Interactions between uracil (Ura) and peptides, glycyl-L-tyrosine (Gly-Tyr) and glycyl-L-glutamic acid (Gly-Glu), in a phosphate-buffered saline (pH = 7.4) were studied in temperature range of 288–313 K by a densimetry method. Using the obtained density data, the apparent molar volumes of Ura, standard apparent molar volumes at infinite dilution and their derivatives with respect to temperature have been calculated. The concentration dependences of the apparent molar volume of uracil have a maximum, the position of which characterizes the 1:1 stoichiometry in the resulting uracil:peptide complexes. The standard apparent molar volume of neutral uracil increases upon complexation with charged peptide particles, such as +GlyTyr− zwitterions and +GlyGlu2− anions. The effects of Ura concentration and temperature on the volume properties were discussed in terms of predominant types of molecular interactions (hydrophilic, hydrophobic, ionic and zwitterionic) on the basis of co-spheres overlap model. The positive values of the second derivative of the standard apparent molar volume of uracil with respect to temperature at constant pressure indicate an increase in the ordering of the solvent in the environment of uracil with the addition of peptides, which may be associated with the blocking of its polar groups due to hydrogen bonding with the peptides. The addition of the more hydrophobic peptide, glycyl-tyrosine, which takes the zwitterionic form in solution, has a stronger structuring effect compared to the more hydrophilic anions of glycyl-glutamic acid.
