Inclusion complexes of cyclodextrins with 4-amino-1,8-naphthalimides

The formation of inclusion complexes between 4-amino-1,8-naphthalimides and cyclodextrins (CDs) was investigated. The naphthalimides used in the study were 4-amino-1,8-naphthalimide (I) and 4-(2-phosphonoethylamino)-N-(2-phosphonoethyl)-1,8-naphthalimide, tetraethylester (II). The CDs employed were alpha-CD, beta-CD, gamma-CD, HP-alpha-CD, HP-beta-CD and HP-gamma-CD (HP = hydroxypropyl). Evidence for complex formation was obtained from the changes in the fluorescence spectra of the dyes in the presence of increasing amounts of the CDs. The most striking changes were observed with HP-beta-CD and HP-gamma-CD. Treatment of the data using Benesi-Hildebrand plots was consistent with a 1:1 inclusion model. The determined stability constants were (K-eq, M-1): 106 (I:HP-beta-CD, pH = 2.0), 193 (I:HP-beta-CD, pH = 7.0), 113 (I:HP-gamma-CD, pH = 7.0), 155 (II:HP-beta-CD, pH = 2.0), 121 (II:HP-beta-CD, pH = 7.0), 301 (II:HP-gamma-CD, pH = 7.0). It can be concluded that compound I forms a more stable complex with HP-beta-CD than with HP-gamma-CD. Compound II, on the other hand, forms a more stable complex with HP-gamma-CD than with HP-beta-CD.