Structure‐staining relationships in histochemistry and biological staining

Correlations between the structural features of dyes and staining performance for elastic fibres were investigated. Dyes studied included the traditional stains (such as Gomori's Aldehyde‐Fuchsin and Weigert's Resorcin‐Fuchsin), acid dyes used from alkaline aqueous‐organic solvent mixtures (the Horobin‐James system), and basic dyes used from acidic aqueous‐ethanolic mixtures (the Taenzer‐Unna system). In all three classes effective elastic fibre stains had large conjugated bond numbers, and were often hydrophobic (i.e. had high Hansch π values). By choosing dyes with conjugated bond numbers at or over a critical value (25 for the TU system, 35 for the HJ) it is possible to select new and effective dyes for use in the HJ and TU staining systems. Mechanistically these results support the view that for typical commercial dyes and also for the traditional stains van der Waals attractions provide the important contributions to dye‐elastic fibre affinities, with hydrophobic bonding playing a subsidiary role. However, supporting the views of Lillie, it was also noted that even hydrophilic dyes of low conjugated bond number could stain elastic fibres, if the dye carried a sufficiently reactive primary amino group as a substituent. The additional substituent groupings needed to generate such reactivity have been specified, for both acidic and alkaline reaction conditions.