DNA-guided self-assembly provides a versatile route for organizing nanoparticles into ordered superlattices with tunable interparticle distances and crystal symmetries. This article outlines the fundamentals of DNA-functionalized nanoparticle crystallization and reviews recent progress with a focus on polyethylene glycol-assisted crystallization of DNA-functionalized gold nanoparticles. Beyond sequence-programmed hybridization, solution composition plays a crucial role in determining nucleation, crystal growth, lattice symmetry, and structural stability. Perspectives for high-quality single-crystal growth and functional materials design based on dimension-controlled nanoparticle assemblies are also discussed.

Water exhibits remarkable polymorphism, with more than twenty crystalline ice phases identified to date. Understanding their crystal structures, hydrogen ordering, and pressure-induced structural transformations requires precise determination of hydrogen positions, making neutron diffraction an indispensable technique. However, the combination of neutron diffraction and high-pressure experiments has long been hindered by severe technical challenges arising from the small sample volumes required at high pressure. This article reviews our efforts to overcome these limitations through the development of advanced high-pressure neutron diffraction techniques, particularly the MITO system for individual pressure and temperature control and the integration of nano-polycrystalline diamond anvil cells with the PLANET beamline at J-PARC. These developments have enabled detailed investigations of hydrogen ordering and dynamics in high-pressure ice polymorphs. Studies on ice XV and the newly discovered ice XIX revealed the existence of partially ordered states and demonstrated that a single hydrogen-disordered phase can transform into distinct ordered phases depending on the pressure–temperature pathway. Time-resolved neutron diffraction measurements of the ice VII–VIII transition further uncovered a crossover between molecular rotational and proton translational motions that governs hydrogen-ordering kinetics. Extending neutron diffraction measurements beyond 100 GPa finally made it possible to directly observe the evolution of the hydrogen distribution toward the symmetrized hydrogen bond in ice X. These achievements illustrate how advances in experimental methodology have opened new opportunities for exploring the structural complexity of ice and hydrogen-bonded materials under extreme conditions.

Nucleic acid structural biology has greatly advanced our understanding of the structural principles governing nucleic acid folding and molecular recognition. This review summarizes my studies on RNA-targeted drug discovery, DNA nanotechnology, and structural biomimetics, illustrating how nucleic acid structural biology has expanded into these research areas. These studies demonstrate that elucidating the structural principles underlying nucleic acid folding and molecular recognition provides a common foundation not only for understanding biological functions but also for the rational design of functional molecules and nanomaterials.

The three Frank-Kasper basic structures of clathrate hydrates have long been theoretically defined, yet the hexagonal form HS-I had remained experimentally elusive for over 70 years. Using a purpose-designed asymmetric quaternary ammonium salt（N4446Cl）, we achieved the first stable synthesis of HS-I, confirmed by single-crystal X-ray diffraction. HS-I shows high CH₄ and CO₂ storage capacity, offering promise for carbon capture applications.

Martyite Zn₃（V₂O₇）（OH）₂・2H₂O is a mineral with a honeycomb lattice of H₂O in a porous framework. Due to geometrical frustration and its two-dimensional nature, H₂O molecules are dynamically disordered at room temperature. In this study, we reveal ferroaxial ordering of H₂O in martyite, driven by the formation of hydrogen-bonded toroidal H₂O hexamers. Our results demonstrate that martyite is a model system for studying 2D ice, which may provide a clue to understanding the polymorphism of H₂O.

Neutron diffraction is one of the most sensitive methods for analyzing hydrogen（especially deuterium）. Clathrate hydrates have a cage-like hydrogen bonding network that encloses guest molecules. The temperature dependence of atomic displacement parameters in hydrogen networks was investigated using powder neutron diffraction as the scattering-length density distribution. Hydrogen sulfide hydrate has a spread from the line between oxygen atoms, and the scattering-length density distribution changes with temperature. The low-temperature phase of ethane hydrate also has a bent hydrogen bonding between oxygen atoms. These anharmonic distributions arise from the thermal motion of guest molecules and the resulting interactions between them and the host structure.

The compositional and structural variety in salt-water systems expands in high-pressure conditions. This leads to the difficulty of modelling planetary interiors. The recent findings of two high-pressure phases of salt hydrate（KCl･H₂O and NaCl･13H₂O）exhibited structural similarity with simple inorganic salt and pure water ice. These provide a deeper understanding of the physicochemical relation between ionic species and water molecules via coordination, electrostatic interaction, and hydrogen bonds.