Interaction of local anesthetics with small phospholipid vesicles investigated by proton NMR spectroscopy

The effects of the local anesthetics tetracaine, procaine (both charged at pH 6), and benzocaine (uncharged) on phospholipid liposomes have been investigated by 500 MHz 1H NMR Spectroscopy. All the drugs reverse the Pr3+ induced shifts of phospholipid resonances in the same sequence as they are shifted by addition of Pr3+: choline POCH2- > choline-CH2N > choline-N(CH3)3 > glycerol Β > glycerol γ > acyl C2 > acyl C3. The drug effects result from incorporation of positive charges (tetracaine and procaine) and from the induction of a conformational change of the phospholipid head group via an action on the lipid glycerol backbone (benzocaine). From titration experiments with tetracaine on liposomes containing Pr3+ inside and outside is derived that the drug passes the bilayer by transverse diffusion. Tetracaine partitions outside/inside at a ratio of 2∶1. Changes in linewidths of the drug resonances when incorporated into the liposomes allow the conclusion that the tetracaine molecule is located in an elongated way between the lipid acyl chains with its nitrogen group near the glycerol backbone. Benzocaine, showing strong effects on the line shapes of the protons on C2 and C3 of the lipid acyl chains is also located near the glycerol backbone, the region with the strongest hydrophobic forces.