Acoustical and Volumetric Properties of L-Tyrosine in Phosphate Buffer at pH 6–8 with 0.1 m Aqueous Urea Solutions

A Study of Solute-Solvent Interactions

Abstract

This paper reports on the acoustical and volumetric behaviour of L-Tyrosine dissolved in phosphate buffer solutions at pH 6, 7, and 8 with 0.1 m aqueous urea as co-solvent, across a concentration range of 0.01 to 0.09 mol kg⁻¹ and temperatures from 303.15 to 328.15 K. Using experimentally determined density (ρ) and ultrasonic velocity (U) data, a comprehensive set of thermodynamic parameters was derived: adiabatic compressibility (βs), specific acoustic impedance (Z), compressibility lowering (Δβs), relative change in adiabatic compressibility (Δβs/β°), relative association (RA), apparent molal volume (φv), and partial molal volume at infinite dilution (φv°). L-Tyrosine presents a unique thermodynamic profile among aromatic amino acids owing to its para-hydroxyphenyl side chain, which confers both hydrophobic aromatic character and a hydrophilic phenolic hydroxyl group. This dual character is reflected in the intermediate behaviour of thermodynamic parameters between purely hydrophobic phenylalanine and the larger indole-bearing tryptophan. Density increases with concentration, decreases with temperature; ultrasonic velocity increases with both variables. Adiabatic compressibility decreases monotonically with concentration. Positive Sv values for L-Tyrosine confirm polar-polar dominated pair-wise interactions. Partial molal volumes at infinite dilution are intermediate between those of phenylalanine and tryptophan, consistent with the intermediate molecular size. The results are discussed in terms of tyrosine hydration characteristics, the role of the phenolic side chain in solvation, and the effect of pH and urea on the interaction thermodynamics.