Inclusion of (aminostyryl)-1-methylpyridinium dyes by β-cyclodextrin and its use for fluorescent-probe studies on association of cationic and neutral molecules with β-cyclodextrin

Abstract

2-(or4-)-[4-(Dimethylamino)styryl]-1-methylpyridinium iodides (2-ASP and 4-ASP) form 1:1 inclusion complexes with β-cyclodextrin (β-CD). The association constant is about 1000 M-1 for both dyes in aqueous solution at 25°C. The complexation results in a 7.6- and 4.9-fold increase in the fluorescence intensities of 2-ASP and 4-ASP, respectively. The formation of inclusion complexes of n-alkylamines (n=4-10), n-alkylammonium ions (n=4-10), n-alkyl alcohols (n=4-9), and n-alkyltrimethylammonium bromides (CnTAB:n=12, 14, 16) with β-CD was studied by a fluorescent-probe method using 2-ASP as a probe. The association constants K were obtained from nonlinear least-square regression analysis of the fluorescence titration data. The logarithm of the 1:1 association constants varies approximately linearly with the number of carbon atoms (nc) of the alkyl chain up to nc=7 and then the dependence of log K on nc is less pronounced for longer alkyl chains: for CnTAB, 1:2 complexation was also observed. Combining the present result with a previous work [15], we found that the increasing order of stability of β-CD complexes of amphiphilic molecules with the same hydrocarbon tail but different head groups is ammonium < sulfonate < alcohol ≅ amine < sulfate. The association constants obtained from this method are compared with those given by other investigators and the discrepancies are discussed. © 1994 Kluwer Academic Publishers.