Primer
Chirality in One Page
Everything in this course follows from a symmetry that is exact in the chemistry and broken in the biology. This page establishes what handedness is and why it is so hard to choose.
What makes a molecule handed
An object is chiral if it cannot be superposed on its mirror image. Your hands are the standard example; so is a screw thread. The commonest molecular case is a carbon atom carrying four different groups: swap any two and you get a structure no rotation will bring back into coincidence.
The difficulty, stated precisely
Two enantiomers have identical energies, identical bond lengths, identical melting points and identical spectra. The electromagnetic interaction that governs chemistry is mirror-symmetric, so there is no thermodynamic reason to prefer one.
It follows that any reaction between achiral starting materials, run in an achiral environment, must produce exactly 50:50. Not approximately — exactly, by symmetry. To get an excess you must introduce handedness from somewhere: a chiral reagent, a chiral catalyst, a chiral solvent, or a chiral seed.
The weak nuclear force is not mirror-symmetric and does make the two enantiomers differ in energy — by something like one part in 10⁷¹ of a bond energy. Whether that is relevant to biology is a live and largely unresolved question; nothing in this course depends on it.
How you tell them apart
Only with something that is itself handed. Plane-polarised light is the classical probe — one enantiomer rotates it clockwise, the other by exactly the same angle anticlockwise, which is how Pasteur did it. Modern practice uses chromatography on a chiral stationary phase, which physically separates the two.
A biological receptor is the same kind of probe, built from handed components, which is why one enantiomer of a drug can be active and the other inert or harmful.
The number everything is quoted in
Composition is reported as the enantiomeric excess:
\[ ee = \frac{[R]-[S]}{[R]+[S]} \]
It runs from 0 for a racemate to 1 for a single enantiomer. Part 1 explains why this combination, rather than either concentration, is the variable the whole subject is written in — and the two derivations later both turn out to be equations in ee alone.