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Trying to squeeze sense out of chemical data

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Substructure Matching, REST style

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I’ve been putting up a number of REST services for a variety of cheminformatics tasks. One that was missing was substructure searching. In many scenarios it’s useful to be able to check whether a target molecule contains a query substructure or not. This can now be done by visiting URL’s of the form


where TARGET and QUERY are SMILES and SMARTS (or SMILES) respectively (appropriately escaped). If the query pattern is found in the target molecule then the resultant page contains the string “true” otherwise it contains the string “false”. The service uses OpenBabel to perform the SMARTS matching.

Using this service, I updated the ONS data query page to allow one to filter results by SMARTS patterns. This generally only makes sense when no specific solute is selected. However, filtering all the entries in the spreadsheet (i.e., any solvent, any solute) can be slow, since each molecule is matched against the SMARTS pattern using a separate HTTP requests. This could be easily fixed using POST, but it’s a hack anyway since this type of thing should probably be done in the database (i.e., Google Spreadsheet).


The substructure search service is now updated to accept POST requests. As a result, it is possible to send in multiple SMILES strings and match them against a pattern all at one go. See the repository for a description on how to use the POST method. (The GET method is still supported but you can only match a pattern against one target SMILES). As a result, querying the ONS data using SMARTS pattens is significantly faster.

Written by Rajarshi Guha

February 3rd, 2009 at 6:01 pm

Update to the REST Descriptor Services

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The current version of the REST interface to the CDK descriptors allowed one to access descriptor values for a SMILES string by simply appending it to an URL, resulting in something like

This type of URL is pretty handy to construct by hand. However, as Pat Walters pointed out in the comments to that post, SMILES containing ‘#’ will cause problems since that character is a URL fragment identifier. Furthermore, the presence of a ‘/’ in a SMILES string necessitates some processing in the service to recognize it as part of the SMILES, rather than a URL path separator. While the service could handle these (at the expense of messy code) it turned out that there were subtle bugs.

Based on Pats’ suggestion I converted the service to use base64 encoded SMILES, which let me simplify the code and remove the bugs. As a result, one cannot append the SMILES directly to the URL’s. Instead the above URL would be rewritten in the form

All the example URL’s described in my previous post that involve SMILES strings, should be rewritten using base64 encoded SMILES. So to get a document listing all descriptors for “c1ccccc1COCC” one would write

and then follow the links therein.

While this makes it a little harder to directly write out these URL’s by hand, I expect that most uses of this service would be programmatic – in which case getting base64 encoded SMILES is trivial.

Written by Rajarshi Guha

January 11th, 2009 at 5:52 pm

Playing with REST Descriptor Services

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As part of my work at IU I have been implementing a number of cheminformatics web services. Initially these were SOAP, but I realized that REST interfaces make life much easier. (also see here) As a result, a number of these services have simple REST interfaces. One such service provides molecular descriptor calculations, using the CDK as the backend. Thus by visiting  (i.e., making a HTTP GET request) a URL of the form

you get a simple XML document containing a list of URL’s. Each URL represents a specific “resource”. In this context, the resource is the descriptor values for the given molecule. Thus by visiting

one gets another simple XML document that lists the names and values of the AlogP descriptor. In this case, the CDK implementation evaluates AlogP, AlogP2 and molar refractivity – so there are actually three descriptor values. On the other hand something like the  molecular weight descriptor gives a single value. To just see the list of available descriptors visit

which gives an XML document containing a series of links. Visiting one of these links gives the “descriptor specification” – information on the vendor, version, reference to a descriptor ontology and so on.

(I should point out that the descriptors available in this service are from a pretty old version of the CDK. I really should update the descriptors to the 1.2.x versions)


This type of interface makes it easy to whip up various applications. One example is the PCA analysis of compound collections. Another one I put together today based on a conversation with Jean-Claude was a simple application to plot pairs of descriptor values for a collection of SMILES.


The app is pretty simple (and quite slow, since it uses synchronous GET’s to the descriptor service for each SMILES and has to make two calls for each SMILES – hey, it was a quick hack!). Currently, it’s a bit restrictive – if a descriptor calculates multiple values, it will only use the first value. To see how many values a molecular descriptor calculates, see the list here.

With a little more effort one could easily have a pretty nice online descriptor calculation application rivaling a standalone application such as the the CDK descriptor GUI

Also,if you struggle with nice CSS layouts, the CSS Layout Collection is a fantastic resource. And jQuery rocks.

Written by Rajarshi Guha

January 7th, 2009 at 7:06 am

Extending the REST PCA Service

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I recently described a REST based service for performing PCA-based visualization of chemical spaces. By visiting a URL of the form

one would get a HTML, plain text or JSON page containing the first two principal components for the molecules specified. With this data one can generate a simple 2D plot of the distributions of molecules in the “default” chemical space.

However, as Andrew Lang pointed out on FriendFeed, one could use SecondLife to look at 3D versions of the PCA results. So I updatesd the service to allow one to specify the number of components in the URL. The above form of the service will still work – you get the first two components by default.

To specify more components use an URL of the form,mol2,mol3

where mol1, mol2, mol3 etc should be valid SMILES strings. The above URL will return the first three PC’s. To get just the first PC, replace the 3 with 1 and so on. If more components are requested than available, all components are returned.

Currently, the only available space is the “default” space which is 4-dimensional, so you can get a maximum of four components. In general, visit the URL

to obtain a list of currently available chemical spaces, their names and dimensionality.


While it’s easy to get all the components and visualize them, it doesn’t always make sense to do so. In general, one should consider those initial principal components that explain a significant portion of the variance (see Kaisers criterion). The service currently doesn’t provide the eigenvalues, so it’s not really possible to decide whether to go to 3, 4 or more components. For most cases, just looking at the first two principal components will sufficient – especially given the currently available chemical space.

Update (Jan 13, 2009)

Since the descriptor service now requires that Base64 encoded SMILES, the example usage URL is now invalid. Instead, the SMILES should be replaced by their encoded versions. In other words the first URL above becomes

Written by Rajarshi Guha

January 3rd, 2009 at 1:14 am

Deep Learning in Chemistry

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Deep learning (DL) is all the rage these days and this approach to predictive modeling is being applied to a wide variety of problems, including many in computational drug discovery. As a dilettante in the area of deep learning, I’ve been following papers that have used DL for cheminformatics problems, and thought I’d mention a few that seemed interesting.

An obvious outcome of a DL model is more accurate predictions, and as a result most applications of DL in drug discovery have focused on the use of DL models as more accurate regression or classification models. Examples include Lusci et al [2013], Xu et al [2015] and Ma et al [2015]. It’s interesting to note that in these papers, while DL models show better performance, it’s not consistent and the actual increase in performance is not necessarily very large (for the effort required). Eakins [2016] has reviewed the use of DL models in QSAR settings and more recently Winkler & Le [2016] have also briefly reviewed this area.

However, simply replacing one regression method with another is not particularly interesting. Indeed, as pointed by several workers (e.g., Shao et al [2013]) input descriptors, rather than modeling method, have greater effect on predictive accuracy. And so it’s the topic of representation learning that I think DL methods become interesting and useful in the area of cheminformatics.

Several groups have published work on using DL methods to learn a representation of the molecular structure, directly from the graph representation. Duvenaud et al [2016] and Kearnes et al [2016] both have described these approaches and the nice thing is that this alleviates the need to choose and select features a priori. The downside is that the learned features are optimal in the context of the training data (thus necessitating large training sets to allow for learned features that are generalizable). Interestingly, on reading Kearnes et al [2016], the features that are learned by the DL model are conceptually similar to circular fingerprints. More interestingly, when they built predictive neural network models using the learned representation, the RMSE was not significantly different from a random forest model using circular fingerprints. Of course, the learned representation is driven by the architecture of the DL model, which was designed to look at atom neighborhoods, so it’s probably not too surprising that the optimal representations was essentially equivalent to a circular fingerprint. But one can expect that tweaking the DL architecture and going beyond the molecular graph could lead to more useful representations. Also, this paper very clearly describes the hows and whys of designing a deep neural network architecture, and is useful for someone interested in exploring further.

Another interesting development is the use of DL to learn a continuous representation of a molecular structure, that can then be modified (usually in a manner to vary some molecular property) and “decoded” to obtain a new chemical structure with the desired molecular property. This falls into the class of inverse QSAR problems and Gomez-Bombarelli et al [2016] present a nice example of this approach, where gradient descent is used to explore chemical space defined by the learned continuous representation. Unfortunately the chemistry represented by the generated structures has several problems as described by Derek Lowe. While this problem has been addressed before (e.g., Wong et al [2009] with SVM, Miyao et al [2016], Skvortsova et al [1993]), these efforts have started with pre-defined feature sets. The current works key contribution is the ability to generate a continuous chemical space and I assume the nonsensical regions of the space could be avoided using appropriate filters.

Winkler & Le [2016] recently reported a comparison of deep and shallow neural networks for QSAR regression. Their results and conclusions are similar to previous work. But more tantalizingly, they make the claim that DNN’s may be better suited to tackle the prediction of activity cliffs. There has been some work on this topic (Guha [2012] and Heikamp et al [2012]) but given that activity cliffs are essentially discontinuities in a SAR surface (either fundamentally or by choice of descriptors), traditional predictive models are unlikely to do well. Winkler & Le point to work that suggests that activity cliffs may “disappear” if an appropriately high dimensionality descriptor space is used, and conclude that learned representations via DL may be useful for this. Though I don’t discount this, I’m not convinced that simply moving to higher dimensional spaces is sufficient (or even necessary) – if it were, SVM‘s should be good at predicting activity cliffs. Rather, it’s the correct set of features, that captures the phenomenon underlying the cliff, that are necessary. Nonetheless, Winkler & Le [2016] raise some interesting questions regarding the smoothness of chemical spaces.

Written by Rajarshi Guha

November 8th, 2016 at 6:23 pm