NMR, surface tension and conductivity studies to determine the inclusion mechanism: thermodynamics of host-guest inclusion complexes of natural amino acids in aqueous cyclodextrins

The assembly of two natural amino acids (viz., L-arginine and L-histidine) as guests with alpha and beta-cyclodextrins as hosts to form inclusion complexes in aqueous medium has been demonstrated which are highly suitable for diverse applications in modern bio-medical sciences. The H-1 NMR study establishes the formation of inclusion complexes, while surface tension and conductivity studies confirm that the inclusion complexes have been formed with 1 : 1 stoichiometry. The nature of the complexes has been established using thermodynamic parameters, based on density, viscosity, and refractive index measurements. The contributions of different groups of the guest molecules towards the limiting apparent molar volume and the viscosity-B coefficient are determined and solvation numbers are calculated. All the parameters support the formation of the inclusion complexes, which are explained based upon hydrophobic effects, H-bonds, electrostatic forces and structural effects.