Effect of Curvature on Nanoparticle Supported Lipid Bilayers Investigated by Raman Spectroscopy

The packing of lipids on silica (SiO(2)) nanoparticles (NPs) was investigated by Raman spectroscopy for 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) as a function of their size, for SiO(2) NPs of 5, 15, 25, 45, and 100 nm nominal diameter. Raman spectral indicators in the C-C and C-H stretching regions were used to determine conformational order and alkyl chain packing for these systems. As the ratio of NP to lipid size decreases, packing in a normal bilayer configuration increases free volume and decreases hydrophobic interaction between the chains. For the 15, 25, 45, and 100 nm SiO(2), for which single supported lipid bilayers (SLBs) are formed around the NPs, the Raman data indicate that there is increased interdigitation and increased lateral packing order between the chains with decreasing NP size, which improves hydrophobic association and decreases the voids that would occur for normal bilayers. For the same size NP, there is increased interdigitation and lateral packing for the DSPC compared with DPPC lipids, as expected based on the greater void volume that would be created for the longer alkyl chain lengths. Another mechanism for filling this void space is the formation of gauche kinks for the terminal methyl groups at the center of the bilayer, which can be monitored by a Raman band at 1122 cm(-1). These gauche defects are most prevalent for the largest size (100 nm) NPs but are observed for all NP sizes. For the 5 nm SLBs, which form aggregates, we hypothesize that bilayer bridging can occur between the NPs. Compared with the 15 nm NPs, the order parameter increases but there are fewer trans conformers, possibly due to chains that are loosely packed or isolated in the interstitial regions.