ALAIN VIEL: In the previous video, you've
learned about the structural diversity that is created by carbon atoms.
Atomic composition is not the only factor that
determines the identity of a molecule.
Some molecules have the same atomic composition,
but have different three-dimensional structure.
In our example here, maleic and fumaric acids have the same composition,
but the double bond blocks the free rotation about the carbon-carbon axis.
Interconversion is prevented unless covalent bonds are broken.
The two molecules are said to have different configurations.
They are also called stereoisomers.
Maleic acid, with its two bulky groups, represented in purple on this diagram,
on the same side of the double bond.
And this molecule corresponds to the cis configuration.
Fumaric acid, with its two bulky groups on opposite sides of the double bond
corresponds to the trans configuration.
A double bond is not a prerequisite to have configurational isomers.
Molecules with the same composition and containing a chiral center
can also exist in different configurations.
So what is a chiral center?
It is a place in a molecule where a carbon is
bound to four distinct chemical groups.
In our example, the chiral carbon is linked to one hydrogen
and to three distinct chemical groups, R1, 2, and 3.
If the carbon was linked to one hydrogen, to the group R2,
and to two identical groups, R1, the carbon would be said achiral.
Depending on how the four groups are arranged around the chiral carbon,
different configurational isomers are found.
Once again, a switch from one configuration to another
requires the breaking of covalent bonds.
The two different configurations of a molecule containing a chiral center
are called enantiomers.
Enantiomers are mirror images of each other, as you can see on this diagram.
No matter how you rotate the original chiral molecule,
you never obtain its mirror image without breaking covalent bonds.
In contrast, you can obtain the mirror image of an achiral molecule
by simple rotation of the chemical groups.
This is a similar concept to our hands and feet.
As you can see on this diagram, our feet are mirror images of each other.
Now take a look at my hands.
If I show you the palm of my two hands, they are mirror images of each other.
My thumbs point toward each other.
I need to rotate one of my hands to have my two thumbs pointing
in the same direction.
But now you can see the palm of one of my hands,
and the back of the other hand.
I would have to cut, rearrange, and glue back some fingers
to show you the palm of my hands and have my thumbs
pointing in the same direction.
And I'm not ready to do it.
So what are the biochemical consequences of having
molecules in different configurations?
The interaction between ligands and their receptors in cells
are stereo-specific.
Molecules with the same composition but different configurations
bind to different receptors, thereby increasing functional diversity.
In our example, R- and S-Carvone are two enantiomers,
and are mirror images of each other.
They bind to distant receptors on nerve cells in your nose.
The binding of R-Carvone to its specific receptor
leads to a sensation of a fresh spearmint smell.
The binding of S-Carvone to its specific receptor
leads to the perception of a pungent caraway smell.
SPEAKER: So double bonds don't allow any rotation, but what about single bonds?
Single bonds do allow free rotation.
The different spatial arrangements of a molecule caused by free rotation
are called conformations, not to confuse with configurations.
The switch from one conformation to another
does not require the breaking of a covalent bond.
In theory, molecules can adopt an infinite number
of possible conformations.
However, some are more stable than others and become prevalent.
Take this molecule of ethane, for example,
and consider two of its conformations, shown on the graph.
These conformations result from a rotation of a methyl group, CH3,
about the carbon-carbon bond located in the center of the molecule.
The graph shows the potential energy of these two extreme conformations,
one having the hydrogen eclipsed with one another, one having the hydrogen
staggered apart.
Due do steric hindrance, the staggered conformation
has a lower potential energy.
As we'll learn in the next videos, it means
that the staggered conformation has a highest
ability than eclipsed conformation.
The free rotation gives flexibility to biomolecules,
and allows biomolecules to reach their most stable conformation.
So in these last two videos, you learned that carbon is a versatile building
block of biomolecules.
Carbon is a key element explaining both the structural and functional diversity
of biomolecules.
In the next video, we'll see a variety of ways
carbon is exchanged between living organisms and their environment.
We'll also introduce the concept of energy transfer,
another key component distinguishing living from inert matter.
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