Food Lipids: Chemistry, Nutrition, and Biotechnology

of electrostatic interactions in the process of ion–membrane binding. Electrostatic
forces also play a strong role in lipid–anion binding with affinity for anions by PC,
following the sequence > I � � SCN � > � Br � >Cl � � � 2�
ClO4 NO3 > SO4
. Here anion
size also has an important role in the process of binding, partly as a result of the
transfer of the local excess charges from the anion to the phospholipid head groups
and vice versa. However, strength of anion binding to phospholipid membranes decreases
with increasing net negative charge density of the membrane [58].
Results of NMR, infrared spectroscopy, and neutron diffraction studies strongly
imply that the inorganic cations interact predominantly with the phosphodiester
groups of the phospholipid head groups [63–67]. On the other hand, inorganic anions
may interact specifically with the trimethylammonium residues of the PC head groups
[68,69].
Temperature may influence binding of ions to phospholipids. Under conditions
of phase transitions, phospholipid chain melting results in a lateral expansion of the
lipid bilayers, which for charged systems is also associated with the decrease in the
net surface charge density. In the case of negatively charged membranes, this transition
leads to lowering of the interfacial proton concentration and decreases the
apparent pK value of the anion phospholipids [70].
B. Protein
Complexes of PC with soy protein have been demonstrated by Kanamoto et al. [71].
In this study, using a linear sucrose density gradient centrifugation analysis, 14 C-PC
was found to be nonspecifically bound to either the 7S or 11S proteins.
C. Iodine
In the presence of phospholipid micelles, iodine changes color in aqueous solution.
It does not undergo color change in the presence of unassociated molecular species.
The color change coincides with the cmc of the substance and is due to formation
�
of the triiodide ion, I 3 [72]. Based on data from laser Raman studies, the reaction
appeared to be related to iodine–phospholipid interaction, as well as to penetration
of iodine into the bilayer membrane, rather than to an ion transport process.
XI. OXIDATION
Unsaturated fatty acids of phospholipids are susceptible to oxidation through both
enzymatically controlled processes and random autoxidation processes. The mechanism
of autoxidation is basically similar to the oxidative mechanism of fatty acids
or esters in the bulk phase or in inert organic solvents. This mechanism is characterized
by three main phases: initiation, propagation, and termination. Initiation occurs
as hydrogen is abstracted from an unsaturated fatty acid of a phospholipid,
resulting in a lipid free radical. The lipid free radical in turn reacts with molecular
oxygen to form a lipid peroxyl radical. While irradiation can directly abstract hydrogen
from phospholipids, initiation is frequently attributed to reaction of the fatty
acids with active oxygen species, such as the hydroxyl free radical and the protonated
form of superoxide. These active oxygen species are produced when a metal ion,
particularly iron, interacts with triplet oxygen, hydrogen peroxide, and superoxide
anion. On the other hand, enzymatic abstraction of hydrogen from an unsaturated
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.