Hydration of a phospholipid appears to be cooperative. A water molecule that
initiated hydration of a site facilitated access of additional water molecules, until the
hydration of the whole site composed of many different interacting polar residues
was completed [46]. Incorporation of the first three to four water molecules on each
phospholipid occurs on the phosphate of the lipid head group and is exothermic [47].
The remaining water molecules are incorporated endothermically.
Neutron diffraction experiments on multilayers containing PC [48,49], PE [50],
and PI [51] have revealed that water distributions are centered between adjacent
bilayers and overlap the head group peaks in the neutron scattering profile of the
bilayer. These results imply that water penetrates into the bilayer head group region,
but appreciable quantities of water do not reach the hydrocarbon core. By combining
X-ray diffraction and dilatometry data, McIntosh and Simon [52,53] were able to
calculate the number of water molecules in the interbilayer space and in the head
group region for dilauroyl-PE bilayers. They found that there are about 7 and 10
water molecules in the gel and liquid crystalline phases, respectively, with about
half of these water molecules located between adjacent bilayers and the other half
in the head group region.
The amount of water taken up by a given phospholipid depends on interactions
between the lipid molecules, including interbilayer forces (those perpendicular to the
plane of the bilayer) and intrabilayer forces (those in the plane of the bilayer). For
interbilayer forces, at least four repulsive interactions have been shown to operate
between bilayer surfaces. These are the electrostatic, undulation, hydration (solvation),
and steric pressures. Attractive pressures include the relatively long-range van
der Waals pressure and short-range bonds between the molecules in apposing bilayers,
such as hydrogen bonds or bridges formed by divalent salts. Several of the same
repulsive and attractive interactions act in the plane of the bilayer, including electrostatic
repulsion, hydration repulsion, steric repulsion, and van der Waals attraction.
In addition, interfacial tension plays an important role in determining the area per
lipid molecule [54]. Thus, as the area per molecule increases, more water can be
incorporated into the head group region of the bilayer. Such a situation is found with
bilayers having an interdigitated gel phase compared with the normal gel phase and
with bilayers having unsaturated fatty acids in the phospholipid compared with saturated
fatty acids [55,56].
The presence of monovalent and/or divalent cations in the fluid phase changes
the hydration properties of the phospolipids. For example, the partial fluid thickness
between dipalmitoyl PC bilayers increased from about 20 A˚ in water to more than
90 A˚ in 1 mM CaCl 2 [57]. In contrast, monovalent cations, such as Na � ,K � ,orCs � ,
decrease the fluid spaces between adjacent charged PS or PG bilayers as a result of
screening of the charge [58,59]. In addition, divalent cations have a dehydrating
effect on PS. The most extensively studied divalent cation, Ca 2� , binds to the phosphate
group of PS [60], liberates water between bilayers and from the lipid polar
groups [61], crystallizes the lipid hydrocarbon chains [59,60], and raises the gel to
the liquid crystalline melting temperature of dipalmitoyl PS by more than 100�C
[59]. In response to these changes, one would expect permeability of the membrane
to be altered.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.