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An STM image of individual L and
D Di-phenylalanine molecules adsorbed onto Cu (110). A human body
has more than one thousand trillion trillion molecules with about
one hundred thousand different shapes and functions. The
researchers have followed the interaction between two molecules to
show the basic mechanism underlying chiral recognition.
Image by Max Planck Institute for
Solid State Research |
If one thinks that there are thousands of
times more molecules forming our body than stars in the universe it is
astonishing how all these molecules can work together in such an
organised and efficient way. How can our muscles contract to make us
walk? How can food be metabolised every day? How can we use specific
drugs to relieve pain?
To work as a perfect machine, our body ultimately
relies on the capability of each little part (molecule) to know a
specific function and location out of countless possibilities. To do
this, molecules carry information in different ways. An international
team at the Max Planck Institute for Solid State Research in
Stuttgart, in collaboration with scientists from the Fraunhofer
Institute in Freiburg and the King's College London are seeking to
find out how the information can be passed on at the very first steps:
from the single molecule level to structures of increasing complexity
and functionality.
The key to understanding all biological processes
is recognition. Each molecule has a unique composition and shape that
allows it to interact with other molecules. The interactions between
molecules let us - as well as bacteria, animals, plants and other
living systems - move, sense, reproduce and accomplish the processes
that keep all living creatures alive.
A very common example of recognition can be
experienced in daily life whenever one meets someone and shakes right
hands. In principle, one can also shake left hands; the fact that we
do it with the right has historically been a sign of peace, used to
show that both people hold no weapon. But, have you ever attempt to
shake the right hand of a person using your left hand? No matter how
the two hands are oriented, you will never fit your left hand with the
right hand of your friend.
Many molecules can recognise each other and
transfer information exactly in the same way, they can either be
"right handed" (D) or "left handed" (L). This property called "chirality"
is a spectacular way to store information: a chiral molecule can
recognise molecules that have the same chirality (same "handedness", L
to L or D to D) and discriminate the ones of different chirality (L to
D and D to L).
Probably one of the most exciting mysteries of
Nature is why the building blocks of life, i.e. amino acids (the
building blocks of proteins) are exclusively present in the chiral L
form and sugars (which constitute DNA) are all in the D form. Once
more, the reason for this preference is "historical", but this time
goes back millions of years till the origins of the biological world.
Scientists believe that current life forms could not exist without the
uniform chirality ("homochirality") of these blocks, because
biological processes need the efficiency in recognition achieved with
homochiral substances. In other words, the separation of molecules by
chirality was the crucial process during the Archean Era when life
first emerged.
Researchers of the Max Planck Institute for Solid
State Research have now used the "nanoscopic eye" of a scanning
tunnelling microscope to make movies following how two adsorbed
molecules (diphenylalanine, the core recognition motif of Alzheimer
amyloid polypeptide) of the same chirality can form structures (pairs,
chains) while molecules of different chirality discriminate and cannot
form stable structures.
As it occurs when you shake the hand of your friend,
the fact that the two homochiral hands are complementary by shape is
not enough, you both have to dynamically adapt and adjust your hands
to reach a better fit, a comfortable situation. By a combination with
theoretical simulations done at Kings College London, the researchers
have shown for the first time this dynamic mechanism of how two
molecules "shake hands" and recognise each other by mutually induced
conformational changes at the single molecule level.
We live in houses, wear clothes and read books made
of chiral cellulose. Most of the molecules that mediate the processes
of life like hormones, antibodies and receptors are chiral. Fifty of
the top hundred best-selling drugs worldwide are chiral. With this
contribution to the basic mechanism of chiral recognition, the
researchers have not only tracked back to the very first steps in the
evolution of living matter but have also shed light on our
understanding and control of synthetic (man-made) materials of
increasing complexity. |
