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Each molecule has its own specialty, structure and function and when these molecules are combined together
they form a compound. Structure and function of a molecule are related to each other and QSARs (Quantitative Structure–
Activity relationships) are based on the criteria that the structure of a molecule must contain the features responsible for
its physical, chemical, and biological properties, and on the ability to represent the chemical by one, or more, numerical
descriptor(s). By QSAR models, the biological activity of a new or untested chemical can be inferred from the molecular
structure of similar compounds whose activities have already been assessed. QSARs attempt to relate physical and
chemical properties of molecules to their biological activities. For this there are so many descriptors (for example,
molecular weight, number of rotatable bonds, Log P) and simple statistical methods such as Multiple Linear Regression
(MLR) are used to predict a model. These models describe the activity of the data set and can predict activities for further
sets of (untested) compounds. These types of descriptors are simple to calculate and allow for a relatively fast analysis.
3D-QSAR uses probe-based sampling within a molecular lattice to determine three-dimensional properties of molecules
(particularly steric and electrostatic values) and can then correlate these 3D descriptors with biological activity.
Physicochemical descriptors, include hydrophobicity, topology, electronic properties, and steric effects etc. These
descriptors can be calculated empirically, statistically or through more recent computational methods. QSARs are
currently being applied in many disciplines, with many pertaining to drug design and environmental risk assessment.