George Mogami

Hydration properties of adenosine phosphate series as studied by microwave dielectric spectroscopy.

Hydration properties of adenine nucleotides and orthophosphate (Pi) in aqueous solutions adjusted to pH=8 with NaOH were studied by high-resolution microwave dielectric relaxation (DR) spectroscopy at 20 °C. The dielectric spectra were analyzed using a mixture theory combined with a least-squares Debye decomposition method. Solutions of Pi and adenine nucleotides showed qualitatively similar dielectric properties described by two Debye components. One component was characterized by a relaxation frequency (f(c)=18.8-19.7 GHz) significantly higher than that of bulk water (17 GHz) and the other by a much lower f(c) (6.4-7.6 GHz), which are referred to here as hyper-mobile water and constrained water, respectively. By contrast, a hydration shell of only the latter type was found for adenosine (f(c)~6.7 GHz). The present results indicate that phosphoryl groups are mostly responsible for affecting the structure of the water surrounding the adenine nucleotides by forming one constrained water layer and an additional three or four layers of hyper-mobile water.

Spatial-Decomposition Analysis of Energetics of Ionic Hydration

Hydration energetics is analyzed for a set of ions. The analysis is conducted on the basis of a spatial-decomposition formula for the excess partial molar energy of the solute that expresses the thermodynamic quantity as an integral over the whole space of the solute-solvent and solvent-solvent interactions conditioned by the solute-solvent distance. It is observed for all the ionic solutes treated in the present work that the ion-water interaction is favorable at the expense of the water-water interaction and that the variations of the ion-water and water-water interactions with the ion-water distance compensate against each other beyond the contact distance. The extent of spatial localization of the excess partial molar energy is then assessed by introducing a cutoff into the integral expression and examining the convergence with respect to the change in the cutoff. It is found that the excess energy is not quantitatively localized within the first and second hydration layers, while its correlations over the variation of ions are good against the first-layer contribution.

Strong Dependence of Hydration State of F-Actin on the Bound Mg2+/Ca2+ Ions

Understanding of the hydration state is an important issue in the chemomechanical energetics of versatile biological functions of polymerized actin (F-actin). In this study, hydration-state differences of F-actin by the bound divalent cations are revealed through precision microwave dielectric relaxation (DR) spectroscopy. G- and F-actin in Ca- and Mg-containing buffer solutions exhibit dual hydration components comprising restrained water with DR frequency f2 (fw). The hydration state of F-actin is strongly dependent on the ionic composition. In every buffer tested, the HMW signal Dhyme (≡ (f1 - fw)δ1/(fwδw)) of F-actin is stronger than that of G-actin, where δw is DR-amplitude of bulk solvent and δ1 is that of HMW in a fixed-volume ellipsoid containing an F-actin and surrounding water in solution. Dhyme value of F-actin in Ca2.0-buffer (containing 2 mM Ca(2+)) is markedly higher than in Mg2.0-buffer (containing 2 mM Mg(2+)). Moreover, in the presence of 2 mM Mg(2+), the hydration state of F-actin is changed by adding a small fraction of Ca(2+) (∼0.1 mM) and becomes closer to that of the Ca-bound form in Ca2.0-buffer. This is consistent with the results of the partial specific volume and the Cotton effect around 290 nm in the CD spectra, indicating a change in the tertiary structure and less apparent change in the secondary structure of actin. The number of restrained water molecules per actin (N2) is estimated to be 1600-2100 for Ca2.0- and F-buffer and ∼2500 for Mg2.0-buffer at 10-15 °C. These numbers are comparable to those estimated from the available F-actin atomic structures as in the first water layer. The number of HMW molecules is roughly explained by the volume between the equipotential surface of -kT/2e and the first water layer of the actin surface by solving the Poisson-Boltzmann equation using UCSF Chimera.

Physical driving force of actomyosin motility based on the hydration effect

We propose a driving force hypothesis based on previous thermodynamics, kinetics and structural data as well as additional experiments and calculations presented here on water‐related phenomena in the actomyosin systems. Although Szent‐Györgyi pointed out the importance of water in muscle contraction in 1951, few studies have focused on the water science of muscle because of the difficulty of analyzing hydration properties of the muscle proteins, actin, and myosin. The thermodynamics and energetics of muscle contraction are linked to the water‐mediated regulation of protein–ligand and protein–protein interactions along with structural changes in protein molecules. In this study, we assume the following two points: (1) the periodic electric field distribution along an actin filament (F‐actin) is unidirectionally modified upon binding of myosin subfragment 1 (M or myosin S1) with ADP and inorganic phosphate Pi (M.ADP.Pi complex) and (2) the solvation free energy of myosin S1 depends on the external electric field strength and the solvation free energy of myosin S1 in close proximity to F‐actin can become the potential force to drive myosin S1 along F‐actin. The first assumption is supported by integration of experimental reports. The second assumption is supported by model calculations utilizing molecular dynamics (MD) simulation to determine solvation free energies of a small organic molecule and two small proteins. MD simulations utilize the energy representation method (ER) and the roughly proportional relationship between the solvation free energy and the solvent‐accessible surface area (SASA) of the protein. The estimated driving force acting on myosin S1 is as high as several piconewtons (pN), which is consistent with the experimentally observed force.

Spatial Distribution of Ionic Hydration Energy and Hyper-Mobile Water

In this chapter, we provide the following two topics. 1: We carry out DRS measurements for divalent metal chloride and trivalent metal chloride solutions and clarify the hydration states. All the tested solutions have hyper-mobile water (HMW) with higher dielectric relaxation frequency f1 (~20 GHz) than that of bulk water (12.6 GHz at 10 °C), and dispersion amplitude of HMW is aligned to Hofmeister series. According to the correlation between an intensity of HMW signal and water structure entropy, HMW can be a scale for the water structure. 2: We carry out the spatial-decomposition analysis of energetics of hydration for a series of ionic solutes in combination with molecular dynamics (MD) simulation. The hydration analysis is conducted on the basis of a spatial-decomposition formula for the excess partial molar energy of the ion that expresses the thermodynamic quantity as an integral over the whole space of the ion–water and water–water interactions conditioned by the ion–water distance. In addition, we examine the correlation between the electric field formed by ion and the number of HMW around ion.

Novel Intermolecular Surface Force Unveils the Driving Force of the Actomyosin System

In this chapter, we discuss the role of water in actomyosin-force generation. We have been investigating the hydration properties of ions, organic molecules, and proteins. These studies revealed that actin filaments (F-actin) are surrounded by a hyper-mobile water (HMW) layer and restrained water layer, while myosin subfragment 1 (S1) has only a typical restrained hydration layer. The understanding of the physicochemical properties of HMW has been greatly advanced by recent theoretical studies on statistical mechanics and solution chemistry. To explain the mechanism of force generation of actomyosin using ATP hydrolysis, we propose a driving force hypothesis based on novel intermolecular surface force. This hypothesis is consistent with the reported biochemical kinetics and thermodynamic parameters for the primary reaction steps. The gradient field of solvation free energy of S1 is generated in close proximity to F-actin.