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One Hundred Years Lock-and-Key Principle.

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ESSAY
One Hundred Years Lock-and-Key Principle **
Albert Eschenrnoser*
When we look at modern organic chemistry and ask ourselves
where we are going. then the spirit of no other organic chemist
of thc past appears to be nearer to our present-day thinking
than that of Emil Fischer. In saying this, I refer, of course,
to Fischer’s legacy of the “lock-and-key” concept, an idea
which together with Pauling’s later extension to transition
states constitutes the foundation on which all our modern
structural modeling of the selectivity of intermolecular communication in chemistry and biology rests. Rightly, therefore, this
symposium commemorates the concept’s centenary, and the organizers are to be congratulated for their insight and initiative.
By the spirit of Emil Fischer, however, I also mean (and
perhaps above all) Fischer’s attitude as an organic chemist
towards biology. It is the attitude that made and continues to
make the biological world the major and deepest source of inspiration for research in organic chemistry, be it in a direct or an
indirect way, or just as a reference for achievement. Organic
chemistry has returned to this attitude only relatively recently
after :i period of splendid isolation in organo-chemical self-complacency for more than half ;Icentury after Emil Fischer’s time.
Today. we marvel at Fischer’s foresight in choosing the research fields in which he decided to spend his lifetime as a
scientist. fields in which he created an oeuvre that makes hiin
today the unique pioneering figure for organic chemistry, biochemistry, and molecular biology alike. His focus on the basic
structural a n d synthetic chemistry of carbohydrates, nucleotides. and polypeptides, the three families of organic matter
that later were recognized as the three pillars on which the living
world is built, his interest in and practical use of enzymes as
chemical catalysts, and his concern about their mode of action
all differentiate him from the natural-products chemists of the
self-complacency period, when the recurring pleasures of successful constitutional analyses and syntheses of nicely crystallizing low molecular weight objects, beautiful colors of pigments,
fine odors of terpenes, and interesting biomedical properties of
alkaloids lured the chemists away from proteins and nucleic
acids. from those constitutionally repetitive and ”nasty” materials that later revealed themselves as the most important part of
(*I
[**I
Prot I h . A. Ewhenmoser
Lahot-,itorium fur Orgamsche Chemir
ETI I-Zcnti-um
Uni\ ersititstrassr 16. CH-XOY2 Zurich (Switzerland)
Int. code + ( l ) 632-1043
Intr,ductor? i-cmarks at the symposium “Supramolecular Chemistry: 100
Yeal-s L.ock-nnd-Key Principle” in Mainr o n August 17, 1994, organired by F.
Diedei-tch and H. Ringsdorf.
the task originally assigned to the chemistry that was named
organic, namely, the task of studying the substances produced
by living organisms.
If Emil Fischer lived in our time, and if he happened not to be
a biologist, he would quite probably be a practitioner of what
today is called supramolecular chemistry, judging by the pronounced flair he had for structure and synthesis. In this field of
chemical enterprise chemists set out to systematically conquer
through synthesis new domains of the structure space of organic
matter.
Let us call to mind that the portion of sequence space occupied by DNA and proteins during biological evolution is
infinitesimally small compared to the overall size of these structure spaces; and the structure space of organic matter as 21 whole
is infinitely greater than the space within which living matter is
evolving. Clearly, moving within a structure space, be it by
Nature herself or by synthetic chemists, demands the process of
srlecrion-selection of structures based on specific criteria. In
this context we can distinguish two types of selection: post-synthesis and pre-synthesis selection. Darwinian selection is the
prototypical example of the first. but also a chemist’s discovery
of an unexpected structure with an unexpected property that is
put to good use is an example. The second type of selection
represents-at least in traditional thinking the molecular scientist’s dream: it is the construction of a specific object based on
theory; that is, the scientist selects the object to be synthesized
in advance. Selection criteria are always utilitarian. be it fitness
for survival, or for technological. scientific. or cultural application. In the supramolecular chemist’s space walks. utility is connected with a given structure’s specific properties; such properties manifest themselves in the transfer of information from the
structure to a receiver, most often another niolecular or
supramolecular structure. And thus, mutual and strictly selective molecular recognition-Fischer’s lock and key is the central issue.
Therein lies that underlying dichotomy in structure space exploration in chemical research: post-synthesis selection (discovery) versus pre-synthesis selection (design). The recent sudden
rise of encoded combinatorial synthesis within chemistry should
make us reflect: we witness the transplant of a principle of
synthesis from the heart of biology into the center of chemistry.
An exciting period is ahead of us in which we shall be able to
watch and compare the power of the two selection modes in
chemical synthesis and their impact on the evolution of chemistry.
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