A P-31-NMR AND H-2-NMR STUDY ON THE STRUCTURAL PERTURBATIONS INDUCED BY CHARGED DETERGENTS IN THE HEADGROUP REGION OF PHOSPHATIDYLCHOLINE BILAYERS

The effect of cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB) on the structure and mobility of phosphatidylcholine membranes was studied by 31P- and 2H-nuclear magnetic resonance. CTAB and DTAB induced a strong increase in the effective phosphorus chemical shift anisotropy of the phospholipid. In phosphatidylcholine which had been specifically labeled by deuterium in the headgroup segments, the detergents caused an increae in the quadrupole splittings from the N-methyl deuterons whereas the splittings from both methylene segments collapsed to a singlet signal at a ratio of about 2 mol of phosphatidylcholine per mol of detergent. The effect of DTAB on the headgroup spectral parameters was largely compensated by the negatively charged detergent sodium dodecylsulfate (SDS). In contrast to the phospholipid headgroup the hydrophobic region of the membrane was almost unaffected by CTAB or DTAB. The trimethylammonium headgroups of membrane-bound detergent molecules were found to be motionally much more restricted than the corresponding headgroup segments of the host phospholipid. From these observations it is concluded that the detergent headgroups are located close to the water/hydrocarbon interface of the phospholipid membrane thereby tilting the phospholipid headgroup dipole from the membrane parallel into a more upright orientation.