Applications of the InChI in cheminformatics with the CDK and Bioclipse
© Spjuth et al.; licensee Chemistry Central Ltd. 2013
Received: 4 December 2012
Accepted: 28 February 2013
Published: 13 March 2013
The InChI algorithms are written in C++ and not available as Java library. Integration into software written in Java therefore requires a bridge between C and Java libraries, provided by the Java Native Interface (JNI) technology.
We here describe how the InChI library is used in the Bioclipse workbench and the Chemistry Development Kit (CDK) cheminformatics library. To make this possible, a JNI bridge to the InChI library was developed, JNI-InChI, allowing Java software to access the InChI algorithms. By using this bridge, the CDK project packages the InChI binaries in a module and offers easy access from Java using the CDK API. The Bioclipse project packages and offers InChI as a dynamic OSGi bundle that can easily be used by any OSGi-compliant software, in addition to the regular Java Archive and Maven bundles. Bioclipse itself uses the InChI as a key component and calculates it on the fly when visualizing and editing chemical structures. We demonstrate the utility of InChI with various applications in CDK and Bioclipse, such as decision support for chemical liability assessment, tautomer generation, and for knowledge aggregation using a linked data approach.
These results show that the InChI library can be used in a variety of Java library dependency solutions, making the functionality easily accessible by Java software, such as in the CDK. The applications show various ways the InChI has been used in Bioclipse, to enrich its functionality.
It is of great importance that chemical structures can be serialized in standard formats in order to enable exchange and linking of chemical information. The IUPAC Chemical Identifier (InChI)  is such a standardized identifier for chemical structures, which lately has seen a great adoption in the cheminformatics community . A recent special issue details this further . Two important use cases are querying for exact matches in databases, and linking chemical structures using semantic web technologies. The official implementation of InChI is in C as a library, in order to provide a single implementation that everyone can use. This however limits its use in other programming languages such as Java. We here describe the packaging of InChI in Java, to enable frameworks and applications written in this language, like the applications mentioned in this paper, BioJava , JOELib , and JChem , to take advantage of the benefits of InChI. We present the integration of InChI in the cheminformatics library the Chemistry Development Kit as well as the graphical workbench Bioclipse. We also provide demonstrations where InChI is used in decision support for chemical liability assessment, for tautomer generation, and for knowledge aggregation using a linked data approach.
Packaging InChI in Java Archives and Maven bundles
JNI-InChI is the packaging of the InChI libraries in portable Java libraries using the Java Native Interface (JNI), available on Sourceforge under GNU Lesser General Public License 3.0 (LGPL) . The JNI-InChI library provides native binaries of the InChI library for 32- and 64-bit Windows, Linux and Solaris, 64-bit FreeBSD and 64-bit Intel-based Mac OS X, covering the most common platforms on which the CDK and Bioclipse are run. The library is available as a regular Jar Archive (.jar file), as Maven bundle from the JNI-InChI project website at http://jni-inchi.sf.net/.
Provisioning of InChI as OSGi bundles
While Maven makes library dependency management a lot easier, it is not the only platform to do so. OSGi  is another standard for dynamic module system in Java, allowing for easy provisioning and interoperability of modules, mainly containing compiled Java code but also associated data. The Bioclipse project has developed OSGi bundles for InChI by wrapping the JNI-InChI libraries, which required some modifications to e.g. class loaders. The OSGi bundles are available from a p2 repository for easy provisioning and integration. Having OSGi bundles with InChI enables easy access from all plugins supporting this module technology. Cheminformatics tools that makes use of the OSGi module system includes KNIME , Cytoscape (as of version 3) , Taverna [11, 12], and Bioclipse . More information and the bundles can be found at http://www.bioclipse.net/inchi-osgi.
The JNI-InChI API
The JNI-InChI library is written to directly make calls to the InChI library. That is, it will make library calls directly, rather than using a command line to access the library. To make this possible with JNI, it defines a JniInchiWrapper class which has a Java API of which some methods are written in Java, and some call native methods in the matching JniInchiWrapper.c class that directly calls the C++ InChI library. This wrapper allows the JNI-InChI user to set up a proper data model for the chemical structure for which the InChI should be calculated, and to set the generation options, allowing users to select, for example, which InChI layers should be generated or if just a standard InChI should be calculated.
Various java methods from the JniInChIWrapper class
Loads the InChI library suitable for theplatform.
Generates an InChI for the given inputstructure, with the InChI options passedwith the input.
Generates a Standard InChI for the giveninput structure.
Generates a structure from an InChI string(without coordinates).
Converts an InChI into an InChIKey.
Check the validity of a (non-standard) InChIeither loosely or strict.
Check the validity of a (non-standard)InChIKey either loosely or strict.
Constructor allowing you to set the InChIgeneration options as a List of Strings.
Adds an atom to the input structure.
Adds a bond to the input structure.
Adds a tetrahedral, bond, or allenestereochemistry element to the inputstructure.
The full API is available as HTML JavaDoc at http://jni-inchi.sourceforge.net/apidocs/. What the API does not do, is support input of chemical structures from chemical file formats, such as the MDL molfile format supported by the InChI library itself. Instead, JNI-InChI encourages cheminformatics libraries to use converters that translate their internal data structure into the JNI-InChI data structure, using the methods of the JniInchiInput class. One library taking this approach is the CDK.
Integration of JNI-InChI into the CDK
The primary purpose of the integration of the JNI-InChI into the CDK is to allow the translation of the CDK data structure into that of JNI-InChI. Using this approach, we can convert the content of any chemical file format the CDK supports into InChIs, overcoming limitations of the InChI library in terms of supported file formats.
While JNI-InChI supports the full range of functionality of the InChI C library, structure-to-InChI, InChI-to-structure, AuxInfo-to-structure, InChIKey generation, and InChI and InChIKey validation, not all of this functionality is available in the CDK library, in version 1.4.13 and later.
The CDK uses this functionality further for generate tautomers, as proposed by Thalheim et al. , and demonstrated later in this paper. Another feature is that the InChI library can be use to generate canonical atom numbers, which is done with the InChINumbersTools class.
Integration of InChI in Bioclipse
Results and discussion
The applications below have additional information on how to install and perform them available on: http://www.bioclipse.net/inchi.
Applications of InChI in cheminformatics
a) Decision support in computational pharmacology
b) Linked data spidering in Bioclipse with Isbjørn
Molecular structures on the internet can be searched using InChI and InChIKeys  directly. However, they can also be used as seed to spider (the process of following links on the world wide web) the Linked Data section of the World Wide Web . We developed a plugin to Bioclipse that searches the Internet for information about a molecule, initiated with the InChI and a web service we developed earlier, providing Universal Resource Identifiers for molecules, available at http://rdf.openmolecules.net/. This service provides a number of initial links to other Linked Data resources, and links to other resources are followed using owl:sameAs and skos:exactMatch predicates.
But by educating Isbjørn about further ontologies we can even, for example, extract drug side effects from the SIDER database , as exposed by the Free University Berlin RDF services, as shown in Figure 3 right. The search results of Isbjørn are presented in Bioclipse as a HTML page and opened in a browser window (not shown).
c) CDK tautomer calculation in Bioclipse
Using this approach we can generate tautomers for any molecules, though it is limited by the heuristic rules implemented by the InChI library. We typically only find a subset of tautomers, rather than a full set. For example, for warfarin it finds only six tautomers out of the 40 reported ones .
The InChI project has chosen the path to rely on a single implementation for standardizing InChI calculations, and it is important that this code is readily available for all cheminformatics software development. This paper describes the packaging of InChI as a Java library using a JNI bridge (JNI-InChI), which is available as a Java Archive (jar file), and as Maven bundles. It further shows the integration into the CDK library and how the JNI-InChI as OSGi bundles renders InChI easily available for software using this dynamic module system, such as the Bioclipse workbench. The various binary packages make the InChI library easily usable in a variety of Java environments.
A feature of the InChI is that it supports various layers of detail in describing the chemical structure, which has confused end users of cheminformatics software. This resulted in a set of chosen layers, resulting in the standard InChI. The CDK supports generation and processing of both the standard and non-standard InChIs. Bioclipse provides a preference page where users can indicate which InChI they like to be calculated by default.
The uses in the CDK and Bioclipse have shown that the InChI is of great utility for uniquely identifying molecular structures in a canonical form, and is therefore well suited for exact matches in database searches, as exemplified in computational pharmacology example. This makes it also highly suitable for mining the internet and the Linked Data network. We demonstrate this with our Isbjørn plugin for Bioclipse, which aggregates knowledge about chemical compounds from an increasing list of disparate sources. The use of the InChI here shows the potential for the common task to collect as much information as possible about a novel chemical structure, uniquely identified by the InChI. But the use of the InChI algorithms is not limited to that purpose, and has further benefits. We demonstrate this with the exposure in the CDK and Bioclipse to generate tautomers.
Our results show that it is possible to overcome the problem that the InChI algorithm is not implemented in Java, but this however comes at a price. Using non-Java code in a Java environment requires a bridge, for which we used JNI, but crossing this bridge is computationally expensive. Furthermore, the integration into the CDK requires bridging two data models: one for the CDK and one for the InChI library. A suite of unit tests is in place to validate that information is correctly translated from the CDK data model into calculated InChIs. However, a full validation using the InChI project test suite has not been completed yet.
Availability and requirements
● Project Name: JNI-InChI
● Project home page: http://jni-inchi.sourceforge.net/
● Operating system(s): Windows, GNU/Linux, OS/X
● Programming language: C and Java
● Other requirements (if compiling): InChI library
● License: GNU LGPL v3 or later
● Any restrictions to use by non-academics: None additional
● Project Name: The Chemistry Development Kit
● Project home page: http://cdk.sourceforge.net/
● Operating system(s): Platform independent
● Programming language: Java
● Other requirements (for the InChI module): JNI-InChI
● License: GNU LGPL v2.1 or later
● Any restrictions to use by non-academics: None additional
● Project Name: Bioclipse
● Project home page: http://www.bioclipse.net/
● Operating system(s): Windows, GNU/Linux, OS/X
● Programming language: Java
● Other requirements (for InChI functionality): JNI-InChI, The Chemistry Development Kit
● License: Eclipse Public License
● Any restrictions to use by non-academics: None additional
We acknowledge Mark Rijnbeek for implementing the InChI-based tautomer generation in the CDK.
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