Biology Reference
In-Depth Information
interior, where it then runs parallel to the sn-1 chain (
Figure 10.25
)
[76]
. As a result the sn-2
chain behaves as if it is shorter than the sn-1 chain. In the gel state the two chain terminal
methyls are out of register by about 1.8
˚
or 1.5 carbon bond lengths. In biological membranes
this is compensated for with the sn-2 chain being about 2 carbons longer than the sn-1 chain,
thus preventing interdigitation.
Methods to Detect Interdigitation
Since diffraction methods can directly measure bilayer thickness, they are the gold stan-
dard for monitoring interdigitation. As discussed in Chapter 9, the electron density profiles
of all membranes look very similar by X-ray diffraction. They consist of a low density bilayer
interior flanked by two high density regions characteristic of the polar head groups (see
Figure 9.5). With interdigitation there is a loss of the deep depression in electron density
that is characteristic of free motion associated with terminal methyls in noninterdigitated
bilayers. Therefore interdigitated electron density profiles are more shallow than those asso-
ciated with noninterdigitated bilayers. Also, interdigitated bilayers are thinner than noninter-
digitated bilayers.
Many other membrane biophysical methodologies have been used to indirectly monitor
interdigitation. For example, freeze fracture EM images of noninterdigitated bilayers show
a smooth surface resulting from fractionation running down the uninterrupted bilayer mid-
plane. In sharp contrast are images obtained from mixed interdigitated gel phase bilayers
indicating discontinuous fracture patterns where the fracture planes are interrupted by up
and down steps. Another approach measures order parameters using ESR (electron spin
resonance)
[77]
where spin probes, particularly DOXYL-stearic acids, are employed. This
homologous series of probes has an unpaired electron (free radical) stabilized in the DOXYL
structure and attached at different locations down the stearic acyl chain.
Figure 10.26
shows
the structure of 5-DOXYL stearic acid.
The measured order parameters decrease from near the head group down the chain to
near the highly disordered chain terminus. In one experiment order parameters at the
ordered 5-position were compared to order parameters at the highly disordered 16-position.
In noninterdigitated bilayers the order parameters for the two positions were distinct, with
the 5-position being considerably higher than the 16-position. However, upon interdigitation,
the 16- and 5-positions reside near the same location and so have similar order parameters.
Above T
m
both probes exhibit fluid phase isotropic motion.
Interdigitation Caused by Perturbations
There are many types of perturbations that can drive noninterdigitated bilayers into a fully
interdigitated gel state. The most studied of these perturbants are short-chain alcohols
CH
3
(CH
2
)
11
CH
2
OH
N-O
O
O
CH
3
CH
3
FIGURE 10.26
5-DOXYL Stearic acid.
