What
follows is a dissection of the first three minutes of Luca Turin's Ted Talks video entitled 'The Science of Scent.'
These first three minutes cover the top and heart notes of the
well-known Estee Lauder perfume Beyond Paradise, gas chromatography of said
perfume as a method of uncovering a rough estimate of the number of molecules
that comprise it, the elements that generally compose scented molecules, and
the molecular structure and naming of cis-3-hexenol, which smells of cut grass.
As most
people (or fragrance fans, anyway) are aware, perfume notes are divided into a
pyramid-like structure: top notes, heart notes, and base notes. A fragrance's development over time is a
function of these distinctions. Top
notes have the lightest molecular weight and thus evaporate from skin and
diffuse through the air the most quickly (they have generally disappeared
within 10-15 minutes after application).
Citrus is a canonical top note.
Heart notes are somewhat heavier molecules, and base notes have the
greatest molecular weight.
After being
unable to get a direct answer from anyone in the fragrance industry about the
number of molecules in Beyond Paradise, he analyzed a sample of it with a gas
chromatograph to get a general answer for himself (about 400 molecules). Synthetics are generally one molecule, whereas
naturals are compositions taken directly from a source found in nature, and are
thus melanges of many different molecules.
For this reason, it is very possible that no one Luca Turin asked had a
very definite idea of how many molecules might be found in Beyond Paradise.
Gas
chromatography is a technique we utilized in my organic chemistry class last
semester in which the relative quantities of the components of a solution (such
as a perfume) are measured. The sample
is injected into the injection port, which is set at a temperature higher than
the boiling points of the sample's components.
Thus, the sample is rapidly heated and the molecules that make it up are
vaporized shortly after injection. A
carrier gas, called the mobile phase (an inert, or unreactive gas or liquid
that the vaporized sample can travel with and through) flows through the
injector and pushes the components of the sample into the gas chromatography
column. The components of the sample, or
the molecules of which it is composed, are separated based on the boiling
points of the molecules and their relative affinities for the stationary phase
(an immobile wax within the column that the molecules travel through). Molecules with a greater number of molecular
interactions with and attractions to the stationary phase will be slowed down
on their trip through the column, whereas molecules that have relatively few
interactions with the waxy stationary phase will be fairly unhindered. Thus, the molecules become separated by how
quickly they can pass through the column (which is dependent on their
interactions with the stationary phase).
The components, after having traveled through the gas chromatography
column, reach a detector which sends a signal to the chart recorder, resulting
in a peak on the chart paper. Each peak
corresponds to a molecule found within the sample, and the area beneath each
peak corresponds to the relative amount of that molecule present in the
original sample. Optimally, each type of
molecule should reach the detector at a different time due to differences in
attraction to the stationary phase.
These
molecules of which fragrances are composed are themselves comprised of building
blocks called atoms. Atoms have
different identities depending on the umber of protons contained in their
nuclei. Each different number of protons
(the atomic number) corresponds to a different element on the periodic table of
elements. The elements that generally
combine to create fragrant molecules are carbon, hydrogen, oxygen, nitrogen,
and sulfur. These elements are found on
the upper right of the periodic table, except for hydrogen, which is the upper
and leftmost corner. Molecules
containing carbon are organic molecules, and are studied in organic chemistry. This is the domain of chemistry that most
molecules with scents fall under.
Luca Turin
uses the example of cis-3-hexenol, which smells of cut grass. This compound contains a cis double bond and
an alcohol (or -OH group). There is a
system for naming and numbering organic compounds. Start with the longest chain of linked carbon
atoms. In this case, the carbon chain is
six atoms long (thus, hexenol). The -ol
suffix alerts us to the presence of an alcohol group, while the fact that the
molecule is hexenol as opposed to
hexanol tells us that the molecule
is an alkene (and contains a double bond) and not an alkane (and made entirely
of single bonds between the atoms). In
this case, the alcohol is the 'highest priority substituent' and numbered
first. The molecule name takes the form
cis-3- hexen-#-ol. The # symbol denoting
the position of the carbon at which the -OH group is attached. Because the -OH group is attached at the end
of the carbon chain, it is attached to carbon 1, and so it is not necessary to
include the number. counting from the
carbon to which the alcohol group is linked (carbon 1) we eventually hit carbon
3, the first carbon of the double bond.
A double bond can be either cis or trans. In a cis double bond, the most important
substituents (with the greatest atomic numbers) are on the same side of the
double bond. In the trans form, the most
important substituent on one carbon atom goes in the direction opposite that of
the most important substituent on the other carbon atom of the double bond. From this information, one can draw the
molecule from the name, or name the molecule when presented with a drawing of
it.
- Pyramid image stolen from Wikipedia.
- Gas chromatography diagram stolen from kutztown.edu
- Cis-3-hexenol image stolen from Wikipedia.
