Sadakatsu Nishikawa

Ultrasonic relaxation due to inclusion complex of amino acid by β-cyclodextrin in aqueous solution

Ultrasonic absorption coefficients in the frequency range of 0.8-95 MHz were measured in aqueous solutions of L-methionine or L-norleucine (guest) in the presence and absence of beta-cyclodextrin (beta-CD, host) at 25 degrees C. A single relaxational absorption was observed only in the solutions containing the guest and host. The ultrasonic relaxation was attributed to a perturbation of a chemical equilibrium associated with an interaction between beta-CD and the amino acid to form the host-guest complex. The kinetic and thermodynamic parameters in the system of L-norleucine with beta-CD were determined from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. Because of the concentration independence of the relaxation frequency in L-methionine system with beta-CD, the equilibrium constant and the standard volume change of the complexation reaction were estimated first from the concentration dependence of maximum absorption per wavelength, and subsequently the rate constants were calculated with the help of the estimated equilibrium constant and the observed relaxation frequency. The results obtained in this study were compared with those for systems of beta-CD with other amino acids or alcohols having comparable hydrophobicity.

Ultrasonic absorption of the oxyethylene group in aqueous nonelectrolyte solutions

The ultrasonic absorption from 6.5 to 220 MHz and velocities at 2.5 MHz have been measured in aqueous solution of triethylene glycol monobutyl ether as a function of concentration at 25°C. A single relaxational excess absorption, observed from 0.60 to 2.5 mol-dm−3, is attributed to a perturbation of an equilibrium associated with solute-solvent interaction. Rate constants for the forward and reverse processes have been determined from the concentration dependence of the relaxation frequency, and the influence on the water structure by an addition of the solute has been considered. Solution densities were also measured in order to obtain the expansivity of the solution. The standard volume and enthalpy changes of the reaction have been estimated from the concentration dependence of the maximum excess absorption per wave length. A linear relationship between the number of oxyethylene groups and the free energy change between bonded water and nonbonded water is established.

Rate and thermodynamic studies of the interaction between water and iso-butyl cellosolve by ultrasonic methods

Ultrasonic absorption and velocity measurements in aqueous solution of iso-butyl cellosolve (ethylene glycol iso-butyl ether) as a function of the concentration are reported. The two relaxational absorptions have been attributed to the perturbation of the equilibria expressed by AB⇌A+B and Aα(1/n)An where A is the solute, B is the solvent, AB is the complex and An is the solute aggregate. The rate constants for each step have been determined. From the concentration dependence of the maximum excess absorption per wave length, the enthalpy change and the volume change for the reaction between the solute and the solvent have been determined for aqueous solutions of butyl cellosolve (ethylene glycol n-butyl ether), iso-butyl cellosolve and propyl cellosolve (ethylene glycol n-propyl ether). The results are consistent with a hydrogen bonding reaction. The effect of the ethers on water structure are considered and it is clear that the fraction of water molecules which can hydrogen bond to the solute decreases with the increasing hydrophobicity of the solute.

Dynamic studies of the interaction between diols and water by ultrasonic methods. Part 4.—3-methylbutane-1,3-diol and 2,2-dimethylpropane-1,3-diol solutions

In order to justify the relationship between the structures of diols in aqueous media and their ultrasonic properties, measurements of ultrasonic absorption, sound velocity, density and viscosity have been made in aqueous solutions of 3-methylbutane-1,3-diol and 2,2-dimethylpropane-1,3-diol, which are isomers of each other. In both solutions, a single relaxational ultrasonic absorption has been observed in the frequency range 15–220 MHz. The absorption mechanism has been interpreted in terms of reaction kinetics associated with the interaction between the solute and solvent. As a result, the effect of the solute on the solvent (water) structure has been estimated, and it has been found that these diols act as water-structure promoters. Furthermore, the greater the hydrophobicity of the solute molecule, the more effectively it promotes the water structure. The trend in hydrophobicity determined from sound absorption has been confirmed from the concentration dependences of the compressibility. The correlation between the solvent structural parameters and the compressibility has been examined. The apparent molar volume has also been determined and is discussed with regard to the ultrasonic parameters.

Dynamic studies of the interaction between diols and water by ultrasonic methods. Part 2.—Pentane-1,5-diol and hexane-1,6-diol solutions

Ultrasonic absorption in the frequency range 15–220 MHz and ultrasonic velocities at 2.5 MHz have been measured in aqueous solutions of pentane-1,5-diol and hexane-1,6-diol at 20 °C. A relaxational excess absorption has been found in solutions of hexane-1,6-diol in the concentration range 1.5–6.0 mol dm–3, while no or little excess absorption has been observed in solutions of pentane-1,5-diol. The concentration dependences of the relaxation frequency and the maximum excess absorption per wavelength have been analysed by the chemical reaction model associated with the interaction between solute and solvent, and the rate and thermodynamic constants have been evaluated.

Ultrasonic relaxation in aqueous solutions of butanediols

Ultrasonic absorption and velocity have been measured to investigate the structural and dynamic properties of aqueous solutions of butane-1,4-diol and butane-1,2-diol at 20 °C. In the former solution no excess absorption was found, and in the latter solution a single relaxational process was observed in the frequency range 15–220 MHz. The excess absorption mechanism has been analysed as a solute–solvent interaction, AB ⇌ A + B, and the rate constants for butane-1,2-diol solution have been determined to be kf= 1.5 × 108 s–1 and kb= 1.6 × 108 dm3 mol–1 s–1 for the forward and backward steps, respectively. The influence of the position of hydroxy groups in the molecules on the water structure is discussed by a comparison with the results for solutions of the two dihydric alcohol.

Dynamic characteristic of amitriptyline in water by ultrasonic relaxation method and molecular orbital calculation.

Ultrasonic absorption coefficients in the frequency range of 0.8-220 MHz have been measured in aqueous solution of amitriptyline (3-(10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5-ylidene)-N,N-dimethyl-1-propanamine) in the concentration range from 0.20 to 0.60 mol dm(-3) at 25 °C. A single relaxational phenomenon has been observed, and the relaxation frequency is independent of the concentration. It has been also observed that the amplitude of the relaxational absorption increases linearly with the analytical concentration. From these ultrasonic relaxation data, it has been concluded that the relaxation is associated with a unimolecular reaction due to a conformational change of the solute molecule, such as a structural change due to a rotational motion of a group in the solute molecule. Molecular orbital semiempirical methods using AM1 (Austin model 1) and PM3 (modified neglect of diatomic overlap parametric method 3) have been applied to obtain the standard enthalpy of formation for amitriptyline molecule at various dihedral angles around one of the bonds in alkylamine side chain. The results have shown the two clear minimum standard enthalpies of formation for amitriptyline. From the difference of the two values, the standard enthalpy change between the two stable conformers has been calculated be 2.9 kJ mol(-1). On a rough assumption that the standard enthalpy change reflects the standard free energy change, the equilibrium constant for the rotational isomers has been estimated to be 0.31. Combining this value with the experimental ultrasonic relaxation frequency, the backward and forward rate constants have been evaluated. The standard enthalpy change of the reaction has been also estimated from the concentration dependence of the maximum absorption per wavelength, and it has been close to that calculated by the semiempirical methods. The ultrasonic absorption measurements have been also carried out in amitriptyline solution in the presence of β-cyclodextrin. However, the ultrasonic relaxation has not been found in the above frequency range. The result has been discussed in relation to the host-guest complex formation between β-cyclodextrin and amitriptyline.

Dynamic study of the interaction between diols and water by ultrasonic methods. Part 3.—2-methylpentane-2,4-diol solution

Ultrasonic absorption, velocity, density and viscosity have been measured in aqueous solutions of 2-methylpentane-2,4-diol at 25 °C as a function of the concentration. A single relaxational absorption has been found whose cause has been attributed to the perturbation of an equilibrium associated with the solute–solvent interaction AB⇌A + B, where A is the solute and B the solvent. The forward and backward rate constants for this interaction have been evaluated from the concentration dependence of the relaxation frequency as kf= 1.1 × 108 S–1 and kb= 1.1 × 108 dm3 mol–1 S–1. The standard volume and enthalpy changes of the reaction have also been determined from the concentration dependence of the maximum excess absorption per wavelength as ΔV= 0.070 cm3 mol–1 and ΔH= 1.7 kJ mol–1. At high concentrations (above 5.5 mol dm–3) another relaxation, which might be due to the interconversion of rotational isomers, has been also found in the same frequency range. This is confirmed from ultrasonic-absorption measurements in toluene mixtures.

Ultrasonic relaxation measurements in aqueous solution and molecular orbital calculation on imipramine.

Ultrasonic absorption coefficients have been measured in aqueous solution of imipramine {3-(10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N,N-dimethylpropan-1-amine} in the frequency range of 0.8-220 MHz at 25 °C. The frequency dependences of the observed absorption was characterized by a Debye-type relaxational equation with two relaxation frequencies, although only one relaxation had been observed in aqueous solutions of the related molecule amitriptyline. Both of the relaxation frequencies in imipramine solutions were found to be independent of the solute concentration and the amplitudes of the relaxational absorptions increase linearly with increasing solute concentration. It was therefore concluded that these two relaxations are associated with unimolecular reactions, such as a structural change due to rotational motions of the bond in the specified group in the imipramine molecule. To analyze quantitatively the source of the relaxations, semiempirical molecular orbital methods have been applied to determine the standard enthalpy of formation of the imipramine molecule at various dihedral angles around the bonds in the alkylamine side chain. According to the results, only one rotational motion of carbon-carbon bond in the side chain was found to be appropriate and the three minima of the standard enthalpy of formation was obtained as a function of the rotational angle. At the three minimum positions, the values of the standard enthalpy of formation are almost the same. With the assumptions (a) that rotational motion is not accompanied by a volume change of the reaction and (b) that the standard free energy change is close to the difference in the values between the standard enthalpies of formation, the equilibrium constants for the rotational isomerization have been calculated to be near unity. Hence, the forward and backward rate constants of the isomerization reactions are nearly the same. If one assumes that there are two kinds of rotational motions in one bond of the molecule, one proceeds with a rate constant on the order of 10(8) s(-1), whereas the other with a rate constant on the order of 10(6) s(-1). The faster and slower processes are also distinguished by the height of the standard enthalpy of formation.

Ultrasonic absorption in aqueous solutions of amino acids at neutral pH.

Ultrasonic absorption coefficients in aqueous solutions of glycine, L-alanine, imidazole, L-phenylalanine, L-histidine and L-tryptophan at neutral pH were measured in the range from 0.8 to 220 MHz at 25 degrees C. A characteristic ultrasonic relaxation phenomenon was observed only in the solution of L-histidine with a relaxation frequency at around 2 MHz at neutral pH. It was proposed from the concentration independent relaxation frequency and the linear concentration dependence of the maximum absorption per wavelength that the relaxation mechanism was associated with a perturbation of the rotational isomeric equilibrium of the L-histidine molecule. The existence of two rotational isomeric forms of L-histidine in water was examined by semiempirical quantum chemical methods, in order to determine the free energy difference between the two states. The forward and backward rate constants were determined from the relaxation frequency and the energy change. Also, the standard volume change of the reaction was estimated from the concentration dependence of the maximum absorption per wavelength. It was speculated that L-histidine fulfills a specific function among amino acids because of the rotational motion in the molecule, in addition to its well-established acid-base properties.