Due to the great scientific recognition (Nobel Prize in chemistry in 2016) and the increasing application potentials of mechanically interlocked, supermolecular structures (e.g. catenanes, rotaxanes), more and more attention is paid to the synthesis and practical application of these unique supermolecules.
Japanese groups at Kumamoto University, in a collaboration with Singapore University researchers, have recently reported on a convenient, one-pot synthesis of poly-catenanes from preformed cyclodextrin-polyrotaxane precursor complex. (Taishi, H. et al : One-pot synthesis of cyclodextrin-based radial poly [n]catenanes Communications Chemistry (2019) 2:78 https://doi.org/10.1038/s42004-019-0180-x |)
The one-pot syntheses (see Figure 1) and isolation of cyclodextrin-based radial polycatenanes with large numbers of cyclic components (>10) attached to a poly-(ethylene glycol)–poly-(propylene glycol) copolymer core, together with detailed structural characterization were communicated. The results reported in this paper could be utilized in the development of advanced supramolecular materials, such as molecular switches, molecular actuators, molecular machines, bio-materials, and drug targeting carriers.
Figure 1. Preparation of a β-CyD-based polycatenane. (Taishi, H. et al, 2019)l
The first cyclodextrin-based catenanes
It may be surprising that catenanes have been known for over 60 years. It has long been accepted in the cyclodextrin literature that the very first catenane synthesis was published by Cramer’s group as early as in 1958, (Lüttringhaus, A.F. Cramer, H. Prinzbach, R.M. Henglein Ann. 613. 185. 1958.)
They prepared an inclusion complex of an aromatic dithiol with sufficiently long aliphatic chain entrapped into a beta-cyclodextrin, followed by oxidizing the two protruding terminal thiol groups to give a disulfide bridge and thus closing a second ring of the catenane.
However, Guzman Gil-Ramirez et al. published in 2015 a comprehensive review on the catenanes (Guzmán Gil-Ramírez, et al Catenanes: Fifty Years of Molecular Links Angew. Chem. Int. Ed. Engl. 2015 May 18; 54 (21): 6110–6150.) and they questioned the successful completion of the cyclisation reaction reported by Cramer’s group. In this review we learned that the first catenane synthesis with sufficiently proven structure was accomplished by Wasserman, who communicated the synthesis of a catenane by statistical threading of a diester through macrocycle during an acyloin condensation. (Wasserman, E. The preparation of interlockin rings : a Catenane, J A C S . 1960, 82, 16, 4433-4434 ,) (See Figure 2). So the first cyclodextrin-based catenane for which evidence was put forward in support of its structure, was synthesized in 1960 by Wasserman.
Figure 2. Scheme of Wasserman’s cyclodextrin-catenane synthesis
(picture taken from Angew. Chem. Int Ed. Engl. 2015 May 18; 54(21): 6110–6150.)
Catenene made by Mother nature:
A few years later after the discovery of the first synthetically generated catenane structure, the discovery of the first naturally occurring catenane composed of DNA strands was revealed. This was isolated from the mitochondria of HeLa cells and human leukaemic leucocytes, being constructed of circular DNA, as shown in Figure 3. (B. Hudson and J. Vinograd, Catenated circular DNA molecules in HeLa cell mitochondria Nature, Vol. 216, 647–652., 1967.)
Figure 3. Electron micrograph (EM) of circular DNA showing a catenane topology, (b) and c) highlighting the two rings of the DNA catenane as a Hopf link.
About catenanes see also: