Kinematic Self-Replicating Machines
© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.
Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.
4.1.4 Self-Assembling Rotaxanes and Catenanes
Noncovalent self-assembly of molecular parts has also been demonstrated. For instance, Fraser Stoddart and colleagues [1510-1515] have extensively investigated the rotaxanes , nanoscale systems in which one molecular part is threaded through a hole or loop in another molecular part, and kept from unthreading by end-groups, like a ring trapped on a barbell. In one system, a ring-shaped molecule slides freely along a shaft-like chain molecule, moving back and forth between stations at either end with a frequency of ~500 Hz, making an oscillating “molecular shuttle” . This shuttling behavior can be controlled by a series of different chemical, electrochemical, or photochemical external stimuli [1513, 1517]. Using another approach, G. Wenz  threaded ~120 molecular beads onto a single, long, poly-iminooligomethylene polymer chain, making a “molecular necklace.” Other groups are pursuing related research [1519-1521] regarding [n]-rotaxanes [1522, 1523] and pseudorotaxanes (mechanically-threaded molecules) [1524, 1525], while still others are using the necklace technique as a means of, for example, self-assembling ~2 nm wide cyclodextrin nanotubes  and more complex molecular machines .
Another interesting molecular system that can be made entirely by self-assembly is the catenanes , which have two or more closed rings joined like the links of a chain. The rings are mechanically linked – there are no covalent bonds between separate links. The first -catenane (2 linked rings) was constructed by Edel Wasserman of AT&T Bell Laboratories in 1960, using simple hydrocarbon rings; the first -catenane was synthesized in 1977. In 1994, Fraser Stoddart and David Amabilino constructed the first -catenane , having ~372 atoms, dubbed “olympiadane” because of its resemblance to the Olympic rings, and in 1997 the same group announced the first deterministically-ordered 7-ring heptacatenane ; another group later reported the spontaneous self-assembly of a 10-component catenane . Much research on larger polymeric chains of linked rings (e.g. oligocatenanes ), supramolecular “daisy chains” [1532, 1533], fullerene-containing catenanes , protein-based catenanes , and supramolecular weaving  is now in progress. The smallest -catenane constructed to date has dimensions 0.4 nm x 0.6 nm . Self-assembled mechanically-interlocked 2-dimensional [1538-1540] and 3-dimensional [1541, 1542] “infinite” arrays of single molecular ringlike species are known. Interestingly, in 1998 it was discovered that many viral coats (which also self-assemble) appear to be a 2-dimensional “chain-mail” weave of mechanically-interlocked protein rings. For instance, the spherical bacteriophage HK97 capsid shell consists of exactly 72 interlinked protein rings, specifically 60 hexamers and 12 pentamers .
Last updated on 1 August 2005