Effects of temperature and PEG grafting density on the translocation of PEGylated nanoparticles across asymmetric lipid membrane.

Plasma membrane internalization of nanoparticles (NPs) is important for their biomedical applications such as drug-delivery carriers. On one hand, in order to improve their half-life in circulation, PEGylation has been widely used. However, it may hinder the NPs' membrane internalization ability. On the other hand, higher temperature could enhance the membrane permeability and may affect the NPs' ability to enter into or exit from cells. To make full use of their advantages, we systematically investigated the effects of temperature and PEG density on the translocation of PEGylated nanoparticles across the plasma asymmetric membrane of eukaryotic cells, using near-atom level coarse-grained molecular dynamics simulations. Our results showed that higher temperature could accelerate the translocation of NPs across membranes by making lipids more disorder and faster diffusion. On the contrary, steric hindrance effects of PEG would inhibit NPs' translocation process and promote lipids flip-flops. The PEG chains could rearrange themselves to minimize the contacts between PEG and lipid tails during the translocation, which was similar to 'snorkeling effect'. Moreover, lipid flip-flops were affected by PEGylated density as well as NPs' translocation direction. Higher PEG grafting density could promote lipid flip-flops, but inhibit lipid extraction from bilayers. The consequence of lipid flip-flop and extraction was that the membranes got more symmetric.