The Nobel Prize in Chemistry 1987
Donald J. Cram, Jean-Marie Lehn, Charles J. Pedersen
14 October 1987
The Royal Swedish Academy of Sciences has decided to
award the 1987 Nobel Prize in chemistry jointly to
Professor Donald J. Cram, University of California, Los
Angeles, USA, to
Professor Jean-Marie Lehn, Université Louis Pasteur,
Strasbourg, and College de France, Paris, France, and to
former research chemist Charles J. Pedersen, Du Pont,
Wilmington, Delaware, USA
for their development and use of molecules with
structure-specific interactions of high selectivity.
Summary
This year's Nobel Prize in Chemistry has been awarded to
Donald J. Cram, USA, Jean-Marie Lehn, France and
Charles J. Pedersen, USA for their development and
application of molecules with highly selective structurespecific
interaction, i.e. molecules that can "recognize" each other and
choose with which other molecules they will form complexes. The
laureates have been rewarded for synthesising organic compounds
of low molecular weight and with very special properties. The
molecules in these compounds are designed principally to bind
cations (positive ions), but also anions (negative ions) and
neutral-molecules, in a specific and selective manner. The three
researchers have studied chemical and physical properties of
these complexes and have elucidated the factors that determine
the ability of the molecules to recognize each other and fit into
one another like a key fits a lock.
Molecules have been produced that mimic the mode of action of
enzymes. The laureates' research has been of great importance for
developments within coordination chemistry, organic synthesis,
analytical chemistry and bioorganic and bioinorganic chemistry,
and has thus laid the foundation for the active interdisciplinary
area of research within chemistry that has now come to be termed
host-guest chemistry or supramolecular chemistry.
Background Information
At the basis of many
biological processes lies the ability of molecules to recognize
each other and to form well-defined complexes. Well-known
examples are substrates bound to enzymes, signal substances bound
to receptors, antibodies bound to antigens and metal ions bound
to ionophores. In most cases, one or more compounds of low
molecular weight bind to a specific region in a
high-molecular-weight compound, most often a protein or a nucleic
acid. The binding is very specific and selective and the low
molecular-weight compound must fit the high like a key in a
lock.
Inorganic chemists have long dreamed of synthesising relatively
uncomplicated organic compounds that perform the same functions
as natural proteins. Great progress towards this goal has been
made over the last 20 years, and it is the pioneering
achievements in this particular area that are now being
recognized.
In 1967, Charles J. Pedersen published two works, which
have now become classics, describing methods of synthesising
cyclic polyethers, which he named crown ethers. Pedersen showed
that these compounds have remarkable and unexpected properties
and that they can even bind the alkali metal ions of lithium,
sodium, potassium, rubidium and caesium into complexes in which
the lithium ion is the smallest and the caesium ion the largest.
He also found that, depending on the structure of the crown
ehter, potassium could for instance be bound before caesium.
Simply expressed, the selectivity is determined by the fact that
different crown ehters include "holes" of different sizes, into
which different spherical metal ions fit.
By building on Pedersen's fundamental discovery, Jean-Marie
Lehn in 1969 developed bycyclic compounds of crown ehter type
which he called cryptands and which show even higher selectivity
when forming complexes.
Jean-Marie Lehn and Donald J. Cram have subsequently each
developed increasingly sophisticated organic compounds which when
forming complexes leave fissures and cavities where
low-molecular-weight compounds with different types of geometry
can be bound. With this work, Pedersen, Lehn and Cram laid the
foundations of what is today one of the most active and expanding
fields of chemical research, a field for which Cram has coined
the term host-guest chemistry while Lehn calls it supramolecular
chemistry.
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| Figure 1. |
Cram in particular, using advanced
organic-sythetic engineering and molecular mechanics
calculations, has designed completely immobile host molecules
that form particularly strong complexes of extremely high
selectivity. Thus, for example, a host molecule with a 420
000-times-stronger tendency to bind sodium ions than to bind
lithium ions has been synthesised. In addition to these alkali
ions and other metal ions, it has been possible to produce host
molecules that bind organic positive ions (cations such as
diazonium and alkylammonium ions), as well as other host
molecules which can bind small neutral molecules or negative ions
(anions such as phosphate ions and organic carboxylate). Through
their detailed investigations of the structures, physical
properties and chemical reactions of the complexes, Lehn and Cram
have increased our understanding of the factors determining the
structure-specific interaction of high selectivity.
These examinations have also contributed to our understanding of
ion transport via biological membranes. Selective cation binding
has already found many applications. Using different types of
host molecule it is possible, for example, to extract radioactive
strontium or toxic cadmium and lead ions without affecting other
ions, which is very interesting in terms of protection of the
environment. Such high selectivity has been achieved that it is
even possible to separate isotopes of the same element. Within
analytical chemistry, selective complex formation has led to the
development of ion-selective electrodes and other types of cation
sensor. Certain transition metal complexes also show catalytic
activity in photochemical processes, for example the
photochemical decomposition of water to hydrogen, which may be of
significance in energy production.
This formation of complexes is also being applied increasingly in
organic synthesis, not least through Cram's success in producing
crown ethers that help in separating the mirror images of
aminoacids.
The goal is to produce synthetic host molecules that recognize
biologically active molecules. Thus Lehn has produced a host
molecule for the signal substance acetylcholine, which is so
important in humans and animals.
The explosive development of the art of organic synthesis has enabled Cram and Lehn to produce hosts that to some extent mimic enzymes such as proteases, ATP-ases and transacylases (see Fig. 2). Supercomplexes which bind organic substrates and metal ions have recently been produced by Lehn (see Fig. 3). It will thus be possible to produce supermolecules which do not suffer from the present limitations on substrate structure and reaction type in, for example, enzymes. Through their work, Cram, Lehn and Pedersen have shown the way.
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| Figure 2 and 3. |
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