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Membrane Structure 361

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FIGURE 3 Charged amphiphiles in lecithin membranes induce a reorientation of the −P-N dipole. The figure shows
deuterium NMR spectra of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine deuterated at the choline headgroup (α-
CD 2 -POPC) without amphiphile (X b = 0), with cationic amphiphile (X b = 0.14), and with anionic amphiphile (X b = 0.4).
Positive charges decrease the quadrupole splitting of the α-segment; negative charges increase it [Seelig et al. (1987).
Biochemistry 26, 7535].

˚
example is shown in Fig. 5 for a membrane composed phase, an average thickness of about 30 A and a thermal
−3
of DPPC at temperatures T > 41 C (liquid crystalline expansion coefficient of−2.510 /K was derived, ingood
◦
phase). The segmental order parameters are approximate- agreement with X-ray diffraction experiments. It should
ly constant for the first nine chain segments, but decrease be noted that this approach is also valid for determining the
towards the central part of the bilayer. The chain order- bilayer thickness of highly unsaturated membranes. The
ing can be explained on the basis of the rotational iso- DPPC order parameter profile (Fig. 5) serves as a “gold
meric model for hydrocarbon chains. In the region of standard” for molecular dynamics simulations of bilayers.
constant order parameters, trans-gauche isomerizations Similarorderprofileshavenowbeenestablishedforalarge
occur only in complementary pairs, leaving the hydro- variety of pure lipid membranes as well as intact biological
carbon chains essentially parallel to each other. This membranes.
leads to well-ordered bilayers with disordered hydro-
carbon chains. The decrease of the order parameter
B. Incorporation of cis-Double Bonds
in the central region is due to increasing contribu-
tions of gauche states. The total number of gauche iso- As mentioned before, almost all biological lipids con-
meric states was estimated to be between 3 and 6 per tain unsaturated fatty acyl chains at the sn-2 position.
chain. The quantitative evaluation of the deuterium data The incorporation of the cis-double bond introduces a
further yields the thickness of the hydrocarbon region of kink in the otherwise straight chain and reduces the gel-
the bilayer. For DPPC bilayers in the liquid crystalline to-liquid crystal phase transition temperature by some