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The hyperchromic effect is an increase in the intensity of an absorption band, for example when a molecule's environment or interactions change. It concerns absorption strength rather than a shift of the band's wavelength. Its converse is the hypochromic effect.^[ref]

At fixed concentration and optical path length, the Beer-Lambert law gives $A=\epsilon c\ell$, so an increase in the molar absorption coefficient increases absorbance. A change of concentration alone can increase absorbance without demonstrating a hyperchromic change in molecular absorption strength.

A motivating measurement is DNA melting. In1962, J. Marmur and P. Doty followed the rise in260nm absorbance as DNA was heated. Their apparatus used glass-stoppered quartz cuvettes in a Beckman DU spectrophotometer, heated by circulating water through thermal spacers. A thermometer measured the cuvette temperature; measurements near the transition allowed about10minutes for equilibration at each step. They corrected absorbance for thermal expansion of the solution and plotted it relative to its value at25degrees Celsius. The midpoint of the hyperchromic rise defined their melting temperature.^[ref]

For an original numerical example, a corrected absorbance increase from0.50 to0.65 is a30% hyperchromic rise. Under a stipulated two-state model with linear absorbance mixing and those fixed end-state baselines,is the fraction in the high-absorbance state. At $A=0.575$, it gives $f=1/2$. Sloping temperature-dependent baselines or intermediate states require a more detailed model; the30% change is not itself a universal conversion from absorbance to the fraction of melted DNA.

RNA hyperchromicity is an increase in an RNA sample's ultraviolet absorption associated with a change in its molecular environment or conformation. The general hyperchromic effect concerns increased spectral-band intensity, rather than a shift of the band to another wavelength.^[ref] In RNA melting measurements, altered base stacking can contribute to the change in absorption; signal interpretation still requires a defined wavelength and sample conditions.

The Beer-Lambert law writes $A=\epsilon lc$, where $A$ is absorbance, $\epsilon$ a molar absorption coefficient, $l$ path length and $c$ concentration. A change in $\epsilon$ can therefore change absorbance even when the amount of RNA is unchanged. Conversely, changed concentration, path length or other optical contributions can mimic a structural signal.

For an original comparison at one stated temperature, suppose extrapolated folded and unfolded absorbances are $A_F=0.40$ and $A_U=0.60$. Defining fractional hyperchromicity relative to the folded state gives $(A_U-A_F)/A_F=0.50$, or50%. Using the unfolded state as denominator would instead give $1/3$. A reported percentage therefore needs its convention.

In a linear two-population optical model,provided $A_U\ne A_F$. With those baselines, $A=0.50$ gives $f_U=0.50$. But with temperature-appropriate baselines $0.48$ and $0.70$, the same absorbance gives $f_U\approx 0.091$. An RNA melting baseline is consequently part of the population estimate, not decorative plotting.

A 2010 tetraloop study measured absorbance-versus-temperature curves to infer folding thermodynamics.^[ref] Such measurements connect an optical response to a model of RNA structure. An absorption increase alone does not count broken base pairs, establish a unique unfolded structure or distinguish every possible chemical change.