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16:0,10:0 PC
Δ
C
CL
(a)
(b)
(c)
fully
interdigitated
mixed
interdigitated
partially
interdigitated
FIGURE 10.24 Lipid interdigitation. Top: An asymmetric phospholipid, 16:0,10:0 PC. Bottom: Three lipid
interdigitated states. (a) partially interdigitated; (b) mixed interdigitated and; (c) fully interdigitated.
Where CL is the length of the longer chain and
D
C is the absolute difference in the chain
lengths (see Figure 10.24 ) and is given by:
2 þ 1 : 5
where n 1 and n 2 are the number of carbons in the sn-1 and sn-2 acyl chains, respectively. The
factor
D
C
¼
n
1
n
1.5 is added to account for the sn-1 chain extending into the bilayer interior ~1.5
carbons deeper than the sn-2 chain (discussed below and shown in Figure 10.25 ). The chain
inequivalence parameter therefore represents the magnitude of the chain length inequiva-
lence normalized by the length of the hydrocarbon region of the bilayer.
For a family of phosphatidylcholines, C18,CX PC where the sn-1 chain is 18-carbons and
the sn-2 chain, X, varies from 18- to 0-carbons. C18,C18 PC is fully non-interdigitated while
C18,C0 PC (a lyso-PC) is fully interdigitated. Other interdigitated states are found for X
between 18- and 0-carbons. The chain inequivalence parameter for C18,C10 PC is close to
0.5, characteristic of the mixed interdigitated state ( Figure 10.24 , B). Mixed interdigitation
requires one chain to be about half the length of the other. Upon undergoing a thermotropic
phase transition, the mixed interdigitated gel phase bilayers transform into partially
þ
 
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