A piece of rope or cord can be tied into different knots with their, often contrasting, properties exploited in a range of applications, from shoelaces to the knots used for climbing, fishing and sailing. 1 Although knots are found in DNA and proteins, 2 and form randomly in other long polymer chains, 3,4 there is currently a lack of methods for tying 5 different sorts of knots in a synthetic nanoscale strand. Here we show that interspersing coordination sites for different metal ions within an artificial molecular strand enables it to be tied into multiple knots. Three topoisomers-an unknot (01) macrocycle, a trefoil (31) knot, [6][7][8][9][10][11][12][13][14][15] and a three-twist (52) knot-were each selectively prepared from the same molecular strand by using transition metal and lanthanide ions to guide chain folding in a manner reminiscent of the role played by protein chaperones. 16 We find that the metal-ion-induced folding can proceed with stereoinduction: for one knot a lanthanide(III)-coordinated crossing pattern only formed with a copper(I)coordinated crossing of particular handedness. In an unanticipated finding, metal ion coordination was also found to be able to translocate an entanglement from one region of a knotted molecular structure to another, resulting in an increase in writhe (topological strain) in the new knotted conformation. The knot topology affects the chemical properties of the strand: while the tighter 52 knot can bind two, different, metal ions simultaneously, the looser 31 isomer can only bind either one Cu(I) ion or one Lu(III) ion. The ability to tie nanoscale chains into different knots offers new opportunities to explore for modifying the structure and properties of synthetic oligomers, polymers and supramolecules.
We report on catalysis by a fuel‐induced transient state of a synthetic molecular machine. A [2]rotaxane molecular shuttle containing secondary ammonium/amine and thiourea stations is converted between catalytically inactive and active states by pulses of a chemical fuel (trichloroacetic acid), which is itself decomposed by the machine and/or the presence of additional base. The ON‐state of the rotaxane catalyzes the reduction of a nitrostyrene by transfer hydrogenation. By varying the amount of fuel added, the lifetime of the rotaxane ON‐state can be regulated and temporal control of catalysis achieved. The system can be pulsed with chemical fuel several times in succession, with each pulse activating catalysis for a time period determined by the amount of fuel added. Dissipative catalysis by synthetic molecular machines has implications for the future design of networks that feature communication and signaling between the components.
We review recent progress in molecular knotting, the chemistry of orderly molecular entanglements. As complex nanotopologies become increasingly accessible they may play significant roles in molecular design.
The anti-HIV nucleoside lamivudine was asymmetrically synthesized in only three steps via a novel surfactant-treated subtilisin Carlsberg-catalyzed dynamic kinetic resolution protocol. The enantiomer of lamivudine could also be accessed using the same protocol catalyzed by Candida antarctica lipase B.
Constructing small molecule systems that mimic the functionality exhibited in biological reaction networks is a key objective of systems chemistry. Herein, we report the development of a dynamic catalytic system where the catalyst activity is modulated through a dynamic covalent bond. By connecting a thermodynamically controlled rearrangement process to resolution under kinetic control, the catalyst system underwent kinetic self-sorting, resulting in amplification of a more reactive catalyst while establishing a catalytic feedback mechanism. The dynamic catalyst system furthermore responded to catalytic events by self-perturbation to regulate its own activity, which in the case of upregulation gave rise to systemic autocatalytic behavior.
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