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Membrane Structure 359

FIGURE 1 Single-crystal structures of three phospholipids. The lipids are 1,2-dilauroyl-sn-glycero-3-
phosphoethanolamine (DLPE) [Hitchcock et al. (1974). Proc. Natl. Acad. Sci. USA 71, 3036], 1,2-dimyristoyl-sn-
glycero-3-phosphocholine (DMPC) [Pearson and Pascher (1979). Nature 281, 49], and 1,2-dimyristoyl-sn-glycero-
3-phosphogycerol (DMPG) [Pascher et al. (1987). Biochim. Biophys. Acta 896, 77]. Structural features which are
carried over into liquid-crystalline membranes: (1) the polar groups are oriented at approximately a right angle to the
hydrocarbon chains, and (2) in DLPE and DMPC the sn-2 fatty acid chain is bent at the C-2 segment while the sn-1
chain is straight. A bent sn-2 chain is a common property of phospholipids in biomembranes. Only one of two possible
conformations is shown for each lipid [Seelig et al. (1987). Biochemistry 26, 7535].

Fig. 2A). It is still detectable at position C-3 (Fig. 2C) but One of the most extensively investigated proteins ac-
averages out at label positions deeper in the hydrocarbon tive at the membrane surface is phospholipase A 2 . This
layer (Fig. 2D, the C-10 segment). enzyme is water soluble, attacks the membrane from the
At a molecular level, the different splittings indicate aqueous phase, and acts specifically on the sn-2 chain. In
that the sn-1 chain extends perpendicularly to the bilayer light of the conformational results presented here, the at-
surface with all segments, while the sn-2 chain starts out tack of phospholipase A 2 is facilitated by the orientation
◦
parallel to the bilayer surface and makes a 90 bend af- of the sn-2 ester bond parallel to the membrane surface.
ter the C-2 segment in order to keep the sn-1 and sn-2
chains parallel to each other in agreement with the crys-
C. Phospholipid Headgroup Response to Ions
tal structure shown for PC and PE in Fig. 1. Further-
more, labeling of the glycerol backbone suggests the pos- The quadrupolar splitting, ν Q , of headgroup-deuterated
sibility of two long-lived conformations of the glycerol PC or PE varies linearly as a function of the total amount
constituent. of electric surface charge, and changes in opposite direc-
The conformation of the two fatty acyl chains near the tions are induced by positive and negative surface charges.
glycerol moiety was further investigated by synthesiz- This indicates that the phosphocholine and the phospho-
ing specifically deuterated 1,3-dipalmitoyl-sn-glycero-2- ethanolamine dipoles are sensitive to electric charges at
phosphocholine.AsseeninFig.2Bthespectrumoftheliq- the membrane surface and function as an electrometer.
uid crystalline phase of sn-glycero-2-phosphatidylcholine As an example, the interaction of α-CD 2 -POPC with an
deuterated at position C-2 shows only a single quadrupo- anionic and a cationic amphiphile is shown in Fig. 3.
lar splitting, indicating that the C-2 segments of both fatty The chemical nature of the ion imparting the mem-
acyl chains are now aligned parallel to the bilayer surface brane surface charge appears to be of secondary impor-
and the chains are bent perpendicularly to the surface only tance. A variety of chemically different, charged com-
after the C-2 segment. pounds including metal ions, local anesthetics, peptides,