Influence of methanol and cyclodextrin cavity size on stoichiometry and binding constant of 3-[2-(9-anthryl)benzoxazol-5-yl]-alanine

A newly synthesized amino acid derivative 3-[2-(9-anthryl)benzoxazol-5-yl]-alanine was applied to study the influence of methanol concentration on equilibrium constant with β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD). For both CDs the formation of ternary CD/guest/methanol complexes were established. The increase of methanol concentration decreases the apparent equilibrium constant for β-CD, whereas for γ-CD an increase was observed up to about 1 M of MeOH and then monotonical decrease with further increase of alcohol concentration. Calculated from the dependence of apparent equilibrium constant on methanol concentration, equilibrium constants of binary complex between β-CD and amino acid is equal to 780 M−1, between β-CD and methanol 2 M−1 and 410 M−1 for ternary complex. The apparent thermodynamic parameters for complex formation of amino acid studied with β- and γ-CD in water/methanol mixture (2%, v/v) are: ΔS=−32.7±2.5 J/mol K, ΔH=−25.3±0.7 kJ/mol; ΔS=24.9±3.3 J/mol K, ΔH=−2.7±1.0 kJ/mol for β-CD and γ-CD, respectively.