Molecular descriptors

Despite the number of available chemicals growing exponentially, testing of their toxicological and environmental behavior is often a critical issue and alternative strategies are required. Additionally, there is the need to predict properties of not yet synthesized compounds to reduce the costs of synthesis, selecting only those that have the maximal potential to be active and nontoxic compounds. In order to evaluate chemical properties avoiding chemical synthesis and reducing expensive and time-demanding laboratory testing, it is necessary to build in silico models establishing a mathematical relationship between the structures of molecules and the considered properties (quantitative structure-activity relationships, QSARs). Molecular descriptors play a fundamental role in QSAR and other in silico models since they formally are the numerical representation of a molecular structure. Molecular descriptors can be classified using different criteria. Among them, there are two main categories, experimental and theoretical descriptors. The basis to understand and perform molecular descriptor calculation, the different theoretical descriptor categories together with their perspectives are described in this chapter.