High hydrostatic pressure in the range of several dozen MPa, are generally assumed to be nonlethal but exert adverse impacts on growth of organisms that are adapted to atmospheric pressure. Deep-sea organisms have established intracellular mechanisms to cope with such extreme environments. A living cell is composed of myriad molecules including nucleic acids, proteins, lipids, ions, and various low molecular compounds. These molecules interact with each other transiently or statically, eventually eliciting innumerable intermolecular interactions even in a small microbial cell. The complexity hampers the understandings of adaptive mechanisms to high pressure employed by deep-sea organisms. Studies with model organisms such as a bacterium Escherichia coli and a yeast Saccharomyces cerevisiae offer breakthroughs to unravel the effects of high pressure on the complex intermolecular networks in living cells. This review summarizes recent advances in high-pressure biology as well as classic issues in this field, especially focusing on remodeling of intracellular systems to adapted to high-pressure environments.

Eukaryotic flagella and cilia generate rhythmical beating motions. When the structure is partially defective, the motility is severely impaired. We have developed a high-pressure microscope that is optimized both for the best image formation and for the stability to hydrostatic pressure up to 150 MPa. Here, we show that high hydrostatic pressure induces vigorous beating motion of Chlamydomonas flagella with defect structures. In addition, our results suggest that application of pressure increased intra-flagella Ca²⁺ concentration. The present technique could be extended to study other protein machineries and cell dynamics.

Hydrostatic pressure is one of the fundamental and significant external stimuli, and thus, has attracted attention in synthetic chemistry and photo-physical and -chemical processes. In considering weak interaction systems such as biological and supramolecular reactions on the basis of thermodynamics, enthalpy and entropy changes exquisitely keep the balance each other according to the enthalpy-entropy compensation law, meaning that it seems to be difficult to control reactions/rates by only either of these parameters. On the other hand, reaction/activation volume change (ΔV)-based hydorstatic pressure (P) control turns out to be an alternative to the conventional control method. In this review, we wish to report our recent hydrostatic pressure-spectroscopic results on ΔV as solvation, conformational change, molecular recognition, and biological reaction.

Recently isotactic poly(4-methyl-1-pentene) (P4MP1) has been found to selectively absorb long-chain alkanes from alkane mixed systems. In this article, we introduce the idea of this study from the pressure-temperature phase diagram. We also show that such selective absorption characteristics can be explained from a simple model, namely Asakura-Oosawa theory, and the relationship with molecular recognition is explained.

Host-guest interactions between a pair of naphthalene-based molecular tubes (i.e., anti-isomer and syn-isomer) and 1,4-dioxiane have been studied to understand selective recognition. Volume changes for complexation of the amide naphthotubes with 1,4-dioxane were investigated using high-pressure fluorescence and high-pressure NMR spectroscopy. We found that the partial molar volume change (ΔV°) for the association of 1,4-dioxane with the naphthotubes was －6.3±0.1 mL mol^－1 for the anti-isomer and 3.2±0.4 mL mol^－1 for the syn-isomer. To elucidate the molecular basis of ΔV°, molecular dynamics simulations of the complexation process were also performed. The difference in ΔV° was attributed to variations in the shape and hydration of naphthotube hydrophobic cavities.

We have begun to study phase transitions of phospholipid bilayer membranes under high pressure from the most representative model-membrane phospholipid, dipalmitoylphosphatidylcholine (DPPC), early in the 1990s and accumulated many bilayer phase diagrams of various kinds of phospholipids depending on pressure and temperature until now. In this review, the comprehensive study on the phospholipid bilayer membranes under high pressure based on the temperature and pressure phase diagrams is described and the correlation among thermodynamic variables, lipid molecular structures and membrane states is explained.

The recent collaboration between theory and experiment has led to the discovery of high-T_c superconductive hydrogen dominant compounds under ultra-high pressure over one million atmospheres which can be attained by the recent advances in technology using diamond anvil cell. Although there are still various difficulties in measuring the physical properties of materials under ultra-high pressure, high-pressure is expected as an important parameter for development of novel functional materials and search for physical properties, which cannot be realized at ambient pressure. In this article, I review the recent results on the search for pressure-induced superconductivity in sulfur hydride system.