Abstract
The staining properties of 69 color indicator dyes and 31 fluorescent indicators in isooctane (2,2,4-trimethylpentane) solution were studied. Thirty one of 56 indicators of pK below approximately 6 stained test tissue sections, 25 with the basic color, 2 with the acid color, and 4 with colors difficult to characterize. Conversely, only 10 of 44 indicators with pK's above 6 stained tissue sections, 5 with the acid color, 3 with the basic color, and 2 with unassignable color. This behavior pattern is independent of the chemical nature of the indicator dye, and appears to reflect a general property of the form of combination between tissue proteins and indicator dyes under the conditions used. The findings suggest that this combination has the nature of a salt-link, that staining by most indicator dyes in a nonpolar solvent is related to acceptance of hydrogen ions (protons) by the tissue proteins, that the different tissue proteins have relatively uniform proton-accepting power under the conditions used, and that they behave in general like relatively weak bases. The ability of tissue proteins to determine the staining color of certain dyes provides a possible explanation for some instances of metachromasia.