Hyperchromaticity

Hyperchromicity is defined as the increase in amount of light absorbed by DNA molecules in solution when double-stranded DNA becomes single-stranded. This can occur upon nuclease digestion or heat denaturation due to the release of nucleotides that absorb more UV light.

Heat denaturation of DNA, also called melting, causes the double helix structure to unwind to form single stranded DNA. When DNA is heated to more than 80 ^oC, the double-stranded DNA unwinds to form single-stranded DNA. The bases become unstacked and can thus absorb more light. In their native state, the bases of DNA absorb light in the 260-nm wavelength region. When the bases become unstacked, the wavelength of absorbance does not change, but the amount absorbed increases by 30-40%.

Hyperchromicity can be used to track the condition of DNA as temperature changes. The temperature where the absorbance of UV radiation is 50% between the maximum and minimum, ie where 50% of the DNA is denatured, is known as T_m (transition/melting temperature).

The value of T_m is an indication of the stability of the DNA molecule.