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Why Use This Engine?

The form you draw is not always the form a molecule takes in water. Moving one hydrogen changes which atoms donate and accept hydrogen bonds, and with it how the molecule binds. RevTaut lists every such form and scores the plausible ones across their conformers. Use it before docking or free-energy work to check that the form you drew dominates, to find a minor tautomer that may be the one that binds, or to compare how a substitution shifts the balance across a series.

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Background

For each tautomer, RevTaut computes the free energy of a conformer ensemble: G=−RTln⁡∑ie−(Ei+Gsolv,i)/RT+GRRHOG = -RT \ln \sum_i e^{-(E_i + G_{\text{solv},i})/RT} + G_{\text{RRHO}} EiE_i is each conformer’s electronic energy, Gsolv,iG_{\text{solv},i} its solvation free energy in water, and GRRHOG_{\text{RRHO}} the thermal correction (zero-point energy, enthalpy and entropy) of the lowest conformer. Populations are a Boltzmann distribution over GG at 298.15 K: pk=e−Gk/RT∑je−Gj/RTp_k = \frac{e^{-G_k / RT}}{\sum_j e^{-G_j / RT}} Zwitterions count as tautomers here.

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How It Works

The default level of theory is:

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Running the Engine

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Inputs

Submit a single molecule. For a salt, submit the parent compound without its counter-ion: RevTaut keeps any counter-ion in the calculation, which changes the results. Files with more than one structure are refused. Any tautomer of the molecule gives the same candidates. Up to 60 heavy atoms; files up to 10 MB.

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Outputs

The results table lists the refined tautomers, most populated first, with their population and free energy relative to the major one. The form you drew is marked Submitted form. Tautomers screened out before refinement are not listed; when the form you drew is one of them, the result says why. Select a tautomer to see its conformers in 3D.

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Reading the Result

When the top two tautomers are within 1 kcal/mol, the result says so: their order is within the method’s error, so treat both as present. Hydrogens held in place marks a tautomer whose every conformer relaxed into another tautomer, so it was optimized with its X-H bonds held at their starting lengths. When such a tautomer holds at least 1% of the population, the result warns that some of these populations need care. This most often happens to a zwitterion optimized in the gas phase. Leave Optimize in water on unless you need a gas-phase comparison.

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Interactive Results Viewer

Two worked examples. In both, the major tautomer is the one measured to dominate in water. 2-Hydroxypyridine, submitted as drawn, comes out 91% 2-pyridone and 9% 2-hydroxypyridine. Adenine stays in its usual form, more than 99.9% of the population, 5.1 kcal/mol below the next tautomer. Select a molecule, then a tautomer, to see its lowest-energy conformer in 3D.