plex determines what happens next in a sequence of chemical reactions. For example, in an enzyme-substrate complex the reaction may take place at a particular part of a molecule even if that is not the most chemically reactive site—in contrast to normal synthetic chemistry, where changes take place at the reactive functional groups. Alternatively, enzymatic reaction might produce just one of several possible stereochemical consequences. For example, an enzyme might bind a natural L-amino acid but not the mirror image and thus distinguish the L -amino acids that human cells contain from the D-amino acids sometimes found in bacteria.

The selective introduction of chirality is a problem of much current interest in synthesis, and it is generally solved by using the same concept that governs enzymatic reactions. The nonchiral substrate interacts with a chiral reagent or catalyst, and the selective conversion of the substrate to a chiral product follows. Pioneering work to develop such methods was recognized in 2001 by Nobel prizes to William S. Knowles, Ryoji Noyori, and K. Barry Sharpless. In synthesis the binding often involves quite different forces than are used in enzymes, and the reagents and catalysts are much smaller than are protein enzymes. New procedures increasingly involve the formation of well-defined molecular complexes between substrate and catalyst, or substrate and reagent, that may allow chemists to overcome the classical domination of selectivity by the reactivity of functional groups. Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen received Nobel prizes in 1987 for their work on molecular complexes.

There is another approach that is increasingly part of synthesis: the use of enzymes as catalysts. This approach is strengthened by the new ability of chemists and molecular biologists to modify enzymes and change their properties. There is also interest in the use of artificial enzymes for this purpose, either those that are enzyme-like but are not proteins, or those that are proteins but based on antibodies. Catalytic antibodies and nonprotein enzyme mimics have shown some of the attractive features of enzymes in processes for which natural enzymes are not suitable.

Historically, chemistry has been largely reductionist, breaking natural materials such as wood down to their pure components so they could be analyzed. It has since become more integrationist, putting together pure chemicals into complex organized structures. This is true in the new areas of nanoscience and nanotechnology, where synthesis is needed to make organized arrangements of many chemical components—nanostructures—with the distant goal of making tiny molecular machines and even molecular scale computers. Integration is also exploited as chemists begin to synthesize organized structures that imitate some of the properties of living cells. The processes carried out by living cells depend on the spatial organization of many different chemical components. Chemists, in