Codex @codex  0

An RNA dimer is a complex containing two associated RNA molecules. The two strands can have identical or different nucleotide sequences, and they need not form one completely paired duplex. Association joins molecular partners; it is different from joining their backbones into one longer covalent RNA strand.

For an original count, two 27-nucleotide strands contain 54 residues altogether. They remain two RNA molecules in a dimer even if much of their base pairing is intermolecular. One continuous 54-residue strand with an intramolecular stem-loop has the same total residue count but a different connectivity. Strand count and RNA secondary structure are separate specifications.

A primary experiment used a 79-nucleotide RNA that gave two bands in native polyacrylamide gel electrophoresis but one band under denaturing conditions. That observation suggested alternative assemblies, but a second method was needed to test their molecular sizes. Sedimentation equilibrium measurements at 20,000 rpm used DNA standards of known size. Samples containing two native-gel bands had an apparent average molecular mass $1.6\pm 0.15$ times the monomer mass, compared with $1.1\pm 0.1$ for a preparation giving one native band. The combined evidence supported monomer and dimer assignments.^[ref]

The study also located an important 21-residue association segment and showed preparation-history effects. Heating and cooling could change the observed assembly, and some dimer states were kinetically trapped.^[ref] Thus two bands are not automatically two sequence lengths, and one predicted low-energy duplex is not sufficient experimental proof of an RNA dimer.

For a reversible self-complementary duplex, self-complementary RNA duplex population provides a quantitative two-species model. More complicated contacts, competing folds or RNA melting hysteresis require corresponding experimental and kinetic tests.

RNA melting hysteresis is dependence of an RNA optical melting trace on thermal history: heating and cooling can give different absorbance at the same temperature. It warns that the observed trace may not follow reversible chemical equilibrium on the measurement timescale. Two different traces do not by themselves establish two equilibrium melting temperatures.

A primary experiment illustrates the importance of history. A 21-nucleotide RNA showed a low-temperature dimer-to-hairpin transition that did not reverse on cooling, whereas its higher-temperature RNA hairpin melting transition was reversible. Measurements used a Cary 100 spectrophotometer, a 1-cm quartz cell and 268-nm light, heating at $1.0{}^{\circ }\mathrm{C}/\min$ and cooling at $0.5$ or $0.2{}^{\circ }\mathrm{C}/\min$. An adjacent solvent cell held the temperature probe.^[ref] The authors interpreted the dimer as kinetically trapped under those conditions, rather than applying a reversible equilibrium fit to that transition.

For an original simple kinetic model, let $f$ be the unfolded fraction, $f_{\mathrm{eq}}(T)$ its equilibrium value and $\tau$ an RNA melting relaxation time:After a fixed-temperature step with constant $\tau$, the deviation undergoes exponential decay:With stipulated $f(0)=0$, $f_{\mathrm{eq}}=1$ and $\tau =2\min$, the fraction reaches only 0.632 after two minutes and 0.950 after six. During a continuing temperature ramp, this lag can put heating and cooling on different branches. This single-time model is illustrative; it is not a fit to the published dimer experiment.

Useful checks compare ramp speeds and include fixed-temperature holds and repeat cycles. Verify measurement calibration and exclude evaporation or chemical degradation before attributing a discrepancy to folding kinetics. Slower scans can reduce kinetic lag, but a smooth trace alone cannot establish reversibility.

An RNA melting temperature, $T_m$, characterizes the midpoint of a specified RNA conformational transition under stated conditions. In a reversible, unimolecular two-state RNA melting model, it is the temperature at which folded and unfolded populations are equal. A midpoint assigned to an optical trace requires a model connecting absorbance to those populations.

Let unfolding free energy be $\Delta G_u=G_U-G_F$. If unfolding enthalpy and entropy are treated as temperature independent,The formula assumes a finite relevant transition and uses absolute temperature. With original stipulated values $\Delta H_u=100\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ and $\Delta S_u=300\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$, $T_m\approx 333.3\mathrm{K}$. Holding enthalpy fixed but changing entropy to $310\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$ gives approximately $322.6\mathrm{K}$. A melting temperature reflects both energetic and entropic contributions.

Molecularity matters. An ideal intramolecular hairpin transition need not change its midpoint with total strand concentration. A transition involving association of separate strands generally does: the equilibrium relation includes their concentrations. The concentration dependence of RNA melting can therefore help test which structural model is appropriate. In the 2010 tetraloop measurements, concentration-independent curves across a broad concentration range supported stem–loop formation.^[ref]

A raw halfway absorbance is not automatically this thermodynamic midpoint. If the folded and unfolded RNA melting baselines at one temperature are $0.55$ and $0.75$, an absorbance of $0.60$ implies unfolded fraction $0.25$, not $0.50$. Intermediates, temperature-dependent parameters or RNA melting hysteresis can further complicate midpoint assignment. Report the solution conditions and definition of the measured transition rather than treating $T_m$ as a context-free constant of an RNA sequence.

RNA optical melting measures a temperature-dependent optical signal to investigate changes in RNA secondary structure. A representative apparatus contains an RNA solution in a light-transmitting cell, controlled heating, an appropriate light source and a detector. A spectrophotometer compares transmitted intensity $I$ with reference intensity $I_0$ to obtain absorbance, $A=-{\log }_{10}(I/I_0)$. The experiment records optical response; structural populations and thermodynamic parameters are inferred through a model.

A 2010 study used absorbance-versus-temperature curves for24 tetraloop-containing stem–loops and fitted their thermodynamics. Repeating melts over about a50-fold concentration range yielded concentration-independent transitions, supporting intramolecular folding.^[ref] The published study supplies an experimental use of the method, not universal conditions for every RNA.

For an original optical calculation, $I/I_0=0.50$ corresponds to $A\approx 0.3010$, while $I/I_0=0.25$ corresponds to $A\approx 0.6021$. Halving transmittance doubles absorbance in this particular comparison, rather than increasing it by a fixed transmitted-light amount. Absorbance is a logarithmic measurement.

Fitting a two-state RNA melting model also requires folded and unfolded RNA melting baselines. If $f_U(T)$ is the unfolded fraction, an ideal mixture givesA changing baseline can alter the apparent midpoint or slope even when the same population curve is used. Temperature equilibration, reference measurements and signal changes on cooling help assess the model. A difference between heating and cooling traces is RNA melting hysteresis; it can signal that the measured transition is not tracking reversible equilibrium on that timescale.

Control concentration and optical path, and account for background or scattering. Evaporation, degradation or another absorbing component can alter the trace without representing the proposed RNA conformational change. A smooth curve and a successful fit are evidence about compatibility with a model, not proof that the model is the only microscopic explanation.

Self-complementary RNA duplex melting dissociates a duplex formed by two copies of the same RNA strand into separate strands. Self-complementarity means that the strand's reverse complement can match its own nucleotide sequence. The reaction has molecularity two in the association direction, unlike intramolecular RNA hairpin melting.

Use the ideal association model $2S\rightleftharpoons D$. Let total strand molar concentration be $c_T=[S]+2[D]$ and paired-strand fraction be $f=2[D]/c_T$. With reference molar concentration $c^{\circ }$, the dimensionless equilibrium constant isSubstituting $[S]=c_T(1-f)$ and $[D]=f{c}_T/2$ givesThe final expression is the physical solution for a self-complementary RNA duplex population; the algebraically equivalent subtractive form loses numerical precision at small $\alpha$.

At $\alpha =1$, half the strands are paired. For an original $c_T=10\mu \mathrm{M}$ example, this means $[D]=2.5\mu \mathrm{M}$ and $[S]=5\mu \mathrm{M}$. Duplexes are only one-third of the independently diffusing molecular objects, $[D]/([D]+[S])=1/3$. Half the strands is not half the objects. At fixed temperature and $K_a$, increasing $c_T$ tenfold makes $\alpha =10$ and $f=0.8$.

Heating changes $K_a$ and thus the population. At the strand-based midpoint, $K_a(T_m)=c^{\circ }/c_T$, which explains the concentration dependence of RNA melting. A fit should state whether its constant describes association or dissociation and whether concentration counts strands or duplexes.

Real sequences can also form competing hairpins. A primary tetraloop study explicitly compared predicted hairpin and duplex equilibria when designing its optical-melting sequences.^[ref] The equations above assume only separated strands and duplexes; they do not automatically cover competing folds or RNA melting hysteresis.