DEER Study of Spatial Arrangement of Spin-labeled Diclofenac in Lipid Bilayers of Different Composition

Diclofenac is a non-steroidal anti-inflammatory drug (NSAID). Here we employ double electron-electron resonance (DEER, also known as PELDOR) to study the interaction of spin-labeled diclofenac (diclofenac-SL) with the model membrane of three types: palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), equimolar mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and this mixture with 20 mol% of cholesterol added. The results suggest that lipid-mediated lateral clustering of diclofenac-SL molecules occurs in all cases, but in very different ways. For the POPC bilayer, alternative clustering takes place in two opposite leaflets, with random distribution of the molecules within the clusters. For DOPC/DPPC and DOPC/DPPC/cholesterol bilayers, diclofenac-SL molecules are separated by a distance of at least 1.4 nm. DOPC/DPPC/cholesterol bilayers are known to form nanoscale liquid disordered and liquid ordered lateral structures, the latter called lipid rafts. For this case, diclofenac-SL molecules were found to be captured by lipid rafts, forming a quasi-regular two-dimensional substructure in them with a "superlattice" parameter of ~ 3.0 nm.