Drug molecules three-dimensional orientation

At this point, it is worth introducing stereoisomers. These are isomeric molecules that have the same molecular formula and hence the same sequence of bonded atoms. However, they differ in the three-dimensional orientation of their constituent atoms. For example, consider the example of the antiarrhythmic drug sotalol. Both d-sotalol and I-sotalol isomers affect 4, that influences cardiac repolarization, but only the I-isomer has beta-blocking effect. Another example is warfarin. The S-isomer is three to five times more potent than the R-isomer, and they are metabolized by different cytochrome pathways, an influence that contributes to variable response and toxicity between batches of the drug and ethnic populations. 

When two molecules approach one another to begin a chemical reaction, the probability of a successful encounter can depend critically on the three-dimensional shapes and the relative orientation of the molecules, as well as on their chemical identities. Shape is especially important in biological and biochemical reactions, in which molecules must fit precisely onto specific sites on membranes and templates drug and enzyme activity are important examples. Characterization of molecular shape is therefore an essential part of the study of molecular structure. 

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