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Carbohydrates
and Metals:
Basic Research for
Medicine and Technology
Carbohydrates · Metals
Among the classes of biogenic products, the carbohydrates have attracted much topical
interest in recent research. The aims of the investigations are quite different. One branch of
research – Glycobiology – deals with the interaction of complex carbohydrate derivatives
(“glycoconjugates”) with proteins as occurs in the course of an immune response. In this field,
the task for chemists is to synthesize such complex carbohydrate structures – both as tools for
biochemical research and as therapeutic agents. In this area of research, the synthesis of one
gram of a substance may meet with difficulties. However, another field of carbohydrate
chemistry deals with ton quantities of products: Cellulose, starch, sucrose and glucose are
renewable resources, which are made available by nature in high purity and at a reasonable
price. In accordance with the aims of “Green Chemistry”, the industrial nations have engaged
in advancing the framework of “Sustainable Development”, carbohydrates may serve as an
important source of raw material. However, each synthesis in carbohydrate chemistry –
whether at a gram or ton scale – is complicated by the mere number of possible reactions of
these polyfunctional molecules. The solution to the problem is catalysis, which, usually means
metal catalysis. But the questions are: How do carbohydrates bind to metal atoms? Are there
well-defined carbohydrate–metal compounds? Moreover, are there new applications for these
new substances? – These are all questions regarding basic research.
Carbohydrates are
variable, or:
What exactly is glucose?
Carbohydrates constitute the most
abundant group of renewable biogenic raw materials. The most
abundant individual substance that
is produced by organisms is also a
carbohydrate: cellulose, the main
constituent of wood. Also, nutritients such as starch from cereals
and potatoes, or sucrose are carbohydrates. In particular, sucrose is an
example of a raw material which is
available in high purity, large quantities, and at a low price. Sucrose
molecules are built up from two
basic units – sucrose is a disaccharide. The building blocks of the
disaccharide are the monosacchararides D-glucose (“dextrose”) and
D-fructose (“fruit sugar”, “levulose”).
The problems that arise in the
course of using carbohydrates as
raw materials for chemistry, may be
demonstrated using D-glucose as an
example (Box 1: Glucose). Briefly
summarized: D-glucose and D-fructose, as well as D-mannose, Dgalactose, and D-ribose, which are
also well-known monosaccharides,
are by no means well-defined substances in terms of molecular structure. Even simple dissolution of Dglucose in water produces two
main isomers, namely a- and b-Dglucopyranose. In small amounts,
more forms are present, which all
are part of a dynamic equilibrium.
This means that a chemical reaction, which preferably uses a minor
component, may nevertheless yield
a major product derived from that
form. The reason is that in the
course of the reaction, the particular form that is consumed is continously resupplied by the transformation of the non-reactive form to
the reactive, keeping its concentration constant. What particular form
will react with an added reagent,
often remains unpredictable. There
are more problems: At each individual molecule, several positions of
very similar reactivity (“functional
groups”) are present, which have to
be addressed in the course of the
preparation of a well-defined product. This specific problem is outlined by the term “over-functionalization”. To deal with this problem, laborious reaction sequences
are performed: the first step is inactivation of that part of the molecule
which is not to be attacked, the
second step is the reaction aimed
Carbohydrates · Metals
Glucose
1
Each of the formulas shown resemble D-glucose
(dextrose) – they are “isomers”. Most atoms are
linked with their neighbors in the same way. As
an example, the carbon atom labelled with the
digit “2” may be considered (carbon atoms are
the fourfold bonded centres; in addition, they
are numbered). In all formulas, the same kind
of neighboring atoms are present in the same
region around carbon-2. Two-coordinate, red
centres are oxygen atoms, the white ends of
the bonds resemble hydrogen atoms. Counting
the atoms yields the formula C6H12O 6 – except
for the bottom-right figure, which results in
C6H 14O 7. All formulas follow the scheme C6H12O 6
= C 6(H 2O)n, here: n = 6. In the 19th century, – misleadingly – the term “carbohydrate” = compound of carbon (C) and water (H 2O) has been
derived from these formulas. The structural
variability shown in the figures has its origin in
the particular reactivity of carbon-1 – the socalled anomeric carbon. This atom is the only
one that binds to two oxygen atoms in all the
formulas. One of these oxygen atoms binds a
hydrogen atom and is hence part of a hydroxy
group (an OH group). The other one is responsible for the variability mentioned. Depending on
the carbon atom that is bonded to this oxygen
atom in addition to carbon-1 – i. e. carbon-5
(top), carbon-4 (mid) or carbon-6 (bottom left)
– the various isomers are formed: On top the
two D-glucopyranoses (left a-, right b-D -glucopyranose), which are the major species in a
dextrose solution; below a- (left) and b-D-glucofuranose (right), which may increase in concentration during the course of chemical reactions. The bottom formulas show two forms
that are hardly detectable in aqueous solutions: on the left a-D-glucoseptanose, on the
right an open-chain form (not the aldehyde but
the aldehyde hydrate is shown, hence this is no
isomer in a narrower sense, since the composition is not the same – according to n = 6, there
is an additional molecule of water incorporated). Whether an individual isomer is denoted a
or b, depends on the position of the hydroxy
group bonded to carbon-1: When looking at the
ring made up from five, six, or seven atoms
from a direction whereby the numbers define a
clockwise rotation, the a-isomer is the one with
its C1-OH group pointing downwards; the other
isomer is the b-form. When one of the isomers
reacts faster in the course of a chemical reaction – either due to its intrinsic reactivity or
due to the application of catalysis –, the product of the reaction will have the same molecular core as that particular isomer, the product is a “derivative” of that isomer.
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Carbohydrates · Metals
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at, followed by a third step to
remove the protective groups. The
entire sequence is termed “protective-group technique”.
How the difficulties may be mastered in a smarter way, is demonstrated by nature itself. Synthesis and
degradation of carbohydrates belong to the most essential part of
nutrient metabolism in every organism. The vast amount of possible
reactions is restricted to the one
intended by opening an energetically favourable path – catalysis
takes place. The natural catalysts
are termed enzymes. When isolated, they can catalyze even outside
of organisms. Enzymatic synthesis
– often very expensive – is acceptable for the preparation of complex
glyco-conjugates since there is a
need for only small quantities of
these products. In special cases, for
example when no expensive reactants are required (“cofactors”),
enzymes are used for the synthesis
of bulk products as well. An example from carbohydrate chemistry is
the technical production of D-fructose from D-glucose by means of
the enzyme xylose isomerase. This
particular enzyme emphatically
stresses that catalysis both in nature
and in technology works rather
similarly in terms of basic principles. Technical catalysts are mainly
metals or metal-based compounds.
In xylose isomerase there are also
metal atoms, namely two manganese atoms, which effect the transformation of glucose in the active centre of the enzyme. The question
arises: Why is the expensive protective-group technique still used
instead of metal catalysis as in
nature in carbohydrate chemistry as
well?
Green Chemistry
as a Special Part of
Sustainable Development
Prior to dealing with this question,
the basic concept behind intensified carbohydrate usage should be
mentioned. The industrial nations
have committed themselves to
develop their economies in a
sustainable way. For chemistry, this
concept means organisation of the
entire life-cycle of a product from
the raw material to waste management within the framework of
sustainable ecological benefit and
preserving resources. Chemical
industry can take care of resources
particularly successfully if renewable
raw materials are used, and are converted into the desired products
using the least possible co-reactants and energy as possible.
aims of Green Chemistry would
have been missed.
More than 90 % of all basic organic chemicals are produced in
metal-catalyzed reactions – why is
there a problem with carbohydrate
reactions?
Metal-binding Sites
of the Carbohydrates
In order to plan metal catalysis in a
rational way, there should be a
basic knowledge of several points:
Fig. 1: A glucose palladium complex. This compound is formed in the course of the
reaction of D-glucose with [(en)Pd(OH)2]. The latter is a complex of palladium with the
nitrogen atoms (blue) of ethylenediamine (en) and two hydroxy groups (OH). The
OH-groups combine with O-bonded hydrogen atoms of the sugar to yield water. The
reaction product is glucose that has been metallated twice. Glucose is found as
a-glucopyranose – compare with the formulas in Box 1. The palladium atoms are located
at the centres of the turquoise bonds
Oxidation of a carbohydrate by
means of oxygen from air, catalyzed by small amounts of an ecologically safe metal, in detergent
production would be close to this
issue. However, usage of biogenic
raw materials does not mean
Green Chemistry in each case. If
the same product is synthesized
starting from the same raw material
but with a reaction sequence that
yields several tons of salts per ton
of detergent as by-products, the
how does an educt molecule bind
to the metal in question; what
reaction proceeds; how does the
product molecules bind to the
metal after the reaction; how can a
product molecule leave the catalytic
centre? (Many technical catalysts
have been developed without such
knowledge or, moreover, with
wrong models in extensive series
of experiments. However, the probability to improve a catalyst by
mere variation of parameters
Carbohydrates · Metals
decreases with increasing educt
complexity.) The first of these
questions cannot usually be answered in the case of carbohydrate
educts. This is also the case for
enzymes. In the case of xylose
isomerase for example, it is known
that two manganese atoms interact
in the active centre of the enzyme –
but there are only speculations
about the course of the individual
reaction steps. The lack of knowledge is similar in the case of technically relevant metal centres. The
important catalytically active metal
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palladium is an example. Basic
information on the palladium-binding sites of glucose has been
published only very recently. Information on the structure of a glucose–palladium complex is given in
Figure 1. This complex, being the
first structurally characterized transition-metal complex of glucose at
all, particularly highlights the substancial lack of basic information in
this field of interest.
What is the reason that basic research has started so late to provide
potential users with basic data of
that kind? First of all, the impressive development of analytical
methods must be mentioned.
Thus, powerful tools for structural
characterization are available – and
are necessary since all the peculiarities that make carbohydrate
chemistry intricate are also valid
when a metal is to be bonded.
Hence, also the binding of a metal
to a monosaccharide is a reaction
that is relatively unclear due to the
same polyfunctionality of the carbohydrate that is aimed to be
mastered by catalysis. To gain in-
Single-crystal
structure analysis
2
Structure analysis performed on single crystals
appears to be the most powerful method of
structure determination. This method is the
backbone of the discipline “Structure Biology”,
which deals with the structural elucidation of
large biomolecules. In chemistry, crystal-structure analysis is mostly used for structure
determination of smaller crystal building units,
which, on the other hand, can be performed
with higher precision (higher “resolution”). The
prerequisite for this method is a single crystal
of typically some tenth of a millimeter in size
(top figure). This crystal is irradiated with
X-rays by means of a “diffractometer” – shown
in the central figure with a Stoe-IPDS operated
by Dr. H. Piotrowski as an example. Usually, the
crystal is cooled to a temperature in the -100 °C
range. Since the wavelength of the radiation
and the periodically repeating distances of the
electron density in the crystal resemble the
same order of magnitude, the phenomenon
of “diffraction” occurs. The bottom figure
shows a diffraction pattern that is obtained
from a well-diffracting crystal in the course of
ca. 1–5 minutes irradiation time. The position of
the diffraction maxima depend on the distance
of equivalent crystal building units in space;
from the intensity of the diffraction maxima,
the structure of the individual molecules can be
extracted. The black spot in the centre of the
diffraction pattern is the shadow of a small
lead cylinder that is used to absorb the main
intensity of the X-ray beam, which is not
diffracted by the crystal.
Zusammenfassung
Reach ThroughPatentansprüche
Carbohydrates · Metals
98
sight into the metal-binding sites
of a carbohydrate, two methods appear to be outstanding – both being
known “in principle” for years, but
both having been markedly developed in the last years: Structure
analysis with single crystals (Box 2:
Single-crystal structure analysis) and
NMR spectroscopy. To establish
rules of metal coordination to carbohydrates, linking the methods is
essential. In a systematic approach,
a crystal structure analysis can show
the structure of the new compounds. Subsequently, NMR spectroscopy is used to study solution
The well-defined structure of the
palladium complex raises the question whether carbohydrate–metal
complexes are significant in their
own right beyond catalysis. Here
are two examples that may answer
this question.
Carbohydrate-RheniumComplexes
in Tumour Targeting
Coordination compounds of the
rare metal rhenium have attracted
interest in nuclear medicine in the
last years. The aim is tumour diagnosis and therapy using radioactive
selves in the course of tumour
growth. Many biomolecules that meet
this prerequisite have carbohydrate
segments. Examples are glycoproteins and glycopeptides. To bind
rhenium atoms selectively to their
carbohydrate part, information is
required about the kind of the
rhenium fragments that are suited
for such a coupling reaction –
however, also in this case there was
not a single example of such a
compound in the literature. By
combining the methods described
above, the first rhenium-carbohydrate complex was synthesized and
characterized (Figure 2). A peculiarity of rhenium facilitates the
synthetic work: There are “cold”,
i. e. non-radioactive isotopes available. Hence laboratory work is not
complicated by antiradiation precautions. Starting from this first
result, research now can proceed in
collaboration with nuclear physicians.
Silicon Transport
in Organisms and
Biomineralization of Silica
Fig. 2: The first structurally characterized rhenium carbohydrate compound. The rhenium
central metal atom is located at the centres of the turquoise bonds. The carbohydrate
ligand is b-methyl-D-galactopyranoside. Compare with b-D-glucopyranose in Box 1. The
oxygen atom in position 4 is pointing up now, changing glucose to galactose. In addition, position 1 does not bear an OH-group, but the H-atom is replaced by a CH3-group; a
glycoside has been formed – hence “methyl…ide”. Due to this modification, the carbohydrate is no longer able to interconvert into another isomeric form. The rhenium atom
bears further ligands: pointing to the right there is an oxygen atom, to the left a
tris-pyrazolyl-hydridoborate ligand (blue: nitrogen, rose: boron)
equilibria. Computational chemistry emerges as another standard
method for this kind of research.
Computational algorithms derived
from density functional theory,
increasingly permit the modelling
of molecules – including their reactivity – of the complexity of the
described palladium compound
even using personal computers.
compounds. Rhenium provides the
active isotopes 186Re and 188Re for
this application. Both main and side
effects can be optimized significantly when the radioactive element is enriched specifically in the
cancerous tissue. One way to
achieve enrichment is coupling of
the rhenium atoms to such biomolecules that are enriched them-
Silicon is obviously essential for
the growth of animal and plant tissue as is deduced from nutritional
and fertilizing experiments. However, understanding on a molecular
level is lacking. The significance of
silicon incorporation is particularly
conspicuous in the case of diatoms
and radiolaria, and also with grass
species and species of the horsetail
genus. Whereas there are very
vague ideas regarding the formation of the biomineral starting from
silicon precursors, there is no knowledge of the chemical nature of the
precursors themselves. Despite the
well-known affinity of silicon
atoms towards oxygen atoms as
binding partners, it was assumed
until very recently, that a carbohydrate and silicon atoms are unable
to form such compounds in an
aqueous medium, which are not
decomposed rapidly by the action
of water to yield the free carbohydrate and silicate (“hydrolysis”).
Carbohydrates · Metals
99
preparations of high bioavailability
or as precursors for technically useful silica forms that follow the
example given by the biominerals.
Literature
Our own work described in this
article has been conducted by Dr.
T. Kunte (palladium complexes),
M. Oßberger (rhenium) and M.
Vogt (Silicon). Their results have
been published in more detail in:
P. Klüfers, T. Kunte:
Polyol Metal Complexes,
37. — A Transition-Metal
Complex of D-Glucose.
Angewandte Chemie 2001,
113, 4356–4358;
Angewandte Chemie, International Edition in English. 2001,
40, 4210–4212.
Fig. 3: Aqueous solutions of cesium hydroxide and D-threitol dissolve silica. From solutions of that kind, the cesium salt of a complex silicate shown in the figure can be
crystallized. The atoms are depicted as spheres in this figure, the colour code is the
same as in the other figures. Six hydrogen bonds are drawn as yellow bars
Thus missing specific transporter
molecules, silicic acid itself is discussed as a possible transport form
– the abundancy of this form is
very low in silicateous soils however. There is an analogy in medicine: Bioavailability of silicon is
extremely low after administration
of silica preparations, which results
in the recommendation of very
high dosages.
However, the hypothesis that carbohydrates are unable to bind silicon in such a favourable way that
results in stability against water
attack, is definitely wrong. Reduced
derivatives of the monosaccharides
– the sugar alcohols – are absolutely capable of forming such
compounds. Figure 3 shows a complex made up from a silicon atom
and three D-threitol molecules. DThreitol is the reduced form of the
rare sugar D-threose, which has
only four carbon atoms. In the
depicted compound, the polyfunctionality of the carbohydrates does
not appear as an obstacle but as a
prerequisite for the unexpected
stabilization of an unusual structure
(usually in silicates the Si-atoms are
surrounded by only four oxygen
atoms): All functional groups are
integrated in a network of bonds –
both as the next neighbours of the
central silicon atom and in hydrogen bonds between OH-groups
and the silicon-binding O-atoms.
Though this six-coordinate silicate
is stable towards hydrolysis, the
solutions have to contain an
unphysiological amount of base.
Due to the very large number of
various carbohydrates there is hope
in future work, to develop even
more suitable binding partners for
silicon. Hydrolytically stable complex silicates bear interest as silicon
P. Klüfers, O. Krotz,
M. Oßberger:
Polyol Metal Complexes.
40. — Oxo-rhenium(v) Complexes
of Carbohydrate Ligands.
European Journal of Inorganic
Chemistry 2002, 6, 1919–1923.
K. Benner, P. Klüfers,
M. Vogt:
Polyol Metal Complexes,
41. — Hydrogen-bonded Sugar
Alcohol Trimers as Hexadentate
Chelating Ligands for Silicon in
Aqueous Solution.
Angewandte Chemie, International
Edition in English 2003, 42,
1058-1062.
Author:
Prof. Dr.
P. Klüfers
Department of Chemistry of the
Ludwig-Maximilians-Universität
Butenandtstraße 5-13
D-81377 München
Fax: +49(0)89 -2180 - 74 07
E-mail: [email protected]