Conformational response of the phosphatidylcholine headgroup to bilayer surface charge: torsion angle constraints from dipolar and quadrupolar couplings in bicelles

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Abstract
The effects of bilayer surface charge on the conformation of the phosphocholine group of phosphatidylcholine were investigated using a torsion angle analysis of quadrupolar and dipolar splittings in, respectively, 2H and 13C NMR spectra of 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC) labelled in the phosphocholine group with either deuterons (POPC‐α‐d2, POPC‐β‐d2 and POPC‐γ‐d9) or carbon‐13 (POPC‐α‐13C and POPC‐αβ‐13C2) and incorporated into magnetically aligned bicelles containing various amounts of either the cationic amphiphile 1,2‐dimyristoyl‐3‐trimethylammoniumpropane (DMTAP) or the anionic amphiphile 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphoglycerol (DMPG). Three sets of quadrupolar splittings, one from each of the three deuteron labelling positions, and three sets of dipolar splittings (13Cα–31P, 13Cα–13Cβ, 13Cβ–14N), were measured at each surface charge, along with the 31P residual chemical shift anisotropy. The torsion angle analysis assumed fast anisotropic rotation of POPC about its long molecular axis, thus projecting all NMR interactions onto that director axis of motion. Dipolar, quadrupolar and chemical shift anisotropies were calculated as a function of the phosphocholine internal torsion angles by first transforming into a common reference frame affixed to the phosphocholine group prior to motional averaging about the director axis. A comparison of experiment and calculation provided the two order parameters specifying the director orientation relative to the molecule, plus the torsion angles α3, α4 and α5. Surface charge was found to have little effect on the torsion angle α5 (rotations about Cα–Cβ), but to have large and inverse effects on torsion angles α3 [rotations about P–O(11)] and α4 [rotations about O(11)–Cα], yielding a net upwards tilt of the P–N vector in the presence of cationic surface charge, and a downwards tilt in the presence of anionic surface charge, relative to neutrality. Copyright © 2004 John Wiley & Sons, Ltd.