Simulation Studies Of Aggregation Behavior Of Gold Nanoparticles And Their Interactions With Cell Membranes

Understanding the interactions of gold nanoparticles (AuNPs) with cellular compartments, especially cell membranes, is of fundamental importance in obtaining their control in biomedical applications.This dissertation investigated the interactions of2.2nm core AuNPs with model cell membranes by coarse-grained (CG) molecular dynamics (MD) simulation. A CG model for AuNPs was first established to make the nanoparticle-cell membrane simulation feasible.Later on the dynamical behaviors of AuNPs in solvents were investigated. Finally, we focused on the interaction of AuNPs with lipid membranes under various conditions.The results provided key information for the designing of these synthetic nanoparticles in their bioapplications.The biomedical applications of gold nanoparticles to be used as delivery, diagnostic and therapeutic agents have been growing rapidly over recent years.Their efficiency of transporting DNA to nucleus is8times higher than that of polyethylenimine.They have also been successfully utilized in novel cancer therapies. In most of these bioapplications,the nanoparticles are required to pass cell plasma membrane either by endocytosis or by direct penetration to reach target cellular compartments, during which significant toxic effect may be induced to the cells.Safe and efficient localization of nanoparticles is critical for these bioapplications, which partly leads to the rising of a new field, nanotoxicology. Controlling the balance between efficiently crossing cell membrane and their potential toxic effect is one of the key challenges in designing and fabricating these synthetic nanoparticles.Numerous experimental studies have been conducted to probe AuNP-cell interactions in the past few years.It has been reported that the sign of surface charges can dramatically influence the uptake of AuNPs.In addition, different shapes, ligand structures and compositions of AuNPs can also lead to different levels of cellular uptake.In particular, it is found that cationic and anionic AuNPs follow different internalization pathways (endocytosis or penetration of cell membrane) to enter cells. Interestingly, surface-structure-regulated anionic AuNPs can bypass endocytosis without overtly disrupting cell membranes.However, the mechanism of AuNP uptake is still poorly understood.On the other hand, while cytotoxicity measurements give astounding results of AuNPs with different physical and chemical characters,the principles underlying AuNPs'cytotoxicity is not yet established. The lack of atomic-level details on AuNP-cell membrane interactions prevents us from gaining an in-depth understanding of the observed phenomena.In addition, the differences in experimental procedures and characterizations of AuNPs make the current results difficult to be normalized, which yield a necessity for a systematic study on the AuNP-cell membrane interactions.For the investigation of AuNP-cell membrane interactions, atomistic-level MD simulation, however, cannot gain enough access in time and space and therefore does not provide sufficient information. To overcome this difficulty, we employed a CG model,which incorporated several atoms into one bead to reduce computational costs. A typical CG model can promote simulation length and time scale by several orders of magnitude.Based on the MARTINI CG frame developed by Marrink et al.,we constructed a CG AuNP model which was able to reproduce both structural and dynamical properties of AuNPs in experiments.CG MD simulations were first carried out to investigate the dynamics of2.2nm monolayer-protected gold nanoparticles in solvents.The effect of ligand length,ligand terminal chemistry, solvents and temperature were examined.It was found that AuNPs with unmodified alkanethiol ligands formed stable aggregates in water in the timescale of hundreds of nanoseconds (eight nanoparticles).In a particular case the AuNPs aggregated into an infinite,one-dimensional chain-like assembly instead of clusters of aggregates.The aggregates of AuNPs with short ligand tails seemed to have an amorphous shape whereas long-tailed AuNPs aggregated into a spherical cluster. The properties of ligand terminals had a dominant influence on the aggregation behavior of AuNPs.Increasing the polarity of the ligand terminals weakened the tendency of aggregation of AuNPs in water. For AuNPs imposed with charged terminals, they did not aggregate even with a high concentration of salt. However the aggregation behavior of AuNPs changed dramatically if the properties of solvent were altered.Temperature increase greatly accelerated the process of aggregation. The results suggest that the dynamics of monolayer-protected AuNP can be controlled by their surface properties as well as the features of solvents.The interactions of AuNPs and model cell membranes were then examined.We constructed lipid bilayers which were aimed at providing the best imitation of living cell plasma membranes.Mammalian cell membranes feature a negative charge due to their composition and the negative proteins embedded on them.In addition,cytosol insides the cells usually have a-60mV potential relative to extracellular fluid.This allowed us to incorporate negative potential on the bilayer. Together, we used negative lipid bilayer in combination with a neutral bilayer to model typical mammalian cell membranes that possess an overall negative electric feature.Simulations reveal that AuNPs with different signs and densities of surface charges spontaneously adhere to the bilayer surface or penetrate into the bilayer interior. The potential of mean force calculations show that the energy gains upon adhesion or penetration is significant. Increasing the surface charge density of AuNPs will result in a higher level of penetration as well as bilayer disruption. It is also found that both the level of penetration and membrane disruption increase as the charge density of the AuNP increases, but in different manners.In the case of AuNPs feature highest surface charge density, it is observed that a nanoscale hole were formed and expanded spontaneously on the peripheral regions of the negative bilayer. The expansion of the hole is on the timescale of hundreds of nanosceonds.The fully expanded hole had a radius of～5.5nm and could transport water molecules at a rate of up to～1100molecule/ns.However holes could not be formed on a larger bilayer. The factors that can eliminate hole formation on the bilayer also include:the decrease of cationic ligands on the AuNP, the reduction of negative lipids in the bilayer, the release of bilayer surface tension, the lowering of temperature and the addition of a high concentration of salt. The results suggest that a hole can only be formed on living cell membranes under the extreme conditions. Penetration and its concomitant membrane disruptions can be a possible mechanism of the two observed phenomena in experiments:AuNPs bypass endocytosis during their internalization into cells;Cytotoxicity of AuNPs.The findings suggest a way of controlling the AuNP-cell interactions by manipulating surface charge densities of AuNPs to achieve designate goals in their biomedical applications, such as striking a balance between their cellular uptake and cytotoxicity in order to achieve optimal delivery efficiency as delivery agents.