Chemistry Reference
In-Depth Information
2.3
The Interaction(s) of Carbohydrates with Silicates
Our focus in this article is on carbohydrates, which constitute the most abundant
organic materials in the biosphere, and in particular their interactions with silicates,
the most abundant materials in the lithosphere. Although no naturally occurring
organosilicon molecule has ever been isolated, such molecules are implied by the
presence of silica as supporting tissue in sponges, diatoms, radiolarians, and some
higher plants. Such biostructures imply a robust silicate biochemistry, which is
largely unknown at present. Progress has been made primarily in understanding the
proteins, called silacateins, which are responsible for biouptake of aqueous silicate
and production of ordered silica nanostructures [
6
,
7
].
The seminal experiments in identifying interactions between the organic world
of carbohydrates and the inorganic world of silicates were reported by the groups of
Kinrade and of Klüfers in 1999 [
8
,
9
-
11
]. Kinrade [
8
] reported that aqueous sodium
silicate reacts with certain glycitols (linear polyols with a single hydroxy group on
each carbon, e.g., HOCH
2
CHOHCHOHCH
2
OH, as well as glyconic acids (open
chain sugars in which the aldehyde has been oxidized to the carboxylic acid, e.g.,
HOCH
2
CHOHCHOHCO
2
H. They used
29
Si NMR spectroscopy to confirm that the
glycitols and glyconic acids are complexed with pentacovalent (negatively charged)
silicon. Klüfers [
10
] inferred that the complexes between silicate and glycitols com-
prise five-membered diolato rings. They confirmed the five-membered structures
with X-ray analysis of the glycitols of mannose, xylose, and threose, [
11
] which
proved to form 3:1 hexacoordinated diolato complexes with silicate.
In contrast to glycitols and glyconic acids, sugars (glycoses) exist as cyclic struc-
tures. Thus the linear form, e.g., HOCH
2
CHOHCHOHCHO, is converted to a ring
form, which can have either five members (furanose) or six members (pyranose), as
illustrated in
1
and
2
for ribose.
H
H
HOCH
2
O
H
H
H
H
O
HO
H
H
H
OH
H
OH
OH
HO
OH
OH
1
2
Four-carbon sugars can exist only as furanoses, and smaller sugars cannot exist in the
cyclic forms. Sugars with five or more carbons exist in both furanose and pyranose
forms. Like the glycitols and glyconic acids, sugars have strings of hydroxy groups,
which conceivably could complex with silicate. Given the stereochemical restric-
tions that Kinrade and Klüfers had found for the glycitols and glyconic acids, it was
