Biochemical Analysis of Solute–Solvent Interactions of Aromatic Amino Acids in Denaturing Environments Relevant to Protein Folding

Abstract

A systematic and comparative thermodynamic investigation of three aromatic amino acids — DL-Phenylalanine, L-Tryptophan, and L-Tyrosine — in phosphate buffer solutions at pH 6, 7, and 8 with 0.1 m aqueous urea has been conducted at concentrations ranging from 0.01 to 0.09 mol kg⁻¹ and temperatures from 303.15 to 328.15 K. Density and ultrasonic velocity data were used to calculate 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°) for all three amino acids. This work provides a unified analysis that highlights how the chemical nature of the aromatic side chain governs the solvation thermodynamics. Phenylalanine, with a purely hydrophobic benzyl side chain, shows negative Sv values indicative of non-polar-polar type interactions; tryptophan and tyrosine, with polar or amphiphilic aromatic side chains, show positive Sv values consistent with polar-polar interactions. Partial molal volumes follow the order L-Tryptophan > DL-Phenylalanine ≈ L-Tyrosine. Adiabatic compressibility decreases with concentration for all three amino acids, and all show RA values greater than unity, but the magnitude of these effects varies systematically with side-chain character. The influence of pH (6–8) and temperature (303.15–328.15 K) on the thermodynamic parameters is analysed and rationalized in terms of the ionization state of the amino acid, the buffer ion interactions, and the thermal disruption of hydration structures. Collectively, these results establish a comprehensive thermodynamic framework linking molecular structure to solution thermodynamic properties of aromatic amino acids in physiologically relevant mixed aqueous media.