Page 102 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Membrane Structure 363

parameters is distinctly smaller in the unsaturated system. in the sn-2 chain than in lipids with only one double
This demonstrates that the presence of a cis-double bond bond.
causes a more disordered conformation of the hydrocar-
bon chains. Considering the relative flexibility within the
C. Effect of Cholesterol on the Order and
palmitic acyl chain, the deuterium resonance data indi-
Motion of the Lipid Hydrocarbon Chains
cate a local stiffening of those segments which are located
in the vicinity of the rigid cis-double bond. An increase The influence of cholesterol on the order and mobility
in temperature leads to a further decrease of the order of lipid bilayers was investigated with both selectively
parameters. deuterated lipids and deuterated cholesterol. Addition of
2
The H-NMR spectrum of POPC membranes deuter- 50% cholesterol to DPPC and DMPC bilayers was shown
ated at the C-9, and C-10 positions of the oleic acyl to lead to an almost twofold increase of the quadrupole
chain shows two quadrupolar splittings, the larger cor- splitting of the labeled fatty acyl chain segment compared
2
responding to the C-9 deuteron and the smaller to the to that of a cholesterol-free bilayer. When [3- H] choles-
C-10 deuteron (Fig. 6). The presence of two quadroupole terol was added to a nondeuterated DPPC bilayer, again a
splittings at the same rigid segments is caused by a tilting large quadrupole splitting of the cholesterol probe was ob-
of the cis-double bond with respect to the bilayer normal served. Both probes lead to the conclusion that a high con-
2
which produces different orientations for the C– H vec- centration of cholesterol induces an essentially all trans-
tors of the 9- and 10-carbon atoms. The angle between the conformation in those hydrocarbon chain segments which
bilayer normal and the C C bond vector was found to be are in contact with the rigid steroid frame. This condens-
7to8 . The order parameter of 1-palmitoyl-2-elaidoyl-sn- ing effect of cholesterol was also observed in monolayer-
◦
glycero-3-choline deuterated at the C-9 and C-10 trans- and neutron-diffraction experiments.
double bond of the elaidic acid chain is also included in The effect of cholesterol on the order parameter pro-
Fig. 6. Due to the symmetry of the trans-double bond the file of individual fatty acyl chains in DPPC bilayers was
two C D vectors at the C-9 and C-10 position make the simulated by means of molecular dynamics calculations
same angle with the C C vector axis. They give rise to as displayed in Fig. 7. A distinct plateau with an order
the same quadrupole splitting, and the evaluation of the parameter of the C D bond vector of S CD =−0.4 was de-
order parameter of the C C axis is straightforward. Tak- tected. This means that the hydrocarbon chains are almost
ing into account the different geometries, the molecular fully extended and that the order parameter of the long
ordering and the angular fluctuations of the cis- and trans- molecular axis, S mol =−2S CD , approaches its maximum
double bonds are identical. In addition, there are no quan- value of S mol = 1.
titative differences between sn-1 and sn-2 chain segments In highly unsaturated lipid mixtures, typical for nerve
at this position in the bilayer. The segmental fluctuations and retinal membranes, cholesterol induces an increase
around the bilayer normal thus only depend on the distance in the order of both saturated and polyunsaturated
from the lipid–water interface but not on the specificseg- hydrocarbon chains. However, the increase in order is
ment geometry. about a factor of 2 smaller for polyunsaturated than for
The effect of cis-unsaturation was also investigated for monounsaturated lipids.
the glycosphingolipid N-(oleoyl-d33)galactosylceramide
incorporated at low concentration into liquid crys-
talline liposomes composed of 1,2-dimyristocyl-3-sn-
phosphatidylcholine (DMPC) and POPC using the
perdeuterated oleoyl chain as the reporter element. The
primary effect of cis-9,-10 unsaturation in glycosphin-
golipids proved to be similar to that of cis-unsaturation in
glycerolipids. It was further shown that the overall dynam-
ics of N-(oleoyl)galactosylceramide in fluid phospholipid
membranes was very similar to that of glycerolipids with
comparable acyl chains.
Increasing sn-2 unsaturation from one to six double
bonds in PC leads to an inhomogeneous disordering along
the neighboring perdeuterated sn-1 chain. As a conse-
FIGURE 7 Effect of cholesterol. Order parameter of the sn-2
quence, the effect of a temperature increase leading to
chain in DPPC bilayers without ( ) and with ( ) 50 mol% choles-
a decrease in the average chain length is somewhat less terol as function of carbon atom. [From Smondyrev and Berkowitz
pronounced in lipids with three or more double bonds (1999). Biophys. J. 77, 2075.]