The Bemis-Murcko scaffold[fn:BM_scaffold] provided by DataWarrior[fn:DataWarrior] retains information about bond order and chirality. Sometimes, however, it suffices to retain only atom connectivity, like an assumption «there are only single bonds». Note, DataWarrior equally offers the export of Bemis-Murcko skeleton, however this simplifies e.g. the scaffold about an imidazole into one of cyclopentane.
After the installation (see below), the script’s general input follows the pattern of
saturate_murcko_scaffolds [-h] inputs [inputs ...]Running from the CLI, this translates for example the SMILES strings about pyridine and benzene to the ones about piperidine and cyclohexane
$ saturate_murcko_scaffolds c1ccncc1 c1ccccc1
C1CCNCC1
C1CCCCC1It equally is possible to provide the input as a list of SMILES in a text file. As an example run in Linux Debian 13:
$ cat test.smi
c1ccncc1
c1ccccc1
$ saturate_murcko_scaffolds test.smi
C1CCNCC1
C1CCCCC1In a mixed input queue, SMILES strings provided via the CLI are processed prior to SMILES provided via one, or multiple input file(s). If wanted, the output to the CLI can be redirected to (piped into) the input of the next command-line utility, or appended to an already existing permanent record, for instance
$ saturate_murcko_scaffolds test.smi > output.smi
$ cat output.smi
C1CCNCC1
C1CCCCC1For normal use, download the most recent Python .whl enclosed in a zip archive distributed on the releases page. Within e.g., an activated virtual environment, the installation proceeds purely locally in the pattern of
pip install saturate_murcko_scaffolds-1.3.1-py3-none-any.whlIntentionally, the Python script and subsequent .whl are set up to work regardless of the underlying operation system (Windows,[fn:PowerShell] Linux, or MacOS) out of the box with the standard library of Python (version 3.10, or higher).
You equally can clone the GitHub repository to then proceed by either command of
pip install .
pip install .eNote (because of pip) this requires a working connection to the
internet during the installation.
If you are interested to locally edit and develop further the
application, pyproject.toml lists additional tools like flake8
and pytest distributed on the PyPI to check and improve source
code quality. Then, the command
pip install pyproject.toml[dev]resolves these dependencies. Finally, a GNU Makefile provides additional analytic tools.
For a collection of organic materials, the Bemis-Murcko scaffolds
were extracted with DataWarrior (then release 5.0.0 for Linux,
January 2019) as listing input.smi including higher bond orders
(see folder demo) with a redirect of the output into file
input_sat.smi. The effect of the «artificial saturation» is easy
to recognize while comparing the scaffold lists (fig. file_diff) in
a difference view.
OpenBabel[fn:openbabel] is used to illustrate the work of the script. The instructions to the CLI follow the pattern of
obabel -ismi test_input.smi -O test_input_color.svg -xc10 -xr12 -xl --addinindexto generate a .svg file (vector representation), or
obabel -ismi test_input_sat.smi -O test_input_sat_color.png -xc10 -xr12 -xl --addinindex -xp 3000to generate a bitmap .png with structure formulae depicted in a
grid of 10 columns by 12 rows. Script series.py automates the
generation of the illustrations about both structure data sets.
It is remarkable how well OpenBabel’s displays the molecular
structures with advanced motifs. In addition to those shown in the
first illustration of this guide, see sub-folder test_data for a
more extensive survey (e.g., the scaffold of cyclophane [entry
#33], sparteine [#38], or adamantane [#50]).
The script provides «saturation» by dropping explicit information
related to double and triple bonds which SMILES encode (=, #
regarding bond order; / (forward slash), \ (backward slash)
regarding (cis)-(trans) relationship around double bonds). While
processing double bonds of e.g., ketones to yield secondary
alcohols, the script refrains from the assignment of new CIP
priorities and a corresponding label. It then depends on the
program used for a visualization, if an explicit wedge is used
(e.g., OpenBabel), or the absence of information is highlighted
(e.g., as question mark in DataWarrior, or the project of CDK
depict[fn:CDKdepict]) as ambiguous. Absolute configuration of
stereogenic centers (indicated in SMILES with the @ sign) already
assigned in the input however is retained.
For a selection of elements (C, N, O, P, S), the implicit
description of aromatic systems (e.g., as c1ccncc1 in pyridine,
c1c[nH]cc1 in pyrrol) is recognized. To offer a «saturation»,
these characters returned as upper case characters to yield e.g.,
piperidine (C1CCNCC1) and pyrrolidine (C1C[NH]CC1).
The script equally preserves up to one single negative, or single
positive charge of these five elements (e.g., [O-]c1ccccc1 about
the phenolate anion, and C[N+](c1ccccc1)(C)C about
N,N,N-trimethylbenzenaminium cation). Here, it can be sensible to
«sanitize» the results this script provides by other libraries as
e.g. RDKit.[fn:rdkit]
The capitalization of the five characters is constrained to prevent
non sensible transformations of e.g., an (implicitly) aromatic atom
of tin [sn] into the invalid form [SN]. Though the script is
going to write tin as [Sn], an adjustment of valence for elements
written with two characters is beyond the current scope of the
script.
A SMILES string may describe more than one molecule. Thus, the
concatenation with ”.” (period character) as seen for example in
descriptions of co-crystals like about 1,4-benzoquinone and
hydroquinone, C1=CC(=O)C=CC1=O.c1cc(ccc1O)O, is retained. The
example is resolved as C1CC(O)CCC1O.C1CC(CCC1O)O.
Norwid Behrnd, 2019, GPLv3.
[fn:BM_scaffold] Bemis, G. W.; Murcko, M. A. The Properties of Known Drugs. 1. Molecular Frameworks. J. Med. Chem. 1996, 39, 2887–2893 (https://doi.org/10.1021/jm9602928).
[fn:DataWarrior] Sander, T.; Freyss, J.; Von Korff, M.; Rufener, C. DataWarrior: An Open-Source Program For Chemistry Aware Data Visualization And Analysis. J. Chem. Inf. Model. 2015, 55, 460–473 (https://doi.org/10.1021/ci500588j). The program, (c) 2002–2024 by Idorsia Pharmaceuticals Ltd., is freely available under http://www.openmolecules.org. For the source code (GPLv3), see https://github.com/thsa/datawarrior.
[fn:openbabel] https://github.com/openbabel/openbabel For the most recent documentation, see https://open-babel.readthedocs.io/en/latest/
[fn:rdkit] For an overview about the freely available RDKit library, see www.rdkit.org. An introduction into the topic of «molecular sanitization» is provided in the section of this very title in the on-line RDKit Book.
[fn:CDKdepict] https://www.simolecule.com/cdkdepict/depict.html For
the mentioned annotation of CIP labels, change No Annotation (second
pull down menu from the left) to CIP Stereo Label.
[fn:PowerShell] Contrasting to cmd.exe, Windows’ PowerShell may
block the execution of scripts. The later is adjustable by the
command set-executionpolicy remotesigned while running in the
administrator mode. For additional details, visit for instance How to
enable execution of PowerShell scripts? on StackExchange/superuser.