Fang Huang

Molecular modeling of interaction between lipid monolayer and graphene nanosheets: implications for pulmonary nanotoxicity and pulmonary drug delivery

Understanding how nanoparticles interact with the pulmonary surfactant monolayer (PSM) is of great importance for safe applications in biomedicine and for evaluation of both health and environment impacts. Here, by performing molecular dynamics simulations, we propose a possible origin of the pulmonary nanotoxicity of graphene-based nanoparticles that comes from a rigidifying effect of graphene nanosheets (GNs) on PSM. This, in reality, indicates that once captured by the PSM, inhaled GNs are hard to be removed from the PSM partially because the expiration or PSM compression is locally restrained, possibly leading to GN accumulation on the PSM. The local rigidifying effect, which is enhanced as multiple GNs approach each other, is found to be dependent on the GN hydrophobicity. In the expiration or PSM compression process, the hydrophilic GN keeps adhering to the monolayer–air interface, while the hydrophobic GN tends to be hosted in the hydrophobic interior and internalize into the PSM via self-rotation. Besides the spontaneous internalization via PSM compression, our pulling simulations indicate that both pulmonary internalization and externalization of GNs can be accomplished by direct translocation across the PSM. The effect of GN hydrophobicity on the direct PSM translocation is well supported by the free energy analysis. This work will help our understanding of pulmonary nanotoxicity of GNs and provide useful guidelines for molecular design of GN-based pulmonary drug delivery materials.

Strong Fermi level pinning induces a high rectification ratio and negative differential resistance in hydrogen bonding bridged single cytidine pair junctions

We propose a high performance single molecule rectifier by sandwiching a deoxycytidine base pair between gold electrodes. The conductance of the single base pair junction can be controlled by its protonation status, with ON/OFF ratios between the protonated (pCC) and deprotonated (CC) junctions of 3-5 orders of magnitude. In the conducting pCC state, we observed a high rectification ratio of two orders of magnitude at bias voltage values around 0.1 V. This rectification ratio surpasses most of the theoretical designs for single molecular rectifiers, while the low working voltage implies significant energy efficiency. Negative differential resistance (NDR) was also witnessed in the protonated state, with a peak to valley ratio of 24. Both the rectifying and NDR effects originate from strong Fermi level pinning effects. The electronic performance offers these single base pair junctions potential applications as a unimolecular rectifier or switch with an NDR effect. The current-voltage response is unique compared with those of the reported canonical A-T and G-C pairs, and provides the possibility to be used for i-motif DNA structure recognition or sequencing.

Combinational biosynthesis of phycocyanobilin using genetically-engineered Escherichia coli.

Genes of the key enzymes for phycocyanobilin (PCB) biosynthesis were cloned into E. coli and combinationally expressed to produce phycocyanobilin, with autologous heme as substrate. Culture conditions were optimized to achieve ~3xa0mg PCB/l. A protocol for the purification of recombinant phycocyanobilin was established using solvent extraction combined with chromatography, which resulted in a final yield of ~0.3xa0mg PCB/l with a purity >95xa0%. Recombinant phycocyanobilin could scavenge hydroxyl radicals with an EC50 of 0.1xa0μM.

Single-Molecule Imaging Demonstrates Ligand Regulation of the Oligomeric Status of CXCR4 in Living Cells

The role of dimerization and oligomerization of G-protein-coupled receptors in their signal transduction is highly controversial. Delineating this issue can greatly facilitate rational drug design. With single-molecule imaging, we show that chemokine receptor CXCR4 exists mainly as a monomer in normal mammalian living cells and forms dimers and higher-order oligomers at a high expression level, such as in cancer cells. Chemotaxis tests demonstrate that the signal transduction activity of CXCR4 does not depend only on its expression level, indicating a close relation with the oligomeric status of CXCR4. Moreover, binding ligands can effectively upregulate or downregulate the oligomeric level of CXCR4, which suggests that binding ligands may realize their pivotal roles by regulating the oligomeric status of CXCR4 rather than by simply inducing conformational changes.

Ultrashort Single-Walled Carbon Nanotubes Insert into a Pulmonary Surfactant Monolayer via Self-Rotation: Poration and Mechanical Inhibition

It has been widely accepted that longer single-walled carbon nanotubes (SWCNTs) exhibit higher toxicity by causing severe pneumonia once inhaled, yet relatively little is known regarding the potential toxicity of ultrashort SWCNTs, which are of central importance to the development of suitable vehicles for biomedical applications. Here, by combining coarse-grained molecular dynamics (CGMD), pulling simulations, and scaling analysis, we demonstrate that the inhalation toxicity of ultrashort SWCNTs (1.5 nm < l < 5.5 nm) can be derived from the unique behaviors on interaction with the pulmonary surfactant monolayer (PSM), which is located at the air-water interface of alveoli and forms the frontline of the lung host defense. Molecular dynamics (MD) simulations suggest that ultrashort SWCNTs spontaneously insert into the PSM via fast self-rotation. Further translocation toward the water or air phase involves overcoming a high free-energy barrier, indicating that removal of inhaled ultrashort SWCNTs from the PSM is difficult, possibly leading to the accumulation of SWCNTs in the PSM, with prolonged retention and increased inflammation potentials. Under certain conditions, the inserted SWCNTs are found to open hydrophilic pores in the PSM via a mechanism that mimics that of the antimicrobial peptide. Besides, the mechanical property of the PSM is inhibited by the deposited ultrashort SWCNTs through segregation of the inner lipid molecules from the outer phase. Our results bring to the forefront the concern of the inhalation toxicity of ultrashort SWCNTs and provide guidelines for future design of inhaled nanodrug carriers with minimized side effects.

Interplay Between Nanoparticle Wrapping and Clustering of Inner Anchored Membrane Proteins.

The receptor-mediated endocytosis of nanoparticles (NPs) is known to be size and shape dependent but regulated by membrane properties, like tension, rigidity, and especially membrane proteins. Compared with transmembrane receptors, which directly bind ligands coated on NPs to provide the driving force for passive endocytosis, the hidden role of inner anchored membrane proteins (IAMPs), however, has been grossly neglected. Here, by applying the N-varied dissipative particle dynamics (DPD) techniques, we present the first simulation study on the interplay between wrapping of NPs and clustering of IAMPs. Our results suggest that the wrapping dynamics of NPs can be regulated by clustering of IAMPs, but in a competitive way. In the early stage, the dispersed IAMPs rigidify the membrane and thus restrain NP wrapping by increasing the membrane bending energy. However, once the clustering completes, the rigidifying effect is reduced. Interestingly, the clustering of longer IAMPs can sense NP wrapping. They are found to locate preferentially at the boundary region of NP wrapping. More importantly, the adjacent IAMP clustering produces a late membrane monolayer protrusion, which finally wraps the NP from the top side. Our findings regarding the competitive effects of IAMP clustering on NP wrapping facilitate the molecular understanding of endocytosis and establish fundamental principles for design of NPs for widespread biomedical applications.

The role of nanoparticle shape in translocation across the pulmonary surfactant layer revealed by molecular dynamics simulations

Airborne nanoparticles (NPs), which vary widely in both shape and size, can be inhaled and deposit in the alveolar region, where they first interact with the pulmonary surfactant (PS) layer to cause toxicological effects and impact the subsequent fate of NPs inside the body. Previous studies on NP–PS interactions have been conducted focusing on spherical NPs, thereby overlooking the role of NP shape. Here, we demonstrate by molecular dynamics simulations the translocation of NPs across the PS layer being influenced by the NP shape. It was found that hydrophilic NPs with all dimensions smaller than 5 nm can rapidly penetrate through the PS layer, being barely affected by the NP shape, while the shape matters for larger NPs in both translocation and PS perturbation. For hydrophobic NPs with at least one dimension smaller than the PS layer thickness, they prefer to be immersed into but hardly transported across the PS layer. If at least one dimension is larger than the PS layer thickness, they can be readily wrapped by the layer under compression, with the steady wrapping state being dominated by the shape-dependent NP rotation. During transport, PS molecules can be recruited by NPs, acting as a corona to influence the biological identity of NPs. Adversely, PS depletion can be induced, together with the perturbed PS arrangement around sharp NP edges to cause destructive PS layer rupture. Our results suggest that all studies of inhalation toxicity and pulmonary drug delivery should consider first the interactions of target NPs with the PS layer, where the shape significantly matters.

Surface patterning of single-walled carbon nanotubes enhances their perturbation on a pulmonary surfactant monolayer: frustrated translocation and bilayer vesiculation

The pulmonary surfactant monolayer (PSM) is a complex material lining the air–water interface of lung alveoli to avoid its collapse by reducing surface tension. Once external particles are inhaled and captured by the PSM, this property might be perturbed to induce inhalation toxicity. However, relatively little is known regarding the detailed interaction between inhaled particles and the PSM. Here, by applying the coarse-grained molecular dynamics simulation method, we probe how inhaled single-walled carbon nanotubes (SWCNTs) interact with the PSM. For pristine SWCNTs, they are found to insert into or be wrapped by the PSM, depending on the tube size and the PSM tension. For hydrophilic tubes, they spontaneously translocate across the PSM, regardless of the tension. In contrast to SWCNTs with unique surface properties, the surface patterning of SWCNTs enhances their perturbation on the PSM. Under expansion, the PSM translocation is frustrated via inducing the ordered or disordered lipid arrangement adhering to the patterned tube surface. Under compression, the lipid rearrangements further self-adjust and grow into bilayers, which protrude along the tube surface and finally develop into vesicles. The stripe width and stripe orientation, among other factors, are found to be the most important factors that determine whether and how the vesiculation takes place.

Interaction pathways between soft lipid nanodiscs and plasma membranes: A molecular modeling study

Lipid nanodisc, a model membrane platform originally synthesized for study of membrane proteins, has recently been used as the carrier to deliver amphiphilic drugs into target tumor cells. However, the central question of how cells interact with such emerging nanomaterials remains unclear and deserves our research for both improving the delivery efficiency and reducing the side effect. In this work, a binary lipid nanodisc is designed as the minimum model to investigate its interactions with plasma membranes by using the dissipative particle dynamics method. Three typical interaction pathways, including the membrane attachment with lipid domain exchange of nanodiscs, the partial membrane wrapping with nanodisc vesiculation, and the receptor-mediated endocytosis, are discovered. For the first pathway, the boundary normal lipids acting as ligands diffuse along the nanodisc rim to gather at the membrane interface, repelling the central bola lipids to reach a stable membrane attachment. If bola lipids are positioned at the periphery and act as ligands, they diffuse to form a large aggregate being wrapped by the membrane, leaving the normal lipids exposed on the membrane exterior by assembling into a vesicle. Finally, by setting both central normal lipids and boundary bola lipids as ligands, the receptor-mediated endocytosis occurs via both deformation and self-rotation of the nanodiscs. All above pathways for soft lipid nanodiscs are quite different from those for rigid nanoparticles, which may provide useful guidelines for design of soft lipid nanodiscs in widespread biomedical applications.

Role of Lipid Coating in the Transport of Nanodroplets across the Pulmonary Surfactant Layer Revealed by Molecular Dynamics Simulations

Hydrophilic drugs can be delivered into lungs via nebulization for both local and systemic therapies. Once inhaled, ultrafine nanodroplets preferentially deposit in the alveolar region, where they first interact with the pulmonary surfactant (PS) layer, with nature of the interaction determining both efficiency of the pulmonary drug delivery and extent of the PS perturbation. Here, we demonstrate by molecular dynamics simulations the transport of nanodroplets across the PS layer being improved by lipid coating. In the absence of lipids, bare nanodroplets deposit at the PS layer to release drugs that can be directly translocated across the PS layer. The translocation is quicker under higher surface tensions but at the cost of opening pores that disrupt the ultrastructure of the PS layer. When the PS layer is compressed to lower surface tensions, the nanodroplet prompts collapse of the PS layer to induce severe PS perturbation. By coating the nanodroplet with lipids, the disturbance of the nanodroplet on the PS layer can be reduced. Moreover, the lipid-coated nanodroplet can be readily wrapped by the PS layer to form vesicular structures, which are expected to fuse with the cell membrane to release drugs into secondary organs. Properties of drug bioavailability, controlled drug release, and enzymatic tolerance in real systems could be improved by lipid coating on nanodroplets. Our results provide useful guidelines for the molecular design of nanodroplets as carriers for the pulmonary drug delivery.