Hypochromism

Hypochromism is the decrease in the absorbance or molar absorptivity of a chromophore arising from electronic interactions between nearby chromophores or from conformational changes. In UV-visible spectroscopy this effect is most familiar in nucleic acids, where base stacking and interactions between adjacent bases alter the electronic transitions responsible for absorption around 260 nm.

In nucleic acids, single-stranded DNA typically absorbs more strongly at about 260 nm than double-stranded DNA. When strands form a duplex, base stacking and excitonic coupling reduce the probability of π→π* transitions, producing a hypochromic effect. The extent of hypochromicity can be quantified as percent hypochromicity, calculated as (A_ss – A_ds)/A_ss × 100%.

Mechanistically, hypochromism is attributed to exciton coupling and electronic interactions among stacked bases that modify energy levels and decrease oscillator strength. Denaturation disrupts stacking and commonly leads to hyperchromism, an increase in absorbance. Temperature, ionic strength, sequence composition, and the presence of intercalating agents or ligands can influence the magnitude of hypochromism.

Applications of hypochromism include monitoring DNA duplex formation and melting curves, as well as assessing interactions with proteins, drugs, or other ligands that affect base stacking or conformation. In broader contexts, hypochromism can be observed in other chromophore systems where electronic coupling or aggregation alters extinction coefficients, such as certain dye aggregates and metal-porphyrin assemblies.