Location of the Diclofenac Molecule in Model Lipid Membranes from Spin-Label-Enhanced NMR Relaxation

Nonsteroidal anti-inflammatory drugs (NSAIDs) can be used to treat a variety of conditions due to their antipyretic, anti-inflammatory, and analgesic effects. Their interaction with lipid membranes can cause various side effects known for these drugs. In this study, we employ ¹H 500 MHz liquid-state NMR to study the spin-lattice relaxation rate, <i>T</i>₁^-1, in model lipid membranes with embedded spin-labeled diclofenac (diclofenac-SL) as a relaxation enhancer. The membranes were unilamellar liposomes of four types: consisting of unsaturated 1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphocholine (POPC), a POPC/cholesterol mixture (80:20 mol %), fully saturated 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphocholine (DMPC), and a DMPC/cholesterol mixture (80:20 mol %). The obtained data for protons in different positions in the lipid molecule showed that, in the absence of cholesterol, diclofenac-SL in the POPC membrane is distributed approximately uniformly throughout the entire depth of the membrane, while in the DMPC membrane there is some shift for the hydrophobic interior. In the presence of cholesterol, a slight shift of diclofenac-SL toward the membrane surface is observed for POPC/cholesterol liposomes, whereas a tendency toward displacement away from the membrane surface is observed in DMPC/cholesterol liposomes.