Color coding molecular mirror images

Date:
September 22, 2021
Source:
Kanazawa University
Summary:
Researchers report a new method for distinguishing between
enantiomers, molecules that are mirror images of each other. The
procedure, relevant for the pharmaceutical industry, involves
the chemical reaction of target enantiomers with color indicator
compounds consisting of one-handed helical polymers, leading to
solutions showing different colors in specific solvents between
the enantiomers.



FULL STORY ========================================================================== Enantiomers are molecules that are each other's mirror image -- like
one's left and right hand. They are said to be chiral, chirality being
the term for 'displaying handedness'. Although a pair of enantiomers have totally the same chemical and physical properties, they often exhibit
different physiological activity towards biological molecules. Being able
to distinguish between enantiomers and detect chirality is important for pharmaceutical purposes - - often, only one of two enantiomers acts as a
drug. Now, Katsuhiro Maeda from Kanazawa University and colleagues have
found a new method for determining the chirality of amines (organic
molecules featuring amino groups (-NH2)). The approach is based on
reactions leading to solutions with different colors depending on the enantiomer present.


==========================================================================
The method of Maeda and colleagues involves the use of special organic 'colorindicator' molecules consisting of one-handed helical poly (diphenylacetylene)s possessing carboxy groups in the side chains
(M-h-poly-1- H and P-h-poly-1-H), which are chiral themselves because
they have so-called one-handed (right- or left-handed) helical structures
(the 'M' and the 'P' refer to the left- and right-handed configurations, respectively). The scientists serendipitously discovered that a pair of enantiomers of particular chiral amines, when reacting with M-h-poly-1-H
using a condensing reagent, displayed completely different colors in
particular solvents (for example, in tetrahydrofuran-acetone, yellow
and red, respectively) depending on their chirality, thereby enabling
easy naked-eye differentiation between the enantiomers.

The researchers tested a whole set of other amines, as well as other
nitrogen- containing organic molecules (specifically, amino alcohols and
amino esters), also showing distinct colorings detectable by the naked
eye. Some solutions had to be cooled down to -60 DEGC, however.

Computer simulations of the compounds together with various experimental analysis provided insights into the molecular mechanisms at play. They
showed that for one enantiomer, intramolecular hydrogen bonding
(attraction between hydrogen atoms within a molecule) does not happen, resulting in a stretched helical structure and a yellow solution, whereas
it does for the other enantiomer, causing the molecular helix to contract, resulting in a red-colored solution.

The scientists used their finding to develop a procedure for obtaining
the so- called enantiomeric excess (ee) of a mixture of chiral molecules,
a measure of the enantiomeric 'purity': an ee of 0% means an equal amount
of left- and right-handed molecules, whereas an ee of 100% corresponds
to the situation of only one type of enantiomer being present. For this,
they quantified the color measurement by recording absorption spectra or
by digital photography by converting to RGB (red, green, blue) values;
these depend on a mixture's ee.

Low-error determinations could be made that were in excellent agreement
with measurements obtained by the current standard technique (called high- performance liquid chromatography).

Maeda and colleagues reckon that they can design other indicator
molecules and expand the method. Quoting the researchers: "This
should be applicable to the on-site, naked-eye determination of ee
of various functional molecules and biologically relevant compounds." ========================================================================== Story Source: Materials provided by Kanazawa_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Katsuhiro Maeda, Daisuke Hirose, Mai Nozaki, Yoichi Shimizu,
Taro Mori,
Kentaro Yamanaka, Koji Ogino, Tatsuya Nishimura, Tsuyoshi Taniguchi,
Munetsugu Moro, Eiji Yashima. Helical springs as a color indicator
for determining chirality and enantiomeric excess. Science Advances,
2021; 7 (27): eabg5381 DOI: 10.1126/sciadv.abg5381 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210922090852.htm

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