Viologen Tweezers to Probe the Force of Individual Donor–Acceptor π-Interactions

Abstract

Donor−acceptor (DA) π-interactions are weak attractive forces that are exploited widely in molecular and supramolecular chemistry. They have been characterized extensively by ensemble techniques, providing values for their energies that are useful for the design of soft materials. For implementation of motions or operations based on these DA π-interactions in wholly synthetic molecular machines, the mechanical strength and force associated with their out-ofequilibrium performance are the key parameters, in addition to their energies obtained at thermodynamic equilibrium. In this context, we have used singlemolecule force spectroscopy as a nonequilibrium technique to determine the mechanical strength of individual DA π-interactions in solution. We designed and synthesized a molecular tweezer that is able to encapsulate π-donors and also demonstrated a precise opening extension. The mechanical breaking of the noncovalent interactions between viologen unitsπ-acceptors commonly employed in mechanically interlocked moleculesand several π-donors afforded a characteristic force−distance signature, revealing the opening of individual viologen tweezers with an unambiguous extension. Single-tweezer host-exchange experiments performed in situ demonstrated the sensitivity of the technique. This simple design could be exploited in quantifying the force of a large range of weak noncovalent bonding interactions as well as the potential work that molecular machines can generate at the single-molecule level.

Multivalent non-covalent interactions lead to strongest polymer adhesion

“…70 to 80 pN for strong donor-acceptor π-interactions in H 2 O, respectively. 53,54 In our multivalent system, there are 12 possibilities for π-π stacking (5•phenyl, 5•catechol, 2•phenyl from DBCO) leading to forces in the range of several 100 pN. In case of cooperative effects (non-linear increase of force with number of bonds) a further increase of the observed force is possible.…”

Side chain interactions govern the response of polypeptide alpha-helices under mechanical stress and prevent the formation of beta-sheets

“…We previously showed that this approach, based on the physisorption of caught PEG tethers onto the tip, provides a sufficiently strong and stable attachment of the molecule onto the tip to obtain reliable information on the system grafted on the surface. [14][15][16][17][18][19] As a result of the different side chains (Figure 1), the ahelices form under opposite pH conditions, pH > 11 for PLys and pH < 4 for PGA. Immediately prior to performing single-molecule force spectroscopy experiments, the gold substrates were dipped in a diluted solution of the molecules and a passivating agent PEG6-SH was added, to enable single molecules to be individually distributed and immobilized onto the cleaned surface (see Methods section).…”

The Rise and Promise of Molecular Nanotopology

“…Molecular links and knots in proteins have been recognized to impact [220][221][222] significantly on their mechanical strength and stability, as revealed by both theoretical simulations and single-molecule force spectroscopy [223][224][225] . In order to harness these features in smart new materials, mechanical bonds are usually incorporated into synthetic polymers, 226 and their influence on the properties of the polymers evaluated.…”