NMR-Chemical Shifts of Methacrylate Molecules Associated with Liposomes

Objectives: Methacrylates are widely used in dentistry as dentures and restorative materials. The inflammatory and allergic effects of methacrylates may be caused by unpolymerized and residual monomers released from the cured resins, since oxygen inhibits methacrylate polymerization. These inflammatory and allergic reactions were previously reported to be due to the high reactivity of the a-carbon of methacryloyloxy groups of methacrylates with hydrophobicity. To extend our recent observations, we studied changes in the 1H and 13C NMR chemical shifts of methacrylate molecules associated with DMPC (L-a-dimyristoylphosphatidylcholine) or DPPC (L-a-dipalmitoylphosphatidylcholine) liposomes, as models for native lipid bilayers, at 30°C, 37°C and 52°C. Methods: An appropriate amount of DPPC or DMPC mixed in chloroform solution was subjected to a vacuum and dispersed by vortexing in D2O buffer solution and sonicated under a N2 atmosphere for 5 min. NMR spectra were obtained on a JEOL Alpha-500 spectrometer. Results: The a-carbon resonance of methacryloyloxy groups in methacrylates showed marked shifts to a higher field, suggesting interactions between the methacrylate molecules and the lipid bilayer. Comparison of the findings for methyl methacrylate (MMA), ethylene dimethacrylate (EDMA) and triethyleneglycol dimethacrylate (TEGDMA) revealed that the interaction of the dimethacrylates (EDMA, TEGDMA) was greater than that of the monomethacrylate (MMA). Study of the interaction of methacrylates with DMPC liposomes by differential scanning calorimetry (DSC) indicated that the interaction was characterized by decreased enthalpy, entropy and transition cooperativity. We used a PM 3 semi empirical MO method to calculate descriptors such as reactivity (HOMO-LUMO energy) and electrostatic function (partial charges) for the interaction between methacrylate molecules and DPPC, and the upfield NMR shifts of the methacrylate resonances were explained well by the descriptors. Conclusion: NMR screening may be valuable in biomaterials development to assess the effects of methacrylates on phospholipid targets.