Chemistry Reference
In-Depth Information
For comparison, included in Table 14.3 are the k
q
values obtained in detergent micelles along
with
k
q
values obtained in homogeneous solvent benzene. As can be seen, the second-order rate con-
stant for
1
O
2
quenching in a liposomal environment is a factor of ~4 lower for
-CAR compared to
the second-order rate constant obtained in the aromatic solvent. While, there is a marked ~80-130
fold difference between the
k
q
values determined in liposomal environments compared to the
k
q
values determined in the aromatic solvent for the XANs.
The present results for
β
β
-CAR incorporated into DPPC vesicles compare favorably with those of
β
-CAR in detergent micelles, this is to be expected because carotene molecules reside in the hydro-
phobic core of the micelle and likewise they reside in the hydrophobic region of the phospholipid
bilayer of liposomes (between the two lipid layers) away from the water interface as depicted in
Figure 14.6 (taken from Burke (2001)). Although the two types of vesicles have somewhat different
structures,
1
O
2
penetration into each type of vesicles is required before
-CAR is able to quench
1
O
2
.
It is known that nonpolar carotenoids, in particular the carotenes, decrease the penetration barrier
for small molecules to the membrane headgroup region of phospholipid vesicles. Most probably,
due to the additional space in the headgroup region, resulting from the pigment-lipid interaction
in the hydrophobic region of the phospholipid bilayer, there is a greater permeability in the head
group region, which aids
1
O
2
diffusion throughout the entire lipid bilayer, by acting as a portal of
entry for
1
O
2
.
The second-order quenching rate constants for the two XANs in DPPC liposomes are quite
different from those reported in micelles. In micelles, where XANs are accommodated in a similar
manner to carotenes, very little variation in the second-order quenching rate constant is observed
(see Table 14.3), but in contrast, a ~26-fold difference in reactivity is observed between the XANs,
LUT, ZEA, and
β
-CAR in a liposomal environment. There are two possible explanations for this;
polar carotenoids such as ZEA and LUT incorporated into liposome bilayers have been shown to
limit molecular oxygen penetration within the lipid bilayer as demonstrated by the pigment-related
decrease of oxygen diffusion-concentration product (Subczynski et al. 1992). Due to their trans-
membrane orientation with both polar end groups anchored at the inner and the outer lipid-water
interface respectively (Gruszecki and Sielewiesiuk 1990), they act as “molecular rivets” rigidify-
ing the lipid membrane by restricting many molecular motions of individual lipid molecules. This
type of interaction reinforces the lipid bilayer and thus restricts the diffusion of small molecules
β
“
Outer” water-lipid interface
HO
HO
A
C
B
HO
OH
OH
OH
“
Inner” water-lipid interface
FIGURE 14.6
Typical orientations of carotenoids within a lipid bilayer (A denotes β-CAR, B LUT, and C
ZEA).
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