Curved single-bilayers in the region of the anomalous swelling : effect of curvature and chain length

We have carried out density and DSC measurements on large unilamellar vesicles (LUVs) of different phosphocholines in the temperature range corresponding to the anomalous swelling regime of multibilayer systems, adjacent to the chain melting transition region. Experiments were performed for DPPC, DMPC and DC13PC, as a function of aggregate size. The steep drop in density occurring around the main transition gets broader as the vesicle radius decreases and a two-step process is clearly observed for DMPC and DC13PC vesicles smaller than 200 nm. No similar effect is observed for DPPC vesicles, whatever the aggregate dimension. DSC measurements corroborate density results, showing a splitting of the main peak that becomes more evident as the chain length decreases and as the curvature of the aggregate increases.