Spectrofluorimetric determination of stoichiometry and association constants of the complexes of harmane and harmine with β-cyclodextrin and chemically modified β-cyclodextrins

Abstract

The association characteristics of the inclusion complexes of the β-carboline alkaloids harmane and harmine with β-cyclodextrin (β-CD) and chemically modified β-cyclodextrins such as hydroxypropyl-β-cyclodextrin (HPβ-CD), 2,3-di- O-methyl-β-cyclodextrin (DMβ-CD) and 2,3,6-tri- O-methyl-β-cyclodextrin (TMβ-CD) are described. The association constants vary from 112 for harmine/DMβ-CD to 418 for harmane/HPβ-CD. The magnitude of the interactions between the host and the guest molecules depends on the chemical and geometrical characteristics of the guest molecules and therefore the association constants vary for the different cyclodextrin complexes. The steric hindrance is higher in the case of harmine due to the presence of methoxy group on the β-carboline ring. The association obtained for the harmane complexes is stronger than the one observed for harmine complexes except in the case of harmine/TMβ-CD. Important differences in the association constants were observed depending on the experimental variable used in the calculations (absolute value of fluorescence intensity or the ratio between the fluorescence intensities corresponding to the neutral and cationic forms). When fluorescence intensity values were considered, the association constants were higher than when the ratio of the emission intensity for the cationic and neutral species was used. These differences are a consequence of the co-existence of acid–base equilibria in the ground and in excited states together with the complexation equilibria. The existence of a proton transfer reaction in the excited states of harmane or harmine implies the need for the experimental dialysis procedure for separation of the complexes from free harmane or harmine. Such methodology allows quantitative results for stoichiometry determinations to be obtained, which show the existence of both 1:1 and 1:2 β-carboline alkaloid:CD complexes with different solubility properties.