Stability constants for complex formation between alpha-cyclodextrin and some amines.

Complex formation of alpha-cyclodextrin with 15 amines (including seven 4-substituted anilines) was studied by the potentiometric method, supplemented by direct UV spectrophotometry and a competitive indicator spectrophotometric method. The data were analyzed in terms of 1:1 and 1:2 complexes (amine-cyclodextrin ratios) and the stability constants K11a, K12a, K11b, and K12b were evaluated; the subscripts indicate the stoichiometry and conjugate acid-base form. For all amines K11b was greater than K11a and K12a was 0. On the basis of the relationship of complex stability to amine structure, it was concluded that the primary binding site in anilines is the 4-substituent.