Chemical evolution as a tool for molecular discovery

In modern academic and industrial laboratories, evolutionary strategies are used routinely to identify biopolymers with novel activities. Large libraries of nucleic acids (approximately 10(15)) or peptides and proteins (approximately 10(13)) can be subjected to multiple rounds of selective pressure, amplification, and diversification, yielding individual sequences with desirable properties. Although the evolutionary approach is a powerful search tool, the chemical nature of biopolymers is not suited for all purposes. Application of evolutionary strategies to libraries of arbitrary chemical composition would overcome this problem, and radically change traditional small-molecule discovery. The chemical make-up of in vitro evolution libraries has necessarily been limited, because library synthesis relies on enzymes. A great deal of current research focuses on expanding the chemical repertoire of in vitro evolution by (a) broadening enzyme substrate specificities to include unnatural building blocks, or (b) developing methods to translate DNA sequences into multistep organic syntheses. We discuss the strengths and weaknesses of the approaches, review the successes, and consider the future of chemical evolution as a tool.