An important milestone in the development of nanotechnology leading to atomically precise manufacturing (molecular manufacturing) is the development of artificial molecular machines that can control molecular transformations. Two scientists from the University of Groningen, Netherlands, published a paper in Science [abstract] earlier this year demonstrating control of a chemical reaction by an artificial molecular machine. They constructed a light-driven molecular motor that catalyses different chemical reactions as the motor is stepped through its rotary cycle. The researchers’ institute has made the full text of “Dynamic Control of Chiral Space in a Catalytic Asymmetric Reaction Using a Molecular Motor” available here.
The authors constructed a rotary motor molecule in which the rotor and stator halves of the molecule rotate about an axle consisting of a carbon-carbon double bond. Rotation occurs in only one direction in a four-stage cycle driven by light absorption and by temperature change. Because the molecule is helical in shape, it is chiral, that is, it exists in two different conformations (shapes) that are mirror images of each other.
The rotor and stator halves of the molecule are each attached to a different chemical function so that when rotation about the axle brings the two functional groups spatially close to each other, they catalyze a chemical reaction. At the four different stages of the rotary cycle, the two groups are either widely separated (two trans configurations) and thus have low catalytic activity, or close to each other and therefore have high catalytic activity (the two cis configurations). In one cis configuration the active catalyst is in one chiral orientation; in the other cis configuration, the catalyst is in the opposite chiral orientation. As expected, when used to catalyze an appropriate chemical reaction that can produce either one of two chiral products, the two trans forms of the motor have low activity and they produce a mixture of the two chiral products. The two cis forms of the motor have high activity. One chiral cis form produces predominantly one chiral product; the other produces predominantly the other chiral product.
The authors conclude:
Coupling of unidirectional switching to catalytic function, as demonstrated here, may prove to be a key design tool in the construction of future catalysts that can perform multiple tasks in a sequential manner.
The molecular specificity of this initial proof-of-principle demonstration is only partial. The differences in catalytic activity and the differences in chiral ratios of the reaction products are only of the order of three- or four-fold. We can hope that continued work in this direction will lead to cleaner reaction specificities resulting from programmable control of artificial molecular machines. Eventually we hope to see arrays of programmable molecular catalysts executing complex reaction sequences, leading to productive nanosysems and atomically precise manufacturing.
