Solute–Solvent Interactions of Glycine, L-Alanine and L-Valine in Aqueous ZnCl₂ and CuCl₂ Media: A Volumetric, Viscometric and Acoustic Study
Author(s): Prof. Kalpana Shahi, Dr. Brameshwar, DPrashant Kumar
Publication #: 2609026
Date of Publication: 12.01.2022
Country: India
Pages: 1-13
Published In: Volume 8 Issue 1 January-2022
Abstract
The thermodynamic and transport properties of three biologically significant α-amino acids — glycine, L-alanine and L-valine — were systematically investigated in aqueous zinc chloride (ZnCl₂) and copper(II) chloride (CuCl₂) solutions over the temperature range 298.15–318.15 K. Apparent molar volumes (Vφ), Jones–Dole viscosity B-coefficients, and isentropic compressibilities (κs) were determined from precise density, viscosity and ultrasonic speed measurements as functions of amino acid concentration (0.05–0.50 mol L⁻¹) and salt molality (0.1–2.0 mol kg⁻¹). Limiting apparent molar volumes (Vφ°) were extracted via the Masson equation, and transfer volumes (ΔtVφ°) were computed by referencing Vφ° values in pure water. The viscosity data were rationalized through the Jones–Dole formalism to yield A and B coefficients, with the B-coefficient serving as a quantitative marker of ion–solvent interactions. Fourier-transform infrared (FTIR) spectra unambiguously confirmed inner-sphere coordination of Cu²⁺ through a bathochromic shift of the asymmetric COO⁻ stretching vibration (Δν = −23 to −25 cm⁻¹), diagnostic of bidentate N,O-chelation, whereas Zn²⁺ produced a smaller shift (Δν = −11 to −14 cm⁻¹) consistent with predominantly monodentate oxygen coordination. Transfer volumes for both ZnCl₂ and CuCl₂ media were substantially larger than the corresponding values previously reported for alkaline-earth chlorides (MgCl₂, CaCl₂), demonstrating the dominant role of partial covalent/chelate character in d-block cation–amino acid interactions. The order ΔtVφ°(Cu²⁺) > ΔtVφ°(Zn²⁺) mirrors the Irving–Williams stability series, providing thermodynamic evidence of stronger coordination with copper(II). The Hepler parameter (∂²Vφ°/∂T²)P was positive for all three amino acids, classifying them as net structure-makers in both electrolyte media. These findings offer quantitative molecular-level insight into protein folding environments relevant to metalloenzyme active sites and bioinorganic chemistry.
Keywords: amino acids; apparent molar volume; viscosity B-coefficient; isentropic compressibility; ZnCl₂; CuCl₂; Irving–Williams series; chelate coordination; transfer volumes; solute–solvent interactions
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