Volumetric, Viscometric, and DFT Studies on the Molecular Interactions of l -Alanine and l -Valine in Aqueous Cytidine at Different Temperatures

Abstract Understanding drug–amino acid interactions in aqueous solution is crucial for predicting physicochemical behavior relevant to drug formulation and biomolecular recognition. This work examines the temperature-dependent interactions of Cytidine (CYTIDINE) with l-valine and l-alanine in water (293.15–308.15 K) using integrated thermodynamic measurements and computational analysis. The results indicate that solute–solvent interactions dominate over solute–solute associations in both ternary systems, with CYTIDINE exhibiting structure-breaking behavior. Interaction strength is consistently greater in the CYTIDINE–l-valine system than in CYTIDINE–l-alanine, attributed to enhanced hydrophobic contributions from the bulkier side chain of l-valine. Gibbs free energy-derived binding affinity and noncovalent interaction analysis from optimized geometries support the experimental observations. This study establishes a direct correlation between temperature-dependent macroscopic thermodynamic behavior and molecular-level interaction patterns in nucleoside–amino acid aqueous systems, providing new insight into hydrophilic–hydrophobic balance relevant to biopharmaceutical solution chemistry.