CONFORMATIONS OF DIBUCAINE AND TETRACAINE IN SMALL UNILAMELLAR PHOSPHATIDYLCHOLINE VESICLES AS STUDIED BY NUCLEAR OVERHAUSER EFFECTS IN H-1 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Conformations of dibucaine and tetracaine in small unilamellar phosphatidylcholine vesicles have been investigated by nuclear Overhauser effects (NOEs) in H-1 nuclear magnetic resonance spectroscopy. Two-dimensional NOE and chemical exchange correlated spectroscopy (NOESY) and rotating frame NOE spectroscopy (ROESY) methods have been applied for obtaining the NOEs. In the NOESY spectra, NOEs between protons within the drug were overwhelmed by spin diffusion even at a short mixing time. This observation reduced the usefulness of the NOESY method on the one hand, however, on the other hand it facilitated remarkably in revealing signals due to the drug, hidden in the broad resonances of the membranes. In the ROESY spectra, the spin diffusion phenomena were less effective; accordingly the conformations of the drugs interacting with membranes were determined by the ROESY method. The observed NOE data showed that dibucaine takes more than two conformations and that both dibucaine and tetracaine are present as a dimer in the membranes. Molecular dynamics calculations supported these findings.