Imaging of Catenated, Figure‐of‐Eight, and Trefoil Knot Polymer Rings

Abstract

Most of the available characterization techniques for macromolecules are based on the determination of the average properties (chemical composition, tacticity, dimensional parameters, melting temperature, etc) of an entire set of molecules in a given system. In this context, imaging techniques could offer a quite different and unique approach that allows the analysis of macromolecules as single objects. However, isolated polymer chains are often too diffuse to be distinctly and directly visualized. To circumvent this problem, a magnification process of the size and density of macromolecules prior to imaging can be used to homogeneously enlarge isolated polymer chains before observation by atomic force microscopy (AFM). Numerous studies have been devoted over the past decades to the investigation of the properties of isolated biomolecules by molecular imaging. The visualization of single DNA molecules [1] has shown that DNA can exist in the form of single and interlocked rings of various complexities. [1,4] Since then, several examples of catenated and knotted single-and double-stranded DNA rings have been reported. [5][6][7][8][9] Continuous progress in this area is clearly associated with the development of imaging techniques that allow the investigation of the molecular structure [10,11] and mechanical properties [12][13][14][15] of isolated macromolecules. Besides biomacromolecules, a limited number of reports have addressed the imaging of isolated polyelectrolyte chains, [16][17][18] and even fewer studies of the AFM investigation of neutral polymer chains at the molecular level have been reported. [19,20] Indeed, neutral linear polymer chains, which have sizes that range from a few to several hundred nanometers, possess a cross section of only a few atoms and are softer than biomolecules and polyelectrolytes. Moreover, their local concentration can widely fluctuate through chain motion because of weaker interactions with substrates. [21][22][23] This fluctuation makes the linear polymer chains highly diffuse objects that cannot be easily observed by imaging microscopy. To increase the contrast of the images, selective adsorption of salts has been used by Minko and co-workers in the case of polyelectrolyte chains. [24] To date, AFM visualization and characterization of single synthetic non-ionic macromolecules has been mostly limited to macromolecules with invariant shape such as carbon nanotubes, comb polymers, [25][26][27][28][29][30][31] and dendrigrafts, [32,33] which possess higher mass densities along their backbone than linear polymers. Therefore, one method to permit molecular imaging of single linear, cyclic, and starlike main-chain polymers could involve magnification of the macromolecules by homogeneously increasing their mass density along the whole of the chain, thus making their cross-section thicker and more readily detectable by the AFM tip. This magnification can be achieved, for example, by uniform grafting of well-defined oligomers onto the backbone of the target macromolecules, a strategy ...

Catenation of Self‐Assembled Nanorings

“…Recently, Deffieux et al. have succeeded in visualizing catenation of grafted polymers by using AFM 19. To our knowledge, the present study is the first time catenation of purely self‐assembled nanofibers has been visualized by AFM.…”

“…[18] Recently, Deffieux et al have succeeded in visualizing catenation of grafted polymers by using AFM. [19] To our knowledge, the present study is the first time catenation of purely self-assembled nanofibers has been visualized by AFM. Arguably, the formation of self-assembled catenanes composed of SMBBs requires the occurrence of a large number of well-defined toroidal aggregates in solution upon cooling for statistical reasons.…”

A supramolecularly templated catenane initiator and a controlled ring expansion strategy

“…Recently, a few studies using atomic force microscopy (AFM) to investigate neutral macrocyclic polymer chains at the molecular level have been reported. 6,17 Here, we have attempted to directly observe the obtained catenated polymers by AFM. The interlocked cyclic structures of individual polymers (e.g.…”

Active‐Metal Template Synthesis of a Molecular Trefoil Knot

“…Although many different approaches to catenanes and rotaxanes have been introduced,1 few strategies have been successfully developed for the synthesis of molecular knots 2. Trefoil knots, the simplest prime knot other than the topologically trivial unknot (i.e., any ring or simple macrocycle),3 have been found in DNA,4 proteins,5 and in synthetic polymers 6. Sauvage and co‐workers prepared the first synthetic molecular knot by using the preorganization of two ligand strands around two tetrahedral Cu I centers as the key template interaction to generate the three crossing points required for a trefoil knot 7.…”