pH-dependent complex formation of amino acids with β-cyclodextrin and quaternary ammonium β-cyclodextrin

Stability constants for the complexes of anionic, neutral (zwitterionic) and protonated forms of l- and d-enantiomers of eight amino acids with beta-cyclodextrin and the positively charged quaternary ammonium beta-cyclodextrin (QA-beta-CD, DS = 3.6 +/- A 0.3) have been determined by spectrophotometric and pH-potentiometric methods. The highest stability constants have been obtained for the aromatic amino acids phenylalanine, tyrosine and tryptophan. Except the dianion of tyrosine and QA-beta-CD, values for the anions in the range of 80-120 have been found, the stability constants for the zwitterionic forms are much smaller and complex formation is negligible with the protonated species. In the case of the other amino acids the differences are less pronounced. The results are interpreted in terms of hydrogen bonding, steric effects and electrostatic interactions between the amino acid moiety and the rims of the cyclodextrins, in addition to the inclusion of the side chain, and are supported by H-1 and C-13 NMR investigations on the systems containing l-phenylalanine and l-tyrosine. The differences between the complex formation constants of the l- and d-enantiomers do not exceed the limits of experimental error in most cases.