Facile dimerization and circular dichroism characteristics of 6-O-(2-sulfonato-6-naphthyl)-β-cyclodextrin

Abstract

A monofunctional derivative of β-cyclodextrin (β-CD) containing a 2-sulfonato-6-naphthyl group at the primary face (1: β-CD-NS) was prepared. The concentration dependences of fluorescence, circular dichroism (c.d.), and NMR spectra of 1 have been investigated and compared with the spectral changes associated with complexation of 6-methoxy-2-naphthalenesulfonate (MNSS) with β-CD. When the concentration of 1 was increased, the following were observed: the enhancement of the concentration-normalized fluorescence intensity; an increase in molar ellipticities of the negative band around 280 nm; a more pronounced exciton coupling band around 230 nm in c.d. spectra; and an upfield shift of NMR signals from aromatic protons. The concentration dependent fluorescent intensity and molar ellipticity data fitted well to the monomerdimer equilibrium of 1 with the dimerization constant of 9700 ± 2500 M -1. The dimerization constant is much larger than the association constants of MNSS with β-CD (510 ± 40 M -1) and 1 with β-CD (430 ± 50 M -1). The naphthyl groups in the dimer do not show an excimer band in fluorescence spectra but exhibit exciton coupling bands around 230 and 330 nm. These indicate that 1 forms a head-to-head type dimer, which is stabilized by mutual inclusion of the 2-sulfonato-6-naphthyl moieties deeply into β-CD cavities of counter molecules. The naphthyl moiety of the monomer of 1 is in a similar environment to that in a 1/noctyl sulfate complex and appears to cap the primary face of β-CD loosely. Compared to the c.d. spectrum of the MNSS/β-CD complex, the spectrum of the 1/β-CD complex is similar in shape, but the rotational strengths of the c.d. bands are much greater.