Home » DataWarrior » Functionality » feature suggestion: .sdf export with explicit hydrogens
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 Re: feature suggestion: .sdf export with explicit hydrogens [message #1411 is a reply to message #1393] |
Thu, 23 September 2021 11:19   |
thomas
Messages: 731
Registered: June 2014
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Senior Member |
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Dear Norwid,
structures in DataWarrior files are stored as canonical encoded strings (idcodes). Therefore, it doesn't matter, whether a user draws a structure with or without hydrogen atoms. They are immediately converted into the canonical form, which is the one without hydrogen atoms. Tetrahedral stereo configurations are encoded as atom parity, such that when displaying the structure, one of the bonds will be drawn as up or down bond to account for the correct parity. This bond may be different from the one originally used to define the parity. As a consequence, when molecules are exported within SD-files, there are no simple hydrogens, but the stereo information is never lost. In order to display or export molecules with the originally supplied coordinates, DataWarrior stores coordinates of all idcode atoms as a separately encoded string. For 3D-coordinates there is a special handling: idcodes are still the same canonical ones without hydrogen atoms, but the coordinate encoding contains 3D-coordinates even for implicit hydrogen atoms, because for conformers hydrogen positions may be important, e.g. for docking. Therefore, SD-Files, if exported with 3D-coordinates, contain hydrogen atoms.
If I provided a setting to also write hydrogen atoms into exported 2D-SD-Files, then I would need to generate new potentially sub-optimal coordinates for the added formerly implicit hydrogen atoms. For 2D-SD-Files it seems common practice to not include implicit hydrogen atoms. Which application do you have in mind that requires hydrogen atoms?
Thomas
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| Re: feature suggestion: .sdf export with explicit hydrogens [message #1422 is a reply to message #1411] |
Sat, 25 September 2021 18:01 |
nbehrnd
Messages: 233
Registered: June 2019
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Senior Member |
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Dear Thomas,
based on recent work with DataWarrior, I retract this feature suggest.
DataWarrior's default assignment of an idcode about a structure is a simplified, yet complete description of a molecule in terms of atom connectivity, as well as stereo chemistry. Thus -- already at the level where libraries of molecules are generated -- DW is able to assign e.g., a a full InChI string, despite at this stage, exported .sdf files lack lines with an explicit label "hydrogen". These .sdf aim for the exchange of information with other programs; including their own (independent from DW) assignment of full InChI strings.
The (re)generation of 3D structures by the conformer generator departing from such a "library .sdf" serves a different purpose. As you mentioned, these subsequent structures are the ones to be used in docking experiments. And these are the .sdf aiming for a visualization of the molecule's shape/geometry in a viewer like Jmol with explicit hydrogen atoms.
As noticed, the set of InChI strings assigned to molecules at level of library generation, and the one about structures past the conformer generation, may differ from each other. This however is because the conformer generator resolves ambiguity if an entry in a "library stage .sdf" did not describe a stereogenic center in either (R), or (S); an axis in (P), or (M); a double bond in (E), or (Z) configuration. The conformer generator's permutation of these choices may be caused by lack of this particular information, i.e. an unknown stereo configuration, or because the .sdf read intentionally describes a racemate (the "enhanced stereo recognition" the structure editor's documentation describes).
Norwid
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