Advances in Chemical Representations and AI in Drug Discovery:
The past century’s technological advancements, especially the computer revolution and high-throughput screening in drug discovery, have necessitated the development of molecular representations readable by computers and understandable across scientific disciplines. Initially, molecules were depicted as structure diagrams with bonds and atoms, but computational processing required more sophisticated representations. Various chemical notations have been developed to encode molecular structures, with early examples like the empirical formula, which provides atomic composition but not connectivity or geometry. The advent of computers facilitated rapid digital storage and modification of chemical data, leading to the development of machine-readable notations and algorithms for 2D and 3D visualization. Modern representations, especially those developed since the 1970s, support small molecules, macromolecules, and chemical reactions, enhancing the efficiency and scalability of cheminformatics.
Applications of AI in Drug Discovery:
In AI-driven drug discovery, chemical representations play a crucial role. Molecular graphs, the most common machine-readable representation, and various other notations are employed to encode structural information for computational analysis. This review highlights the importance of these representations in AI applications, providing examples where AI techniques, such as ML models, are applied to cheminformatics and drug discovery. The review is an essential guide for researchers and students in chemistry, bioinformatics, and computer science, emphasizing the dependency of representation choice on the specific task. While not exhaustive, the review directs readers to further literature on AI applications in cheminformatics, showcasing how modern computational techniques are revolutionizing drug discovery by enhancing data handling and analysis capabilities.
Introduction to Molecular Graph Representations:
Understanding molecular graphs is essential for grasping chemical representations used in drug discovery. A molecular graph maps atoms to nodes and bonds to edges, representing molecules in a structured way. Formally defined as a tuple of nodes (atoms) and edges (bonds), these graphs can be visualized using various software. Nodes and edges are often encoded into matrices: an adjacency matrix for connectivity, a node features matrix for atom identity, and an edge features matrix for bond identity. Graph traversal algorithms ensure consistent node ordering, which is crucial for generating reliable representations. This flexibility allows encoding 3D information, offering advantages over linear notations.
Connection Tables and MDL File Formats:
Connection tables (Ctabs) and MDL (now BIOVIA) file formats are crucial in molecular graph representation. Ctabs consist of counts, atoms, bonds, atom lists, Stext, and properties blocks, efficiently describing molecular structures by specifying atom and bond details. They avoid explicit hydrogen representation, reducing file size. MDL formats, built on Ctabs, include Molfiles for single molecules and extend to SD, RXN, RD, and RG files for additional data and reactions. These formats are widely used for compact, systematic chemical information storage and transfer, supporting diverse cheminformatics applications.
Contemporary Notations: SMILES and InChI:
SMILES, developed in 1988, is an intuitive and popular notation for encoding molecular structures. It assigns numbers to atoms and traverses the molecular graph using depth-first search, allowing multiple representations of the same molecule. Unique SMILES can be designated through canonicalization. SMILES can encode stereochemistry and other complex structures but struggle with organometallic compounds and ionic salts. The International Chemical Identifier (InChI), introduced in 2006, provides a standard, open-source canonical notation with multiple layers for detailed molecular representation. InChIKeys offer unique, searchable, hashed versions of InChIs, enhancing accessibility for chemical information.
Summary of Chemical Representations:
Chemical representations encompass various methods to model molecules, reactions, and macromolecules. Structural keys like MACCS and CATS encode the presence of specific chemical groups. Hashed fingerprints like Daylight and ECFP use hash functions to represent molecular patterns. Reactions are described using formats like Reaction SMILES, RInChI, and CGR. Macromolecules, including proteins and peptides, utilize sequence-based notations and structures from repositories like the PDB. These diverse methods facilitate accurate analysis and prediction in chemical informatics and drug discovery.
Graphical Representations for Molecules and Macromolecules:
Graphical representations of molecules, crucial for visualization and analysis, include 2D depictions and 3D models. 2D depictions show skeletal structures, often using standardized IUPAC guidelines, but still face challenges in layout and rendering. Tools like RDKit and CDK have improved 2D visualizations. For macromolecules, depictions focus on polymer or peptide structures, with tools like the Pfizer Macromolecule Editor aiding visualization. 3D depictions, using software such as Avogadro and PyMOL, include ball-and-stick, cartoon, and van der Waals models, facilitating studies in docking, protein-ligand interactions, and mechanistic studies. These representations enhance understanding of cheminformatics and drug discovery.
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