= RNA hyperchromicity
{c}
\b[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.<Hyperchromic effect in IUPAC Gold Book>[H02923, definition.]{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 $A=\varepsilon lc$, where $A$ is <absorbance>, $\varepsilon$ a <molar absorption coefficient>, $l$ path length and $c$ concentration. A change in $\varepsilon$ 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,
$$
A=(1-f_U)A_F+f_UA_U,\qquad
f_U=\frac{A-A_F}{A_U-A_F},
$$
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 $f_U\approx0.091$. 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.<Thermodynamic characterization of naturally occurring RNA tetraloops by Justin P. Sheehy and others>[Materials and Methods, optical melting and curve fitting.]{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.
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