Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. They are thus able to interact with oppositely charged tissue constituents. The groups that are responsible for the ionising capability are the auxochromes. Amphoteric dyes have both positively chargeable groups and negatively chargeable groups present on the molecule. Depending on the charge actually present, these dyes may interact as either positively charged ions (basic dyes) or negatively charged ions (acid dyes).
The determining factor in compounds of this nature, including amino acids and proteins, is the pH involved. Each such compound has a pH at which the overall charge on the molecule is zero. At this pH the negatively charged groups and the positively charged groups exactly cancel each other out. This is the isoelectric point, and the compound is considered to form an internal salt at this pH, otherwise known as a zwitterion. At pH levels below the isoelectric point the positively charged groups are favoured, and the compound is considered to be a cation. At pH levels above the isolectric point the negatively charged groups are favoured, and the compound is considered to be an anion.
For most staining methods, the reactions of tissue constituents require pH levels within the range of about pH 2 to pH 10. As it happens, for most dyes containing both positively and negatively charged groups, a pH outside this range is needed to change their overall charge. For that reason the amphoteric nature of such dyes is theoretical, and is of little practical application, although Edward Gurr proposed a classification system for dyes based on dye auxochromes including classes of amphoteric dyes.
Edward Gurr, (1971)
Synthetic dyes in biology, medicine and chemistry
Academic Press, London, England.