Counterpropagating Gradients of Antibacterial and Antifouling Polymer Brushes.

We report on the formation of counterpropagating density gradients in poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) brushes featuring spatially varying quaternized and betainized units. Starting with PDMAEMA brushes with constant grafting density and degree of polymerization, we first generate a density gradient of quaternized units by directional vapor reaction involving methyl iodide. The unreacted DMAEMA units are then betainized through gaseous-phase betainization with 1,3-propanesultone. The gas reaction of PDMAEMA with 1,3-propanesultone eliminates the formation of byproducts present during the liquid-phase modification. We use the counterpropagating density gradients of quaternized and betainized PDMAEMA brushes in antibacterial and antifouling studies. Completely quaternized and betainized brushes exhibit antibacterial and antifouling behaviors. Samples containing 12% of quaternized and 85% of betainized units act simultaneously as antibacterial and antifouling surfaces.

Supramolecular Functionalization for Improving Thermoelectric Properties of Single-Walled Carbon Nanotubes-Small Organic Molecule Hybrids.

Single-walled carbon nanotube (SWCNTs-P)-small organic molecule hybrid materials are promising candidates for achieving high thermoelectric (TE) performance. In this study, we synthesized rod-coil amphiphilic molecules, that is, tri(ethylene oxide) chain-attached bis(bithiophenyl)-terphenyl derivatives (1 and 2). Supramolecular functionalization of SWCNTs-P with 1 or 2 induced charge-transfer interactions between them. Improved TE properties of the supramolecular hybrids (SWCNTs-1 and SWCNTs-2) are attributed to increased charge-carrier concentration (electrical conductivity), interfacial phonon scattering (thermal conductivity), and energy difference between the transport and Fermi levels (ETr - EF; Seebeck coefficient). SWCNTs-2 exhibited a ZT of 0.42 × 10-2 at 300 K, which is 350% larger than that of SWCNTs-P. Furthermore, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped SWCNTs-2 showed the highest ZT value of 1.96 × 10-2 at 300 K among SWCNTs-P/small organic molecule hybrids known until now. These results demonstrated that the supramolecular functionalization of SWCNTs-P with small organic molecules could be useful for enhancement of TE performance and applications in wearable/flexible thermoelectrics.

Effect of lipid shape on toroidal pore formation and peptide orientation in lipid bilayers.

Amphiphilic peptides of different lengths were simulated with lipid bilayers composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoMPC) in different ratios. Simulations of lipid bilayers without peptides show that the bilayers with more lysoMPC become more disordered and thinner. Amphiphilic peptides added to this simulation do not insert into the DMPC bilayer at a low peptide/lipid ratio (P/L ≤ 1/50), while they do insert into the DMPC/lysoMPC bilayer and form a toroidal pore even at such a low P/L ratio, where the pore edge is surrounded by lysoMPC rather than by DMPC. In particular, upon pore formation, peptides migrate toward the edge of a pore and become tilted, showing transmembrane alignment regardless of the peptide length, in qualitative agreement with experiments. This pore formation occurs more frequently in larger bilayers that allow greater curvature, indicating that bilayer curvature is important for pore formation. These results indicate that the addition of lysoMPC induces a thinner bilayer with greater curvature, and thus the bilayer with lysoMPC can be more easily penetrated by peptides, leading to the formation of a toroidal pore stabilized by peptides and lysoMPC. These findings help explain experimental observations of the effect of the inverted cone-shaped lyso-lipid on pore formation and peptide orientation, and also support the experimental suggestion regarding the formation of an iris-like ring of helices lining a toroidal pore.

Aggregation and insertion of melittin and its analogue MelP5 into lipid bilayers at different concentrations: effects on pore size, bilayer thickness and dynamics.

Melittin and its analogue MelP5 (five mutations T10A, R22A, K23A, R24Q, and Q26L of melittin) were simulated with lipid bilayers at different peptide/lipid molar ratios using all-atom and coarse-grained (CG) force fields. In CG simulations, both melittin and MelP5 insert into the bilayer at high concentration, while at low concentration only MelP5 can do so, showing the increased membrane permeability of MelP5 because five mutations weaken the electrostatic repulsion between peptides and strengthen the hydrophobic interactions between peptides and lipid tails, in quantitative agreement with experiments. In particular, aggregation of 6-8 MelP5 leads to pore formation, as also suggested by experiments. All-atom simulations, starting with atomic coordinates converted from the final configurations of CG simulations, show that MelP5 peptides bring more water molecules into the pores than do melittin peptides, indicating that MelP5 peptides form larger pores. Also, MelP5 peptides more effectively disorder lipids and thus increase the lateral mobility of lipids than do melittin peptides, leading to thinner bilayers. These findings indicate that differences of only five sequences can influence peptide aggregation and insertion, and bilayer thickness and dynamics, which helps explain experimental observations of the higher extent of antimicrobial activity and macromolecular leakage for MelP5 than for melittin, and also support experimental suggestions regarding the number of aggregated MelP5 and different effects of melittin and MelP5 on pore formation.

All-atom simulations and free-energy calculations of coiled-coil peptides with lipid bilayers: binding strength, structural transition, and effect on lipid dynamics.

Peptides E and K, which are synthetic coiled-coil peptides for membrane fusion, were simulated with lipid bilayers composed of lipids and cholesterols at different ratios using all-atom models. We first calculated free energies of binding from umbrella sampling simulations, showing that both E and K peptides tend to adsorb onto the bilayer surface, which occurs more strongly in the bilayer composed of smaller lipid headgroups. Then, unrestrained simulations show that K peptides more deeply insert into the bilayer with partially retaining the helical structure, while E peptides less insert and predominantly become random coils, indicating the structural transition from helices to random coils, in quantitative agreement with experiments. This is because K peptides electrostatically interact with lipid phosphates, as well as because hydrocarbons of lysines of K peptide are longer than those of glutamic acids of E peptide and thus form stronger hydrophobic interactions with lipid tails. This deeper insertion of K peptide increases the bilayer dynamics and a vacancy below the peptide, leading to the rearrangement of smaller lipids. These findings help explain the experimentally observed or proposed differences in the insertion depth, binding strength, and structural transition of E and K peptides, and support the snorkeling effect.

Molecular dynamics studies of PEGylated α-helical coiled coils and their self-assembled micelles.

We performed coarse-grained (CG) molecular dynamics simulations of trimeric α-helical coiled coils grafted with poly(ethylene glycol) (PEG) of different sizes and conjugate positions and the self-assembled micelle of amphiphilic trimers. The CG model for the trimeric coiled coil is verified by comparing the α-helical structure and interhelical distance with those calculated from all-atom simulations. In CG simulations of PEGylated trimers, the end-to-end distances and radii of gyration of PEGs grafted to the sides of peptides become shorter than those of free PEGs in water, which agrees with experiments. This shorter size of the grafted PEGs is also confirmed by calculating the thickness of the PEG layer, which is less than the size of the mushroom. These indicate the adsorption of PEG chains onto coiled coils since hydrophobic residues in the exterior sites of coiled coils tend to be less exposed to water and thus interact with PEGs, leading to the compact conformation of adsorbed PEGs. Simulations of the self-assembly of amphiphilic trimers show that the randomly distributed trimers self-assemble to micelles. The outer radius and hydrodynamic radius of the micelle, which were calculated respectively from radial densities and diffusion coefficients, are ∼7 nm, in agreement with the experimental value of ∼7.5 nm, while the aggregation number of amphiphilic molecules per micelle is lower than the experimentally proposed value. These simulations predict the experimentally measured size of PEGs grafted to the trimeric coiled coils and their self-assembled amphiphilic micelles and suggest that the aggregation number of the micelle may be lower, which needs to be confirmed by experiments.

Electrochemical characteristics of zinc oxide-polyethyleneglycol complex films using EQCM.

ZnO-PEG-ZnO complex film was fabricated by forming ZnO thin film on the Polyethyleneglycol (PEG) thin film. ZnO thin films were formed by an electrostatic method and ZnO-PEG complex films were fabricated by adsorbing PEG on the ZnO thin films surface with hydrogen bond. The electrochemical characteristic of the ZnO-PEG-ZnO film was analyzed by EQCM techniques. The resonance frequency, resistance and current changes were measured simultaneously with scan rate 100 mV/s, sweep range -1.4-1.2 V in 5 mM ZnCl2 aqueous solution. The electrochemical characteristic of the ZnO-PEG-ZnO complex film was compared with that of the ZnO thin film, and the possible electrode applications of ZnO-PEG-ZnO complex films were examined.