Induction of metachromasia in cationic dyes and fluorochromes using a clay mineral: a potentially valuable model for histochemical studies.

The use of simple model substrates to analyze conditions and mechanisms of metachromatic reactions is an important strategy in histochemical studies. In this study we show that in the presence of the three-layered silicate clay montmorillonite, diluted solutions of thionine and toluidine blue develop an immediate metachromatic shift (from blue to reddish violet). Likewise, hypoemission and red shift in the fluorescence spectra of acridine orange, pyronine Y and ethidium bromide appear when montmorillonite is added to the dye solution. Cationic dyes could insert as stacked structures into the negatively charged interlamellar spaces of the clay. These spectral results indicate that on account of its strong aggregating capacity, montmorillonite is a suitable model substrate to study metachromatic reactions.