combinatorial chemistry
Combinatorial chemistry is the art of making enormous numbers of compounds by mixing and matching building blocks in every possible combination. If you have ten options at one position and ten at another, you can in principle make a hundred products; with three positions you reach a thousand, and so on. It is a multiplication machine for molecular diversity.
Classic combinatorial methods exploit this combinatorial explosion through techniques like split-and-pool synthesis on solid-phase beads, where each bead ends up carrying a single distinct compound. Modern incarnations include DNA-encoded libraries, in which each molecule is tagged with a unique DNA barcode that records how it was assembled, letting billions of compounds be screened together and the hits decoded by sequencing.
The early 1990s hype promised that sheer numbers would fill drug pipelines, but raw library size proved less valuable than library quality. Many combinatorial libraries were built from easy reactions yielding flat, druglike-poor molecules, and screening them gave disappointing hit rates. The field matured by favoring smaller, well-designed, property-aware libraries over brute-force vastness.
A DNA-encoded library combining three sets of 1,000 building blocks contains a billion barcoded compounds screened in a single tube.
Three positions, a billion compounds.
Combinatorial chemistry produces a library; whether each member is a discrete compound or part of a mixture depends on the method (one-bead-one-compound vs. pooled formats).