Dye-tissue interactions: mechanisms, quantification and bonding parameters for dyes used in biological staining

Staining of tissues by dyes is accomplished through various types of bonds, some of which have been poorly defined in traditional biological literature. Here, basic principles of bonding are reviewed to establish uniform terminology and definitions consistent with the field of chemistry. The concept of charge - its presence or absence, magnitude, extent of delocalization and potential for being displaced by outside forces - underlies all bonding phenomena. These same attributes influence solubility and resistance to extraction during dehydration of tissue sections. Covalent bonds involve shared electrons; they are very strong and essentially irreversible under conditions encountered during staining. Polar covalent bonds within dye molecules generate partial atomic charges that create the potential for hydrogen bonding. This is measured by the hydrogen bonding parameter (h), the number of groups bearing charges within the ranges -0.15 to -0.50 eV or +0.15 to +0.30 eV. The potential for ionic bonding is indicated by net charge (Z), while the strength of such bonds is a function of charge site geometry on both bonding partners. Charge delocalization owing to conjugation, electron influencing groups, and resonance creates soft charge sites in which the ionic charge is spread over a large volume. Poorly delocalized charges or point charges are hard (small in volume). Firm bonds result from hard-hard or soft-soft pairs. Hard-soft combinations are weak, readily displaced in competitive interactions, and disrupted by solvents. Coordinate bonds with certain metals are involved with mordant staining and metal chelation dyes. Three different van der Waals attractions comprise the remainder of bonding types, all involving dipoles: Keesom (dipole-dipole) forces, Debye (dipole-induced dipole) forces and London (induced dipole-induced dipole) forces. Potentials for engaging in any of these is quantified by measures of polarity (dipole moment, d), polarizability (crudely with pi atoms describing the size of the conjugated system, or more directly with alpha), hydrophobicity (with the octanol-water partition coefficient, log P or the more convenient Hydrophobic Index, HI), and the number of halogen atoms (X). By using molecular modeling software, quantitative measures of bonding potential (bonding parameters) have been determined for over 400 dyes.