The following account of Willstätter's work is by Professor O. Hammarsten, Chairman of the Nobel Committee for Chemistry of the Royal Swedish Academy of Sciences
By
its property of making possible the assimilation of carbon dioxide under the influence
of sunlight and hence introducing the synthesis of organic substances in the green
parts of the plant, chlorophyll - as is well known possesses extraordinarily great
biological significance and has an extremely important task to fulfil in the economy
of Nature. The elucidation of the nature and the mode of operation of this substance
is therefore a task which is of the highest degree of importance. The difficulties,
however, which confront research scientists in this field have been so great that
until very recently they have prevented a successful study of the problem of chlorophyll.
Willstätter is the first, jointly with several of his students, to have been
successful in overcoming these difficulties by working out new and very valuable
methods and by extensive investigations carried out with masterly experimental
skill. By the new and important discoveries resulting from these investigations
he has been able to elucidate in all its essential parts the question of the chemical
nature of chlorophyll.
It is true that earlier investigators had
observed that chlorophyll contains magnesium, besides other mineral substances.
Willstätter, however, has the merit of having been the first to recognize
and to prove with complete evidence the fact that magnesium is not an impurity,
but is an integral part of the native, pure chlorophyll - a fact of high importance
from the biological point of view. He has shown that magnesium is held within
the chlorophyll molecule in a manner which is very similar to the way in which
iron is held in haemoglobin; this bond is so firm that the magnesium is not liberated
even by the action of a strong alkali. On the other hand, it can be removed by
an acid without injury to the remainder of the chlorophyll molecule, and the magnesium-free
chlorophyll which can be obtained in this way is well suited to certain investigations.
Willstätter has made use of this circumstance to test to what extent chlorophyll
can be the same in different kinds of plants. Investigations carried out on more
than 200 different plants, both phanerogamia and cryptogamia, showed that the
chlorophyll was the same in all the kinds so far examined. This chlorophyll is,
nevertheless, not a chemically homogeneous substance. It is a mixture of two somewhat
different but yet closely related chlorophylls, one of them being blue-green,
the other yellowgreen, and the former occurring more richly in the leaves than
the latter.
The fact that chlorophyll in the ordinary sense is a
mixture of two green pigments had, it is true, already been shown to be probable
by Stokes in 1864, and both Tsvett and Marchlevski had brought forward important
support for this view. It is Willstätter, however, who has here produced
the certain and conclusive proof.
To prepare chlorophyll in an unchanged,
pure state and in such large quantities that it can be the subject of complete
chemical analysis has of course been one of the most important tasks of chlorophyll
research; at the same time, it was one of the most difficult of all. By the successful
solution of this task Willstätter has also been able to prepare the two above-mentioned
different types of chlorophyll in a pure state and so supply exact proof of their
existence. In doing so he has been able to carry out a thorough investigation
of the large amount of the various derivatives which can be produced from these
two different chlorophylls, and as a result of this means he has brought a desirable
clarity and lucidity into a field of chlorophyll chemistry, which was previously
very complicated and confused. By elaborating methods for the preparation of pure
chlorophyll in rather large quantities he has also created new and rich possibilities
for further fruitful research in this field.
The most important part
of Willstätter's investigations is, nevertheless, that relating to the detection
of the chemical structure of chlorophyll. He has shown that chlorophyll is an
ester, which on saponification with alkali can be split up into a previously unknown
alcohol called "phytol", which represents about one third of the molecule, and
a colour component called "chlorophyllin", containing magnesium, which forms the
remaining part. He has more closely investigated these two components both individually
and for their transformation and decomposition products. Furthermore, he has found
that this splitting-up of chlorophyll into the two mentioned main components can
also take place as a result of the action of an enzyme occurring in the leaves,
which he has called "chlorophyllase", and hence he has been able to elucidate
the nature of the crystallized chlorophyll. He has established that this is not,
as some investigators have assumed, the pure, unchanged natural pigment in the
leaves. The crystallized chlorophyll is a laboratory product, an alkyl ester,
which lacks phytol. The amorphous chlorophyll, containing phytol, is the unchanged
natural pigment in the green parts of the plant.
A very important
section of Willstätter's work on the chemical structure of chlorophyll is
represented by his investigations into the colour components, the "chlorophyllin",
and other "phyllins" and derivatives formed from it. These investigations are
of particular interest with regard to the question of the relationship between
blood pigment and chlorophyll.
From the iron-containing red blood
pigment, haemoglobin, substances can be prepared, purple in colour and free from
iron, which are known as porphyrins, and the one which has been known longest
of these is haematoporphyrin. A substance very closely related to this, with regard
to optical properties, has been prepared from a chlorophyll derivative by Hoppe-Seyler,
who called this chlorophyll pigment phylloporphyrin on account of the similarity
between the two substances. Schunck and Marchlevski have shown later that a chemical
relationship does exist between blood pigment and chlorophyll, but in this case,
too, it is Willstätter who has conducted the completely conclusive investigations.
In these investigations, which concerned the pigment nucleus both in chlorophyll
and in haemoglobin, he has made several new and important observations regarding
the pyrroles and their position in this nucleus; in particular, however, he has
shown that from these two pigments the same parent porphyrin, "aetioporphyrin",
can be prepared, whose molecule has retained the essential characteristics of
the pigment nucleus. By doing this he has produced the most interesting and decisive
proof of the relationship between the two most biologically important pigments
in Nature - haemoglobin and chlorophyll.
He has also prepared in
a pure state and studied exhaustively the yellow pigments, the so-called carotenoids,
which occur together with chlorophyll in the leaves of plants. By means of the
results obtained regarding both these yellow pigments and the chlorophylls he
has paved the way for new biological researches into the part played by the different
leaf pigments in the assimilation of carbonic acid.
He has also studied
with great success another group of plant pigments, namely: the blue and red pigments
of flowers, the so-called "anthocyanins". He has isolated the characteristic pigment
and investigated its chemical nature from a rather large number of flowers, such
as cornflower, roses, pelargonia, larkspur, hollyhock, etc., as well as from some
fruits, such as bilberries, black grapes and cranberries. As a result, the anthocyanins
have been shown to be glycosides, which can be split up into a kind of sugar -
in most cases glucose - and a colour component, a "cyanidin". Willstätter
has elucidated the chemical structure of these cyanidins; he has proved in what
their difference consists in the various flowers or fruits, and has also proved
their close relationship with the yellow pigments, occurring in Nature, of the
flavone or flavonol group. By the reduction of one such yellow pigment, quercetin,
he has obtained the cyanidin which occurs in roses and cornflowers, and by chemical
synthesis he has succeeded in preparing the cyanidin of the pelargonia, pelargonidin.
He has shown the dependence of the flower pigments upon the reaction of the plant
sap and has thus explained how one and the same anthocyanin can have a different
colour in different flowers, as is the case with roses and cornflowers. The anthocyanin
is in both cases the same, but in the rose it is bound to a plant acid and is
therefore red, whereas in the cornflower it is bound to an alkali and is therefore
blue.
By extending his investigations to the yellow pigments of flowers
as well, and by quantitative determination of the anthocyanins in certain kinds
he has shown that the difference in the colour which the flowers assume in Nature
or under the care of the grower depend upon several different circumstances, such
as the appearance of several different anthocyanins in the same kind, great variations
in anthocyanin content, different reaction of the cell sap and the simultaneous
presence of different quantities of yellow pigments, which latter can again differ
from one another in types.
In this field of plant-pigment chemistry,
Willstätter's investigations can also be regarded as pioneering; the most
comprehensive and the most important are, however, his investigations on chlorophyll,
by which he has not only succeeded in unravelling the chemical structure of this
substance, but also laid the sound scientific foundation for continued successful
research into this extremely important field of plant chemistry.
From Nobel Lectures, Chemistry 1901-1921, Elsevier Publishing Company, Amsterdam, 1966
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