phenanthridinium

Phenanthridinium refers to a family of positively charged nitrogen-containing heteroaromatic cations derived from phenanthridine. The core structure is a fused three-ring system containing a pyridine-like nitrogen; when the nitrogen is protonated or quaternized, the molecule carries a delocalized positive charge over the ring system. In practice, phenanthridinium compounds are encountered as salts, such as chloride or bromide, and also as N-substituted variants (for example N-methylphenanthridinium) formed by quaternization of the ring nitrogen.

Preparation and examples

Phenanthridinium cations can be formed by protonation of phenanthridine or by N-alkylation with methylating reagents to yield N-substituted phenanthridinium salts. The neutral parent phenanthridine serves as the precursor, while the resulting cations are isolated as halide or sulfate salts, among others. Notable derivatives include the fluorescent phenanthridinium dyes such as proflavine and, most famously in biology, ethidium bromide—an N-phenanthridinium salt used as a DNA intercalator and fluorescent stain.

Properties and applications

Phenanthridinium cations are planar, highly conjugated species that intercalate between nucleic acid base pairs due to their positive charge and aromatic surface. This makes them useful as DNA stains and fluorescence probes in molecular biology and microscopy. Variants differ in color, absorption, and emission properties, enabling a range of diagnostic and research applications. Safety considerations are important; many phenanthridinium salts, including ethidium bromide, are mutagenic and require appropriate handling and disposal. Related compounds include phenanthridine (the neutral precursor) and other heteroaromatic cations such as acridinium derivatives.