Search results for "Mechanical bond"
showing 6 items of 6 documents
Vibrational circular dichroism spectroscopy for probing the expression of chirality in mechanically planar chiral rotaxanes
2020
Mechanically interlocked molecules can exhibit molecular chirality that arises due to the mechanical bond rather than covalent stereogenic units. Developing applications of such systems is made challenging by the absence of techniques for assigning the absolute configuration of products and methods to probe how the mechanical stereogenic unit influences the spatial arrangements of the functional groups in solution. Here we demonstrate for the first time that Vibrational Circular Dichroism (VCD) can be used to not only discriminate between mechanical stereoisomers but also provide detailed information on their (co)conformations. The latter is particularly important as these molecules are now…
Does mechanical locking-base ceramic brackets reduce cracks at debonding?
2012
Objectives: Debonding of chemically bonded ceramic brackets has been mostly led to enamel damages. This study aimed to evaluate enamel cracks followed by debonding ceramic brackets with mechanical retention. Study Design: Sixty extracted human premolar teeth were randomly divided into two groups. Metallic brackets and recently available ceramic brackets with mechanical retention were bonded to the teeth in first (MB) and second (CB) group respectively. After a week, the brackets were debonded using a pair of bracket-removing pliers assembled to a testing machine. Optical stereomicroscope and trans-illumination techniques were used to calculate the cracks and measure their length before bond…
Programmable assembly of peptide amphiphile via noncovalent-to-covalent bond conversion
2017
Controlling the number of monomers in a supramolecular polymer has been a great challenge in programmable self-assembly of organic molecules. One approach has been to make use of frustrated growth of the supramolecular assembly by tuning the balance of attractive and repulsive intermolecular forces. We report here on the use of covalent bond formation among monomers, compensating for intermolecular electrostatic repulsion, as a mechanism to control the length of a supramolecular nanofiber formed by self-assembly of peptide amphiphiles. Circular dichroism spectroscopy in combination with dynamic light scattering, size-exclusion chromatography, and transmittance electron microscope analyses r…
The Right Answer for the Right Electrostatics: Force Field Methods Are Able to Describe Relative Energies of DNA Guanine Quadruplexes
2014
Different force fields and approximate density functional theory were applied in order to study the rotamer space of the telomeric G-quadruplex DNA. While some force fields show an erratic behavior when it comes to the reproduction of the higher-order DNA conformer space, OPLS and MMFF implementations are able to reproduce the experimentally known energy order. The stabilizing effect of the AA (anti−anti) versus SA (syn−anti) conformer is analyzed applying mechanical bond strength descriptors (compliance constants). The fact that we observe the correct energy order using appropriate force fields is in contrast with results previously reported, which suggested the general inappropriateness o…
Dual-stimuli pseudorotaxane switches under kinetic control
2021
A series of dumbbell-shaped sec-ammonium salts with bulky (pseudo)stoppers (‘speed bumps’) were tested for their ability to form pseudorotaxanes with a redox-switchable, tetrathiafulvalene (TTF)-decorated [24]crown-8 ether. Depending on the size of the pseudostoppers, fast (less than ten minutes), slow (hours to days) and very slow (no pseudorotaxanes observed) threading has been observed. NMR spectroscopy as well as tandem mass spectrometry indicate the formation of non-threaded face-to-face complexes prior to pseudorotaxanes formation. Both isomers can be distinguished by their substantially different stability in collision-induced dissociation (CID) experiments. Two external stimuli affe…
Damming an electronic energy reservoir: ion-regulated electronic energy shuttling in a [2]rotaxane
2021
We demonstrate the first example of bidirectional reversible electronic energy transfer (REET) between the mechanically bonded components of a rotaxane. Our prototypical system was designed such that photoexcitation of a chromophore in the axle results in temporary storage of electronic energy in a quasi-isoenergetic “reservoir” chromophore in the macrocycle. Over time, the emissive state of the axle is repopulated from this reservoir, resulting in long-lived, delayed luminescence. Importantly, we show that cation binding in the cavity formed by the mechanical bond perturbs the axle chromophore energy levels, modulating the REET process, and ultimately providing a luminescence read-out of c…