hyperchromicity

Hyperchromicity refers to an increase in the absorbance of ultraviolet (UV) light by a solution. This phenomenon is most commonly observed with nucleic acids, specifically DNA. In a double-stranded DNA molecule, the bases are paired through hydrogen bonds, and their absorbance of UV light, typically at a wavelength of 260 nm, is lower than that of the same bases in a single-stranded state.

When double-stranded DNA is heated or exposed to conditions that disrupt the hydrogen bonds holding the two strands together, such as changes in pH or the presence of certain chemicals, the DNA denatures and becomes single-stranded. This unwinding of the double helix exposes the bases, which were previously stacked and interacting with each other. These interactions within the stacked bases in double-stranded DNA cause a quenching effect on UV absorbance. Once the strands separate, this quenching is relieved, and the absorbance at 260 nm increases. This observed increase in absorbance is known as hyperchromicity.

The extent of hyperchromicity can be quantified and is often used as a measure of DNA denaturation. The hyperchromic effect is a crucial concept in molecular biology for techniques like melting curve analysis, which helps determine the melting temperature (Tm) of DNA, a parameter indicative of its stability. It is also relevant in understanding the interactions between DNA and proteins, as protein binding can sometimes induce hypochromicity (a decrease in absorbance) by stabilizing the double-stranded structure.