2D H-1-NMR CONFORMATIONAL STUDY OF PHOSPHATIDYLSERINE DILUTED IN PERDEUTERATED DODECYLPHOSPHOCHOLINE MICELLES - EVIDENCE FOR A PH-INDUCED CONFORMATIONAL TRANSITION

The conformation of phosphatidylserine (DMPS) diluted in perdeuterated dodecylphosphocholine micelles (DPC) has been investigated by 1D and 2D proton NMR spectroscopy. Chemical shift pH dependence showed that t he pK relative to the serine carboxyl titration (3.4 +/- 0.05) was nea rly identical to that measured in bilayers. Chemical shift and NOE dat a revealed that the phosphatidylserine molecule undergoes a conformati onal transition upon titration of the serine carboxyl group. The NOE n etwork observed between the different parts of the molecule was suffic iently abundant to allow, in combination with molecular modeling metho ds, an assessment of the conformational changes. The conformational ch anges mainly involve the glycerol backbone, which is parallel to the w hole molecule, that is, to the layer normal, at low pH and becomes per pendicular to the whole molecule at neutral pH. In both cases, the con formations are remarkably close to those observed for the crystal form s of zwitterionic and negatively charged phospholipids. Two-dimensiona l proton NMR study of phospholipids, diluted in perdeuterated DPC mice lles, appears to be a simple and relevant method to obtain complete an d direct information on their conformations in a model membrane-soluti on interface.