Biology Reference
In-Depth Information
Micelles and Biological Membranes in 1973 and The Hydrophobic Effect in 1980
[10,11]
. The
Hydrophobic Effect is attributed to intra-molecular H-bonding between water. Charles Tan-
ford is also the author of Ben Franklin Stilled the Waves, an interesting account of the early
history of membrane studies
[12]
.
H-bonds are the result of sharing a hydrogen between two electronegative atoms (e.g. F,
Br, O, N, S etc.). H-bonds cannot form between water and a hydrocarbon as C and H have
nearly equal affinity for electrons. If the hydrocarbon hexane is placed in a box of water in
the absence of gravity, hexane will form a perfect sphere indicating minimal contact
surface area with water. Entropy, a measurement of the degree of randomness, would
predict that without any other forces, hexane by itself should disperse, decreasing its
order. However, observation clearly shows that in water hexane prefers to exist in what
appears to be an unfavorable, highly organized spherical structure. Remove water from
theboxandhexanewoulddisperse.Thereforethecohesiveforcesthatkeepwater
together (H-bond forces) are much stronger than the hexane entropic forces that would
disperse the sphere. Very weak van der Waals forces that would keep the hexane mole-
cules together are also insignificant when compared to the cohesive forces of water.
Hexane molecules stay together in water because it is essentially more favorable for the
water molecules to hydrogen bond to each other than to engage in very weak interactions
with non-polar hexane molecules. The bulk waters are surrounded by, and H-bonded to,
4 other waters, while the waters at the hexane interface are surrounded by, and H-bonded
to, only 2 other waters (
Figure 3.5
). The reduced number of H-bonds available to waters at
the hexane interface makes them energetically less favorable. As a result, it takes work to
drive a hydrocarbon (fatty acyl) chain into water, creating a new, unfavorable interfacial
surface.
The Hydrophobic Effect is the major driving force for all biological structure, including
that of membranes. The hydrocarbon tails of membrane phospholipids segregate away
from water, making the membrane interior dense with hydrocarbon chains, but devoid of
water. Energetics of membrane formation is similar to the previously discussed case of
a hexane sphere separating from water.
Figure 3.9
depicts the 5 major forces contributing to membrane bilayer stability in water:
the Hydrophobic Effect; head group
head group interac-
tions; entropy of the caged tails; and van der Waals forces. The hydrophobic effect is by far
the most important in membrane stabilization. In contrast to the hexane sphere discussed
above, membranes are composed of amphipathic lipids that also have a polar, often charged
head group that can favorably interact with water, further stabilizing the membrane. Addi-
tional stabilizing forces can come from ionic interactions between the head groups of adjacent
phospholipids. Another force driving membrane formation is related to the motion (entropy)
of the bilayer acyl chains. The hydrocarbon chains of phospholipids exposed to water would
be totally surrounded by a stiff water cage high surface tension where their motion would be
highly restricted (low entropy, unfavorable). Upon shedding the water cage, the acyl chains
are segregated into the hydrophobic bilayer interior, resulting in a large increase in favorable
acyl chain motion (increase in entropy).
Although the Hydrophobic Effect is by far the major force stabilizing membranes, the
membrane interior is also held together very weakly by van der Waals forces. Van der Waals
forces are the result of a collection of very close molecular and surface interactions discovered
water interactions; head group
e
e
