A contribution to the theory of biological staining based on the principles for structural organization of biological macromolecules.

Biological staining is to a large degree explainable based on the principles governing folding and aggregation of macromolecules in aqueous solution. Most macromolecules are polyions, which, except for heteropolysaccharides, have a large proportion of nonpolar or only slightly polar residues. Because they are amphiphilic, they react in water by a complex set of hydrophobic interactions involving charged residues, nonpolar residues and water molecules. The hydrophobic interactions lead to complex folding systems or micelle-like structures. Dyes are amphiphilic molecules with a tendency to form micelles, but with limitations due to geometric constraints and charge repulsion. Macromolecules and dyes react with each other in aqueous solution following the same principles as for the structural organization of macromolecules, as in protein folding for example. Dye binding requires near contact between nonpolar groups in both the dye and macromolecule, and this is accomplished by choosing a pH at which the dye and macromolecule have opposite net charges. Charge attraction is insufficient for binding in most cases, but it is directive because it determines which macromolecules a given dye ion is able to contact. These considerations apply to the staining of globular (cytoplasmic) proteins and to nucleic acid staining. The staining mechanism is by hydrophobic interactions. Above approximately pH 3.5, DNA may also bind dyes by hydrophobic intercalation between the bases of the double helix; at lower pH the double helix opens and dye binding is as for RNA and globular proteins. Heteroglycans (mucins) have virtually no nonpolar groups, so nonpolar interactions are restricted to the dye molecules. Metachromatic staining of heteroglycans is due to hydrophobic bonding or micelle formation between the monovalent planar dye molecules aided by charge neutralization by the negatively charged heteroglycans. Alternatively, as the charge attraction increases with the number of closely placed charges, acidic heteroglycans may be stained by a polycation such as alcian blue or colloidal iron. For elastic fiber and collagen staining, actual hydrophobic interactions are less important and hydrogen bonding and simple nonpolar interactions play a major role. These macromolecules may therefore be stained using a nonaqueous alcoholic solution.