Chiral chemistry
Inducing chirality.
Researchers at Case Western Reserve University developed a "top-down" approach to introduce chirality into a nonchiral molecule
by using a macroscopic blunt force to impose and induce chirality. "The key is that we used a macroscopic force to create
chirality down to the molecular level," said Charles Rosenblatt, professor of physics at Case Western Reserve University in
Cleveland, Ohio, in a December 2011, press release and senior author of a recent paper on the research (3).
Formulation development forum: nanosized dendrimers
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Specifically, the researchers imposed a macroscopic helical twist on an achiral nematic liquid crystal by controlling the
azimuthal alignment directions at the two substrates (3). On application of an electric field, the director rotates in the
substrate plane. This electroclinic effect, which requires the presence of chirality, is strongest at the two substrates and
increases with increasing imposed twist distortion (3).
The researchers treated two glass slides so that cigar-shaped liquid crystal molecules would align along a particular direction.
They then created a thin cell with the slides, but rotated the two alignment directions by approximately a 20-degree angle,
according to the university release. The 20-degree difference caused the molecules' orientation to undergo a right-handed
helical rotation, or a so-called imposed "chiral twist." Because of the higher energy needed to maintain the naturally left-handed
molecules in the crystal, some of the left-handed molecules in the crystal became right-handed, with this shift being the
induced chirality. To test for chirality, the researchers applied an electrical field perpendicular to the molecules. If there
were no chirality, there would be nothing to see. If there were chirality, the helical twist would rotate in proportion to
the amount of right-handed excess. The result was a model involving a trade-off among bulk elastic energy, surface anchoring
energy, and deracemization entropy that suggested the large equilibrium director rotation induced a deracemization of chiral
conformations in the molecules or "top-down" chiral induction (3).
Stereochemical analysis
. Researchers at Carnegie Mellon University successfully used nuclear magnetic resonance (NMR) to analyze the stereochemical
structure of gold nanoparticles, a potentially important advance in drug development. Determining a nanoparticle's chirality
is a key step toward developing them as chiral catalysts.
The researchers reported on the chirality in gold nanoclusters by NMR spectroscopic probing of the surface ligands. The Au38 (SR)24 and Au25 (SR)18 (where, R = CH2CH2Ph) were used as representative models for chiral and nonchiral nanoclusters, respectively (4). The researchers compared the
NMR signal from the hydrogen atoms in the nonchiral gold nanoparticle with the NMR signal from the hydrogen atoms in the chiral
gold nanoparticle. The NMR method overcame the limitations of circular dichoism spectroscopy in determining the chirality
of gold nanoparticles in a racemic mixture. The nanoparticles' chiral core induced the methylene group's two hydrogen atoms
to give off different frequencies, a phenomenon known as diastereotopicity.
The researchers compared the NMR signal from the hydrogen atoms in the nonchiral gold nanoparticle with the NMR signal from
the hydrogen atoms in the chiral gold nanoparticle. The nonchiral nanoparticle's NMR spectrum did not reveal any differences,
but the chiral nanoparticle's NMR spectrum revealed two different hydrogen signals, providing a simple and efficient way of
telling whether the particle is chiral or not, even for a 50/50 mixture of isomers, according to a Dec. 7, 2011, Carnegie
Mellon University release. The researchers concluded that NMR spectroscopy can be a useful tool for investigating chirality
in gold nanoclusters. Since the diastereotopicity induced on the methylene protons by chiral nanoclusters is independent of
the enantiomeric composition of the chiral particles, NMR can probe the chirality of the nanoclusters even in the case of
a racemic mixture while circular dichroism spectroscopy is not useful for racemic mixtures (4).
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