Interaction of urea with fluorophores bound to cyclodextrins. Fluorescence of p-toluidino naphthalene sulphonate

Abstract

The effect of urea on the fluorescence properties of p-toluidino naphthalene sulphonate (TNS) bound to α- and β-cyclodextrin (CDx) has been studied by steady-state and time-resolved emission spectroscopy. From the observed emission yield, lifetime and emission maxima the polarity of the α-CDx and β-CDx environment is estimated. Addition of urea to aqueous CDx solutions decreases the fluorescence yield and lifetime of TNS. The effect is more pronounced for α-CDx and less for β-CDx. The results are interpreted in terms of the model that urea increases the local polarity of the CDx surface and, in the case of α-CDx, displaces TNS molecules form the cyclodextrin interface. The TNS molecules are easily accessed by the urea molecule in the smallest α-CDx where a major part of the bound TNS molecule is projected outside the cavity. In β-CDx two kinds of TNS: β-CDx complexes (1:1 and 1:2) are formed. Addition of urea causes removal of TNS molecules bound to CDx in the 1:1 complex, but TNS in the 1:2 complex remains more or less unaffected because urea cannot approach it.