Naturally occurring and synthetic cyclic macromolecules

Abstract

Macrocyclic polymers are fascinating molecules that have triggered the curiosity of biologists, chemists and theoreticians since the discovery of cyclic DNA in living cells, more than 50 years ago [1,2]. The topological restriction imposed by the cyclic architecture and the absence of chain ends in ring polymers result in original properties, some of which have yet to be completely elucidated. For example, it has been shown that cyclic polymers have smaller hydrodynamic volumes and radii and are less viscous than their linear analogues. Other specific properties such as diffusion and viscoelastic behaviors, which should also be strongly influenced by the cyclic topology, have been only marginally explored due to the limited availability of the corresponding high molar mass macrocycles.Much less is known about more complex ring topologies such as knotted and catenated macromolecular rings (Scheme 1) that are also found in nature. Their mechanism of formation and their function are still unclear, and increasing efforts are being conducted to tentatively prepare and investigate their specific characteristics and properties.Various examples of cyclized and pluri-cyclized DNA macromolecules have been reported in the literature since the pioneer work of Clayton [3] who identified circular forms of mitochondrial DNA in extracts of human leukemic leucocytes, both in the form of circular dimers and as catenanes made up of interlocked rings.The winding of DNA in higher-order forms such as knots and catenanes was also shown on electron micrographs of DNA molecules coated with Echerichia coli RecA protein. It was found that DNA topoisomerase I of E. coli generates an equal mixture of (?) and (-) duplex DNA knots, as well as catenanes [4]. Together, RecQ helicase and topoisomerase III (Topo III) of E. coli comprise a potent DNA strand passage activity that can catenate covalently closed DNA [5]. The structure of the catenated DNA species formed by RecQ helicase and Topo III was directly assessed using atomic force microscopy (AFM) [6] (Fig. 1).The mechanism by which topoisomerase mediates knotting of supercoiled DNA was investigated by Wasserman [7]. Knotting was explained by DNA contacts stabilized by enzyme molecules.DNA catenanes and large circular DNA have also been prepared by the reaction of T4 DNA ligase with linear DNA in the presence of nicked DNA [8,9].In addition to naturally occurring macrocycles and the investigation of their formation mechanisms, the preparation and the study of well-defined synthetic ring polymers as models of natural rings, has been implemented. Preliminary studies were first oriented towards polymer systems bearing reactive functions on the backbone chain, such as polydimethylsiloxanes [10], polyethers [11], polyesters [12], and so forth.The synthesis of cyclic polymers by end-to-end coupling of chains, first proposed by Casassa [13], was experimentally performed in 1980 [14,15] by the ring closure of living a, x-difunctional polymer chains in the presence of a difunctional coupling agent, u...

Synthesis of Cyclic Polystyrenes Using Living Anionic Polymerization and Metathesis Ring-Closure

“…The synthesis and characterization of macrocyclic polymers have presented fascinating challenges and rewards for polymer chemists and theoreticians. − Macrocyclic polymers are of intrinsic interest because of their unusual architecture (topology), i.e., their lack of end groups and the consequences of their reduced hydrodynamic volumes. For example, macrocyclic polymers exhibit lower solution viscosities, higher SEC elution volumes, lower mean square radii of gyration, higher glass transition temperatures ( X n < 400), lower theta (Θ) temperatures, ,, higher melt densities, lower zero shear viscosities, faster relaxation times, absence of an entanglement plateau, and faster diffusion coefficients compared to their linear analogues.…”

Force distribution in a semiflexible loop

“…However, loops at the molecular level constantly undergo fast, random thermal fluctuations. Ring-shaped or crosslinked polymers, cyclic macromolecules [1,2], and DNA loops [3][4][5] are some common examples of molecular loops. Polymers whose ends are held at fixed points also belong to this category, which can be realized in single-molecule pulling experiments [6,7].…”

Design and synthesis of catenated rings based on toroidal DNA structures

“…Catenated DNA structures commonly refer to interlocked entities of double-stranded circular DNA in which there is absence of a fixed pattern of physical interactions between their components of DNA circles. [1][2][3][4][5][6][7][8][9] Achievement of proximity between two distant DNA segments through proper means has been deemed to be one of the most crucial prerequisites for synthesizing interlocked DNA structures. [10][11][12] Generation of negatively supercoiled DNA by the action of DNA gyrase, for example, was utilized to drive some distant DNA segments closer to each other, which in turn led to the generation of catenated DNA rings upon subsequent enzymatic actions.…”