Kentaro Shiraki

Carbon nanotubes (CNTs) exhibit exceptional properties, including high thermal conductivity, mechanical flexibility, and near-infrared luminescence. These properties can be further tuned through CNT surface functionalization. Aryl diazonium salts are widely used for such functionalization because they react under mild conditions. Although the CNT functionalization efficiency depends on the solvent, the factors contributing to these solvent effects remain insufficiently understood. This study systematically investigated the effect of five representative organic solvents—dimethylformamide, isopropyl alcohol, tetrahydrofuran, N-methyl-2-pyrrolidone, and dimethyl sulfoxide (DMSO)—on the degree of CNT functionalization using aryl diazonium reagents. The degree of functionalization was evaluated using photoluminescence spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The solvent effect on CNT functionalization was attributable to three factors: CNT dispersibility, the chemical stability of the aryl diazonium reagent, and the binding affinity of the reagent to CNTs; the relative contributions depend on the reagent concentration. Among the solvents tested, DMSO was the most effective for CNT functionalization over a wide range of reagent concentrations. Furthermore, a homogeneous CNT–protein composite film exhibiting near-infrared photoluminescence was fabricated using DMSO. These findings provide mechanistic insights into solvent effects in diazonium-based CNT functionalization and offer guidelines for solvent selection in CNT functionalization and CNT-based composite fabrication.

Molecules in solution continually change their orientation through thermal motion. The rate of this reorientation reflects the friction between solutes and their surroundings at the local environme...

Liquid–liquid phase separation (LLPS) of intrinsically disordered proteins is highly sensitive to environmental conditions, yet the molecular basis of sharp temperature responsiveness remains poorly understood. Here, we investigate a sequence-encoded mechanism underlying the pronounced temperature sensitivity of phase separation using Sup35NM, the intrinsically disordered domain of the yeast prion Sup35. We show that a tyrosine-rich local structural region within this domain encodes strong temperature responsiveness of droplet formation. Mutational analyses reveal that tyrosine residues mediate both intramolecular interactions that stabilize local structure and intermolecular interactions required for LLPS. Substitution of the tyrosine residues with alanine disrupts local structure and weakens intermolecular interactions, thereby diminishing temperature sensitivity. In contrast, substitution with phenylalanine promotes rapid droplet gelation, abolishes internal fluidity, suppresses amyloid formation, and confers resistance to temperature-induced dissolution. Based on these findings, we propose a molecular model in which finely tuned, moderately weak aromatic interactions among tyrosines enable reversible local compaction that is sensitive to temperature, thereby generating a sharp phase transition. These results suggest that amino acid sequences encode not only phase separation propensity but also the sensitivity of condensates to environmental perturbations, providing a framework for understanding how intrinsically disordered proteins act as molecular sensors of cellular conditions.

Protein and nucleic acid play central roles in biology and pharmaceuticals. Both share a similar architecture made of a backbone and side chains. Protein has a peptide backbone and various side chains, whereas nucleic acid has a phosphate backbone and aromatic side chains. However, they are significantly different in the chemical properties of the backbone and side chains. The protein backbone is uncharged, while nucleic acid backbone is negatively charged. The protein side chains comprise widely different chemical properties. On the other hand, the nucleic acid side chains comprise a uniform chemical property of aromatic bases. Such differences lead to fundamentally different folding, molecular interactions and co-solvent interactions, which are the focus of this review. In regular protein secondary structures, the peptide groups form polar hydrogen bonds, making the interior hydrophilic. The side chains of different chemical properties are exposed on the outside of the protein secondary structures and participate in molecular and co-solvent interactions. On the other hand, hydrophobic/aromatic nucleobase side chains are located inside the typical double helix or quadruplex structures. The charged phosphate groups of the nucleic acid backbone are located outside, participating in electrostatic interactions. The nucleobases are also involved in molecular interactions, when exposed in breaks, hairpins, kinks and loops. These structural differences between protein and nucleic acid confer different interactions with commonly used co-solvents, such as denaturants, organic solvents and polymers.

Protein and nucleic acid play central roles in biology and pharmaceuticals. Both share a similar architecture made of a backbone and side chains. Protein has a peptide backbone and various side chains, whereas nucleic acid has a phosphate backbone and aromatic side chains. However, they are significantly different in the chemical properties of the backbone and side chains. The protein backbone is uncharged, while nucleic acid backbone is negatively charged. The protein side chains comprise widely different chemical properties, while the nucleic acid side chains comprise a uniform chemical property of aromatic bases. Such differences lead to fundamentally different folding and molecular interactions and co-solvent interactions, which are the focus of this review. In regular protein secondary structures, the peptide groups form polar hydrogen bonds, making the interior hydrophilic, while the side chains of different chemical properties are exposed on the outside and participate in molecular interactions and co-solvent interactions. On the contrary, hydrophobic/aromatic nucleobase side chains locate inside the typical double helix or quadruplex structures and charged phosphate groups of the nucleic acid backbone locate outside participating in molecular interactions. The nucleobases are also involved in molecular interactions, when exposed in breaks, hairpins, kinks and loops. These structural differences between protein and nucleic acid confer different interactions with commonly used co-solvents, such as denaturants, organic solvents and polymers.

Proteins occupy various states, including liquid-like condensates, amorphous aggregates, and amyloid fibrils, each linked to distinct biological functions and pathologies. While the transition from liquid-like condensates to amyloids has been extensively studied, kinetic relationship between amyloid and other metastable solid states remain unclear, as bulk-scale experiments inevitably lead to the conver-sion of metastable phases into the most stable phase. We developed a droplet-based microfluidic sys-tem that quantifies amyloid nucleation and metastable amorphous aggregate (AA) formation. Using the yeast prion protein Sup35, we found that condensates convert into both amyloids and AAs, and that AA formation suppresses amyloid formation in size-dependent manner at the micrometer scale. Further-more, we demonstrated the well-known amyloid inhibitor (−)-epigallocatechin-3-gallate paradoxically promoted amyloid formation at low concentrations by modulating AA and amyloid nucleation kinetics. These findings provide fundamental insights into protein phase transitions in vivo and may inform nov-el therapeutic strategies targeting metastable aggregates of amyloidogenic proteins.

Immunoglobulin A (IgA) is the most abundant antibody in the human body and plays a critical role in defending against bacteria and viruses that invade through mucosal surfaces. Leveraging this property, recombinant IgA has been developed as a preventive drug for infectious diseases such as COVID-19, influenza, and tuberculosis. Recombinant IgA is typically purified using columns packed with protein-immobilized resins, such as jacalin, Protein L, or nanobody-based materials. However, these methods have several drawbacks, including high costs due to expensive resins and mobile phases, the need for multiple purification steps, and the potential leakage of immobilized proteins. To address these issues, the present study proposes an alternative IgA purification method using zirconia (ZrO₂) particles. Purification was performed for three forms of IgA─monomeric, dimeric, and secretory component-bound IgA─using both batch adsorption and chromatography. All three forms were successfully purified with high purity. The method employs phosphate buffers at mildly acidic and neutral pH as the mobile phase, which helps preserve the structure of IgA. Because ZrO₂ particles are both cost-effective and chemically and mechanically robust, the proposed method overcomes the limitations associated with conventional protein-immobilized resins in IgA purification.