RNA hyperchromicity

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.