Molecular Dynamics of the Interaction of Some Alkaline-Earth Cations with Some Pentoses Undergoing 4C1 ⇌ 1C4 Isomerization

Abstract

Ultrasonic relaxation spectra in the frequency range 0.08−400 MHz of the ions Ca2+ or Ba2+, interacting with the pentoses d-(−)-ribose, d-(−)-lyxose, and d-(−)-arabinose at concentrations of 0.5 M and molar ratio (pentose/metal2+) = 1 at 25 °C, are reported. For Ca2+ or Ba2+ with d-(−)-ribose ultrasonic relaxation spectra at various concentrations up to c = 0.7 mol/dm3 at 25 °C are also reported. The ultrasonic spectra can be interpreted by the sum of three Debye relaxation processes. The one at the lowest relaxation frequency (∼0.07−0.25 MHz) is ascribed to the 4C1 ⇌ 1C4 isomerization of the carbohydrates. The relaxation process at intermediate frequency (∼10−12 MHz for Ca2+ or 50−60 MHz for Ba2+) is interpreted as due to the partial desolvation of the cations by the carbohydrate. The “fast” relaxation process at 120−160 MHz is attributed to the diffusion-controlled approach and initial desolvation of the cation and carbohydrate. The two faster processes are interpreted according to the multistep Eigen−Tamm complexation mechanism: M2+ + carbohydrate ⇆ (k1/k-1) M2+···carbohydrate ⇆ (k2/k-2) M2+ carbohydrate.