Chemistry Reference
In-Depth Information
likely that sugars would have similar or stronger restrictions, as the rings in which
they exist are conformationally more restricted. In an early report, Kinrade and
co-workers [
12
] found that ribose forms a complex with silicate.
Lambert and co-workers reported the first comprehensive examination of sugar
silicates in 2004 [
13
]. They examined all four aldopentoses, seven of the eight
aldohexoses, all four ketohexoses, several disacchraides, and the 1-
O
-methyl gly-
cosides of several sugars. Of these, the monosacchraides ribose, xylose, lyxose,
talose, psicose, fructose, sorbose, and tagotose, and the disaccharides lactulose,
maltulose, and palatinose successfully formed sugar silicate complexes, as signi-
fied by observation of
29
Si signals in the pentacoordinate region. Notable failures
include the monosacchrides arabinose, glucose, mannose, and galactose, the disac-
charides sucrose and turanose, and all the 1-
O
-methyl glycosides. For a positive
result, three structural factors had to be operative: (1) The anomeric oxygen must
be free, that is without
O
-methyl glycosidation. The anomeric hydroxy hydrogen is
the most acidic in the molecule and probably initiates the reaction. (2) The carbon
adjacent to the anomeric carbon must bear a hydroxy group
cis
to the anomeric
hydroxyl. By a chelating effect, silicate complexes with two of the sugar hydroxyls,
and
cis
-diols are the most amenable. (3) These elements must reside in a furanose,
not a pyranose, ring. Apparently puckering of the six-membered ring sterically in-
hibits formation of the five-membered oxalato ring. The resulting complex would
take the form
3
for D-ribose.
O
O
O
Si
O
CH
2
OH
O
O
OH
3
The common sugars glucose and mannose lack significant amounts of the key
furanose form. All five disaccharides contain furanose rings, but in sucrose the two
anomeric carbons are bonded through a glycosidic linkage, so the key anomeric
hydroxyl is blocked. In tagotose the 2 hydroxyl is linked to the second sugar ring, so
the molecule lacks a cis diol structure that includes a furanoid anomeric hydroxyl.
Since such a structure is available in lactulose, maltulose, and palatinose, those
three disaccharides form sugar silicates.
The final element of the structure of the complexes involved the question of stoi-
chiometry, which for structure
3
could be a 1:1 combination of sugar and silicate in
which the remaining three linkages to silicon are hydroxyls, or a 2:1 sugar/silicate
combination in which one of the linkages is hydroxyl and the other two are a second
sugar. Electrospray mass spectral examination of the complexes indicated that all
are 2:1 complexes, such as
4
for ribose [
13
].
