76 Science & technology The Economist October 9th 2021
Around 1980 Dr Parisi found some of the
rules that govern apparently random phe
nomena. He studied a type of material
called “spin glass”, in which, for example,
iron atoms are mixed at random into a ma
trix of copper atoms. The iron atoms each
behave as tiny magnets but, whereas in a
normal lump of magnetised metal their
northsouth poles all point in the same di
rection, in a spin glass they do not. Dr Par
isi devised a way to understand how they
find their optimal orientations. His mathe
matical ideas not only help explain some
of the complex systems of Earth’s climate,
as described by his two fellow laureates,
but also illuminate other apparently ran
dom phenomena in fields as diverse as ani
mal behaviour, neuroscience and machine
learning.
This year’s physics prize is the first sci
entific Nobel awarded for understanding
of the climate. Asked if this was a notso
subtle message to world leaders ahead of
the upcoming cop26 climate summit in
Glasgow, members of the award commit
tee said the prize was meant to celebrate
the discoveries themselves. But, they add
ed, it also showed that the modelling of the
climate and the notion of global warming
rest on solid physical science. Human be
ings can no longer say they did not know
how or why Earth is heating up.
Ringing the changes
The chemistry prize was shared by Benja
min List, of the Max Planck Institute for
Coal Research, in Mülheim an der Ruhr,
and David MacMillan, of Princeton Univer
sity. Their prizewinning work, published
in 2000, was conducted independently,
and unknown to each other at the time, but
with the same end in mind. This was to
break the stranglehold of enzymes and
transition metals on the field of catalysis.
Some chemical reactions proceed with
alacrity. Most, though—including many
that are industrially important—need a
helping hand in the form of a catalyst. Evo
lution has provided a goodly range of these
in the form of enzymes, which are large,
complicated and sometimes temperamen
tal protein molecules, but which have the
advantage that they can create pure ver
sions of what are known as optical iso
mers. These are molecules that have two
forms which are mirror images of each
other. This is important in the drug indus
try, for the different versions, known as en
antiomers, can have different effects in the
body. Also, if you choose the right en
zymes, it is often possible to carry out mul
tistep reactions in only a few stages.
Transition metals are those in the mid
dle of the periodic table—copper, nickel
and iron, for example. The structures of the
electron shells surrounding the nuclei of
their atoms are complicated, meaning they
are chemically versatile. This is what
makes them good catalysts. Some transi
tionmetal catalysts are the metals them
selves. More often, they are small mole
cules that include a transitionmetal atom.
Transitionmetal catalysts can be easier to
handle than enzymes, but usually fail to
distinguish between enantiomers. Also,
transitionmetal compounds are frequent
ly toxic, with all the environmental conse
quences that entails. And multistep reac
tions involving them can be longwinded.
Dr List and Dr MacMillan found a way to
have the best of both worlds: smallmole
cule catalysts that have no metal atoms in
them, can turn out pure enantiomers, and
often simplify multistep reactions. That
has significant industrial implications.
Dr List worked on an enzyme called al
dolase A. This catalyses what is known as
the aldol reaction, an important way of
forging molecular bonds between carbon
atoms. Aldolase Ais made of 350 amino ac
ids, the building blocks of proteins, but the
bit that does the work consists of only
three of these: lysine, glutamic acid and ty
rosine. The rest of the enzyme is packag
ing. He therefore wondered if he could iso
late the enzyme’s active centre and yet pre
serve its activity. In fact, he did better. He
showed that the aldol reaction can be cata
lysed by a single amino acid, proline. And,
crucially, this retains the enantiomeric pu
rity of the enzymemediated reaction.
Dr MacMillan came from the other end
of the problem. He wanted to remove the
metal (in this case copper) from the cata
lyst involved in a different process, the
Diels–Alder reaction. This is a way of join
ing two molecules into a sixcarbon ring.
One of the reagents contributes four car
bon atoms to the ring and the other con
tributes two. Sixcarbon rings are ubiqui
tous in organic chemistry, and by putting
different side groups onto the reagents a
vast variety of them can be turned out. Dr
MacMillan found he could catalyse Diels
Alder reactions using a type of metalfree
molecule called an imidazolidinone to ac
tivate the twocarbon component, mean
ing that it combines enthusiastically with
its fourcarbon compadre.
The result of these two pieces of work is
a field called asymmetric organocatalysis
(the asymmetric part of the name referring
to its ability to generate pure enantio
mers), that is now rippling through indus
trial chemistry. And, since industrial
chemistry, in one form or another, under
pins most economic activity, it is also rip
pling, however invisibly, through life.
Sense and sensibility
The idea that there are five senses goes
back at least as far as Aristotle. But it is not
quite true. Four of the senses are obvious,
if only because each is associated with a
particular organ: sight with the eyes, hear
ing with the ears, taste with the tongue and
smell with the nose. But the fifth classical
sense, touch, is distributed over the whole
surface of the body, albeit that it is concen
trated in the fingertips.
Touch, moreover, is only one such dis
tributed sense. Others perceived con
sciously include pain, heat and cold. And
modern science has shown there are also
unconsciously perceived senses, known
collectively as proprioception. These keep
track of the position and movement of the
body and its parts. This year’s Nobel prize
for physiology or medicine went to the dis
coverers of the molecular mechanisms of
two of these distributed senses—tempera
ture and mechanical stimulation.
The winners were David Julius of the
University of California, San Francisco and
Ardem Patapoutian of Scripps Research, a
biomedical institute in San Diego. Dr Julius
did the pioneering work on temperature.
He and Dr Patapoutian, acting indepen
dently, then advanced this work. After that,
Dr Patapoutian moved on to look at me
chanical stimulation.
Dr Julius’s chosen tool for his investiga
tion, which he began in the late 1990s, was
capsaicin. This is the active ingredient of
chilli peppers. By a chemical coincidence
(as was then assumed and is now known)
capsaicin reacts with, and thus stimulates,
one of the body’s heatreceptor proteins.
Dr Julius set out to discover what this pro
tein was. To do so he made millions of frag
ments of genetic material for proteins
known to be active in heatreceptor cells.
He then introduced these fragments into
other cells, to encourage them to manufac
ture the relevant protein fragments. That
done, he tested the modified cells for sen
sitivity to capsaicin.
The fragments which induced capsa
icin sensitivity turned out to be parts of a
protein now called trpv1. This belongs to a
class of proteins called ion channels,
which do many jobs in the body. As pre