Anion-Dependence of Fast Relaxation Component in Na‑, K‑Halide Solutions at Low Concentrations Measured by High-Resolution Microwave Dielectric Spectroscopy

High-resolution microwave dielectric spectra of NaX, KX (X: F, Cl, Br, I) aqueous solutions of <i>c</i> = 0.05 and 0.1 M measured in the frequency range 0.2–26 GHz at 10 °C are analyzed. The dielectric relaxation (DR) spectrum of each solution, which deviates slightly from the bulk-water spectrum, is mathematically divided into the bulk-water spectrum and the spectrum of solute particles covered with a water layer using a mixture theory by assuming the existence of continuous bulk-water phase. The solute spectra above 3 GHz are fitted with a linear series of pure water component (γ dispersion with DR frequency <i>f</i>_w), fast Debye component–1 with DR frequency <i>f</i>₁ (><i>f</i>_w), and slow Debye component–2 with DR frequency <i>f</i>₂ (<<i>f</i>_w). Component–2 is only found for the fluorides. The sum of dispersion amplitudes of γ and components −1 and −2 for NaX and KX are found to be almost irrespective of X and equal to the pure water level, indicating that components −1 and −2 are from the water modified by ions, thus denoted as “hypermobile water” and “constrained water” (not rigidly bound to ion), respectively. Below 3 GHz, sub-GHz dispersion component is detected and assigned as a relaxation response of counterion cloud. The resulting limiting-molar conductivities of NaX and KX are in good agreement with the literature data measured at much lower frequencies. The estimated number of hypermobile water molecules is found to increase from 9 to 31 for NaX and from 9 to 37 for KX with increasing anion size. Thus, except for the fluorides, it is reported that the modified water by salt ions exhibits only Debye component–1 other than γ dispersion, indicating the existence of a water–ions collective hypermobile mode in each solution

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 <i>f</i>₂ (<<i>f</i>_w: DR frequency of bulk solvent, 17 GHz at 20 °C) and hypermobile water (HMW) with DR frequency <i>f</i>₁ (><i>f</i>_w). The hydration state of F-actin is strongly dependent on the ionic composition. In every buffer tested, the HMW signal <i>D</i>_hyme (≡ (<i>f</i>₁ – <i>f</i>_w)­δ₁/(<i>f</i>_wδ_w)) of F-actin is stronger than that of G-actin, where δ_w 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. <i>D</i>_hyme value of F-actin in Ca2.0-buffer (containing 2 mM Ca²⁺) is markedly higher than in Mg2.0-buffer (containing 2 mM Mg²⁺). Moreover, in the presence of 2 mM Mg²⁺, the hydration state of F-actin is changed by adding a small fraction of Ca²⁺ (∼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 (<i>N</i>₂) 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 –<i>kT</i>/2e and the first water layer of the actin surface by solving the Poisson–Boltzmann equation using UCSF Chimera