Rotaxane

A rotaxane (from Latin rota 'wheel' and axis 'axle') is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation).

The earliest reported synthesis of a rotaxane in 1967 relied on the statistical probability that if two halves of a dumbbell-shaped molecule were reacted in the presence of a macrocycle that some small percentage would connect through the ring.

[4][5] Recently, Leigh and co-workers described a new pathway to mechanically interlocked architectures involving a transition-metal center that can catalyse a reaction through the cavity of a macrocycle.

This dynamic complex or pseudorotaxane is then converted to the rotaxane by reacting the ends of the threaded guest with large groups, preventing disassociation.

[8] The clipping method is similar to the capping reaction except that in this case the dumbbell shaped molecule is complete and is bound to a partial macrocycle.

The partial macrocycle then undergoes a ring closing reaction around the dumbbell-shaped molecule, forming the rotaxane.

Leigh and co-workers recently began to explore a strategy in which template ions could also play an active role in promoting the crucial final covalent bond forming reaction that captures the interlocked structure (i.e., the metal has a dual function, acting as a template for entwining the precursors and catalyzing covalent bond formation between the reactants).

[22] The enhanced stability of rotaxane dyes is attributed to the insulating effect of the macrocycle, which is able to block interactions with other molecules.

Graphical representation of a rotaxane
Structure of a rotaxane that has a cyclobis(paraquat- p -phenylene) macrocycle . [ 1 ]
(a) A rotaxane is formed from an open ring (R1) with a flexible hinge and a dumbbell-shaped DNA origami structure (D1). The hinge of the ring consists of a series of strand crossovers into which additional thymines are inserted to provide higher flexibility. Ring and axis subunits are first connected and positioned with respect to each other using 18 nucleotide long, complementary sticky ends 33 nm away from the center of the axis (blue regions). The ring is then closed around the dumbbell axis using closing strands (red), followed by the addition of release strands that separate dumbbell from ring via toehold-mediated strand displacement. (b) 3D models and corresponding averaged TEM images of the ring and dumbbell structure. (c) TEM images of the completely assembled rotaxanes (R1D1). (d) 3D models, averaged and single-particle TEM images of R2 and D2, subunits of an alternative rotaxane design containing bent structural elements. The TEM images of the ring structure correspond to the closed (top) and open (bottom) configurations. (e) 3D representation and TEM images of the fully assembled R2D2 rotaxane. Scale bar, 50 nm. [ 7 ]
Rotaxane synthesis can be carried out via a "capping," "clipping, "slipping" or "active template" mechanism
Structure of a rotaxane with an α- cyclodextrin macrocycle . [ 11 ]
Animation of a pH-controlled molecular rotaxane shuttle