Chemistry Reference
In-Depth Information
A similar effect is invoked for amphiphilic membranes in that they will have a
rest state, or curvature in this case, that is determined by the geometric packing
requirements of the constituent molecules that will try to maintain the minimum-
energy state. If the membrane is distorted, the packing energy of constituents will
try to force the membranes back into its minimum-energy state. Like the spring, if
the restoring capacity of the membrane structure is exceeded, or if a component
modification changes the net energy of the system, irreversible membrane rupture
may occur. In biological systems, such rupture may be involved in such vital pro-
cesses as cell division and the entry and expulsion of specific components such as
ions, hormones, and enzymes. It may also be involved in unwanted activities such
as the penetration of viruses or toxins into target cells.
Interfaces formed by amphilic molecules can take on three configurations:
(1) they can be curved toward the hydrophobic region of the molecule, a conforma-
tion usually referred to as ''type 1 curvature''; (2) they can curve toward the polar
or hydrophilic region, the ''type 2 curvature''; (3) or they can be essentially planar.
The three basic categories are illustrated in Figure 5.2.
The effect of molecular geometry on the probable aggregation structures of
amphiphiles was introduced in Chapter 4. As indicated, type 1 curvature is favored
by amphiphiles for which the head group, S, is relatively more bulky than the aver-
age cross-sectional area of the tail, R, such as single-chained charged detergents.
Type 2 is favored by molecules with bulky hydrophobic chain regions, such as
double-chain surfactants or single-chain materials in high salt solution. Planar, or
approximately so, assemblies are found for particular situations in which the critical
packing parameter for the system is essentially unity. Most biological lipids, for
example, must be delicately balanced in their hydrophobic-hydrophilic geometries,
so that their phase diagrams at biologically relevant physical conditions favor
almost flat or planar lamellar mesophases, since cell membranes require such struc-
tures for cell integrity and function. In a real system, however, the concept of a
well-defined defect-free, planar lamellar state is an idealization, and such systems
are usually delicately balanced and ready for transformation from one curved type
to the other as required by the proper functioning of the system.
Type 1 Type 2 Type 3
Figure 5.2. The three ''senses'' of curvature of amphiphilic interfaces and membranes.
