Comparative Physicochemical Investigation of the Inclusion Compounds of Cyclodextrins with Arginine and Histidine Stereoisomers

The inclusion complexes of α-, β-cyclodextrin and 2-hydroxypropyl-α-cyclodextrin with two amino acid stereoisomers (L-, D-arginine and L-, D-histidine) were studied by using differential scanning calorimetry, thermogravimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy methods. The solid inclusion compounds were prepared in a 1:1 molar ratio of the host and guest using the co-precipitation method. The pH measurements and structural visualization of complexes were carried out. The obtained results proved the formation of the complexes and revealed that the size and the symmetry of the cyclodextrin (CD) and also the structure and flexibility of the amino acid molecule had a significant influence on the complexation interaction. The correlation of the experimental data shows that βCD has a preference to form more stable complexes with levogir isomers of amino acid than αCD and 2-hydroxypropyl-α-cyclodextrin. The complexation of the amino acids isomers was accomplished by partial inclusion of the guest molecule in the CD cavity, and it was observed that CDs could better discriminate between histidine isomers than between arginine isomers.