Thermodynamic and spectroscopic studies of alanine and phenylalanine in aqueous β-cyclodextrin solutions

Ultrasonic speed, u and density, ρ have been measured for dl-alanine (Ala) and l-phenylalanine (Phe) in aqueous β-cyclodextrin (β-CD) at 298.15, 303.15, 308.15, and 313.15 K. The complexation of Ala and Phe with β-CD has been studied by means of UV-vis and thermodynamic (ultrasonic speed and density) studies. Using the measured ultrasonic speed and density data the apparent molar compressibility (κS,φ), apparent molar volume (Vφ), limiting apparent molar compressibility (κS,φ0), limiting apparent molar volume (Vφ0), their constants (SK and Sv), and hydration number (nH) have been obtained. The positive values of transfer properties at infinite dilution for Ala and Phe in β-CD is the outcome of the balance between released water molecules from β-CD cavity and hydrophobic groups of Ala and Phe that enter into the macrocycle β-CD cavity. The experimental results have also been discussed on the basis of UV-vis absorbance. The results indicate the formation of a more stable host-guest complex between Phe-β-CD than between Ala-β-CD.