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 use double electron–electron resonance (DEER, also known as PELDOR) to study the interaction of spin-labeled diclofenac (diclofenac-SL) with three types of model membranes consisting of palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), an equimolar mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and this mixture with the addition of 20 mol% cholesterol. The results suggest that lipid-mediated lateral clustering of diclofenac-SL molecules occurs in all cases. 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.