NYU Chirality Switch

from the yet-another-actuator dept.
Jeffrey Soreff writes "Richard Terra and Christine Peterson originally pointed me towards an article on a chirality-switching molecule from NYU described at link

Quoth the press release web page:

A New York University team led by chemist James W. Canary has developed a molecule with switchable chirality*. (FOOTNOTE: Nearly all biomolecules are chiral compounds. That is, they exist in two forms (enantiomers) which are non-superimposable mirror images of each other. …) …The investigators were then able to switch the molecule's chirality by the addition or removal of an electron.

For more analysis by Jeffrey Soreff, click "Read More" below. This press release is a summary of an article in Science titled "Electron-Induced Inversion of Helical Chirality of Copper Complexes of N,N-Dialkylmethionines". (I do not yet have access to this article.) Now different oxidation states of a molecule cannot be enantiomers of each other, they differ in all sorts of ways besides handedness. It turns out that a better description of the work is at one of Canary's lab's web pages: link

It turns out that the changes in the complex due to the change in oxidation state are indeed quite a bit more dramatic than a shift from one enantiomer to another. The copper connects to different ends of the methionine ligand in the Cu(I) (cuprous) and Cu(I) (cupric) complexes. Quoth the web page:

It is well known that Cu(II) ions form very stable complexes with carboxylates, whereas Cu(I) favors sulfide ligands. It thus seemed plausible that compound Met1, a derivative of the amino acid methionine, might display a different coordination chemistry in Cu(II) and Cu(I) complexes (Fig. 4). The Cu(II) complex should coordinate the three nitrogen atoms and the carboxylate. For the (S)-enantiomer, this would result in negative chirality for the orientation of the chromophores, and should give rise to a (-)-couplet in the ECCD spectrum. The Cu(I) complex, on the other hand, should show coordination by the dialkyl sulfide ligand in place of the carboxylate, which would invert the twist of the molecule, and yield a (+)-couplet in the ECCD spectrum. The inversion of the twist of the molecule would come about as a result of the pivoting of the amino acid arm about the C-N bond (Fig. 4), which would invert the direction of the C-H bond. This would in turn cause the other two methylene groups to rotate, resulting in inversion of the overall twist of the structure, and therefore a change in chirality of the orientation of the quinoline chromophores with respect to one another.

so the molecule does indeed switch the overall twist in the complex and therefore changes the sign of the ECCD (a variant of circular dichroism) couplet (hence the chirality switch) but this isn't a switch between enantiomers, but rather between complexs with opposite overall twist, but with other differences (Cu-O vs. Cu-S bonding!) as well.

From the perspective of nanotechnology, I see this as another redox-based actuator. A charge is changed, and part of a molecule moves with respect to another part. Nice chemistry, but I don't see it as terribly different from similar things Stoddart did in '96, e.g.

Gomez Lopez, M. Preece, J.A. Stoddart, J.F.

"The art and science of self assembling molecular machines" [Nanotechnology 7:183-192 Sept. 1996]

In this review, we show how noncovalent bonding interactions between pi-electron rich aromatic ring systems (e.g. hydroquinone) and the pi-electron deficient tetracationic cyclophane, cyclobis(paraquat p phenylene) can be used to self assemble novel molecular architectures which are not only interesting to us, because of their fascinating topologies, but also because they have the potential to be developed into molecular structures with switchable properties on the nanometre scale. The high efficiency observed in the self assembly of a (2)catenane, and its dynamic properties in solution, represent the first step in the design and self assembly of other molecular assemblies better suited for the study of molecular switching processes. Therefore, a series of (2)rotaxanes, mechanically interlocked molecular compounds, consisting of a linear pi-electron rich dumbbell shaped component and the pi-electron deficient tetracationic cyclophane as the cyclic component, have been self assembled and evaluated. All of the so called molecular shuttles show translational isomerism and one of them, comprising benzidine and biphenol recognition sites as the non degenerate pi-electron rich sites, shows molecular switching properties when it is perturbed by external stimuli, such as electrons and protons. The versatility of our approach to nanoscale molecular switches is proven by the description of a series of molecular assemblies and supramolecular arrays, consisting of pi-electron rich and pi-electron deficient components, which display molecular switching properties when they are influenced by external stimuli that are photochemical, electrochemical and/or chemical in nature. However, the molecular switching phenomena take place in the solution state. Therefore, finally we describe how simple molecular structures can be ordered on to a solid support at the macroscopic level using Langmuir Blodgett techniques. This is a necessary condition which must be fulfilled if we wish to construct supramolecular structures with device like properties at the macroscopic level. [emphasis added to highlight actuator]

There is a bit of novelty in that that change in copper bonding to methionine gives a shove to the quinoline groups, which do the actual rotating of polarized light. The combination of an actuator with a mechanical link (albeit a short one) is a step towards molecularly precise mechanical systems. This is the same phenomena as allosteric interactions in e.g. hemoglobin. Any comments on what the longest designed atomically precise mechanical link is? "

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