Hierarchical assemblies of biomolecular subunits can carry out versatile tasks at the cellular level with remarkable spatial and temporal precision. As an example, the collective motion and mutual cooperation between complex protein machines mediate essential functions for life, such as replication, synthesis, degradation, repair and transport. Nucleic acid molecules are far less dynamic than proteins and need to bind to specific proteins to form hierarchical structures. The simplest example of these nucleic acid-based structures is provided by a rod-shaped tobacco mosaic virus, which consists of genetic material surrounded by coat proteins. Inspired by the complexity and hierarchical assembly of viruses, a great deal of effort has been devoted to design similarly constructed artificial viruses. However, such a wrapping approach makes nucleic acid dynamics insensitive to environmental changes. This limitation generally restricts, for example, the amplification of the conformational dynamics between the right-handed B form to the left-handed Z form of double-stranded deoxyribonucleic acid (DNA). Here we report a virus-like hierarchical assembly in which the native DNA and a synthetic coat undergo repeated collective helicity switching triggered by pH change under physiological conditions. We also show that this collective helicity inversion occurs during translocation of the DNA-coat assembly into intracellular compartments. Translating DNA conformational dynamics into a higher level of hierarchical dynamics may provide an approach to create DNA-based nanomachines.
Surface properties of materials are strongly dependent on surface chemistry and surface structures. The fabrication of hierarchical surface nanostructures will endow solid surfaces with new functionalities and properties. In this research, we propose the polymerization-induced surface self-assembly (PISSA) approach for surface reconstruction. In this approach, two macro-CTAs, one grafted on silica particles and the other molecularly dissolved in solution, were used in reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization, and surface micelles (s-micelles) with different morphologies and sizes on silica particles were fabricated. Kinetics studies demonstrate that there are two critical points on a plot of ln([M]0/[M] t ) vs polymerization time, corresponding to the onsets of surface assembly and the self-assembly of block copolymers. The morphology of s-micelles is dependent on the monomer conversion and the length of macro-CTA. For macro-CTA with short chain length, with an increase in monomer conversion the s-micelles experience a morphology change from spherical s-micelles to layered structures. For macro-CTA with long chain length, the average size of s-micelles increases with monomer conversion. In this research, we demonstrate PISSA can be used as a versatile method for surface modification.
Studies on the fabrication of polymer–protein hybrid self-assemblies have aroused great interest over the past years because of a broad range of applications of the materials in drug/protein delivery, biosensors, and enhancement of protein stability. The hybrid assemblies are usually fabricated from polymer–protein bioconjugates, which may suffer from the damages to the protein structures and the loss of functionalities in the synthesis. Herein, we report a simple and efficient approach to the fabrication of vesicle-like structures based on coassembly of homopolymer chains and protein molecules. At room temperature, poly(N-isopropylacrylamide) (PNIPAM) and bovine serum albumin (BSA) are able to form complexes through hydrophobic interactions in aqueous solution. Upon heating to a temperature above the cloud point of PNIPAM, vesicle-like structures with collapsed PNIPAM in the walls and BSA at the surfaces are formed. The size and membrane thickness of the assemblies can be tuned by the molar ratio of PNIPAM to BSA. The hydrophobic interaction between PNIPAM and BSA plays a key role in the complex formation and self-assembly process. The complexes and assembled structures are analyzed by using micro differential scanning calorimetry, light scattering, and transmission electron microscopy. BSA in the assemblies retains over 90% of its activity, and the protein stability is enhanced because of the hydrophobic interaction between proteins and polymers. This approach allows us to prepare polymer–protein assemblies without bioconjugate synthesis. Meanwhile, possible damages to the protein structures and the loss of bioactivities of proteins can be avoided.
Synthesis and self-assembly of bioconjugates composed of proteins and synthetic molecules have been widely studied because of the potential applications in medicine, biotechnology, and nanotechnology. One of the challenging research studies in this area is to develop organic solvent-free approaches to the synthesis and self-assembly of amphiphilic bioconjugates. In this research, dialysis-assisted approach, a method based on unimer-aggregate equilibrium, was applied in the coassembly of lysozyme and conjugate of cholesterol and glutathione (Ch-GSH). In phosphate buffer solution, amphiphilic Ch-GSH conjugate self-assembles into vesicles, and the vesicle solution is dialyzed against lysozyme solution. Negatively charged Ch-GSH unimers produced in the unimer-vesicle exchange equilibrium, diffuse across the dialysis membrane and have electrostatic interaction with positively charged lysozyme, resulting in the formation of Ch-GSH-lysozyme bioconjugate. Above a critical concentration, the three-component bioconjugate molecules self-assemble into bioactive vesicles.
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