Chemistry Reference
In-Depth Information
Lambert and co-workers reported the realization of this possibility in what they
termed the bottom-up synthesis of sugar silicates (in distinction to the direct reac-
tion between, or top-down synthesis of, sugars with silicate) [
15
]. They compared
various formose reactions either with sodium hydroxide as catalyst (classic formose
conditions) or with sodium silicate as catalyst. They found that formaldehyde (C1)
alone produces low yields under both conditions, as it lacks the alpha hydrogen to
initiate the condensation. They studied the reactions of glycolaldehyde (C2) and
glyceraldehyde (C3), either alone or mixed with the other or with C1. These sugars
exist in the straight chain and other forms, but they do not have enough carbons to
form a furanose ring. In the case of C3 alone, the reaction occurs with either catalyst
to form C6 oligomers within seconds at room temperature. Under standard formose
conditions, however, the oligomers decompose quickly, so that within 12 h little
sugar product remains. When the same reaction is carried out with C3 in the pres-
ence of sodium silicate as catalyst, solely C6 products are formed, remaining almost
unchanged for 12 h or more. Similar results occur with C2 alone. The reaction of C2
and C3 together is more complex, because it can proceed from simple dimerization
to give C4 and C6, but also by the cross reaction to give C5. It appears that C5 prod-
ucts are the most abundant, and, again, the products are unstable in the presence of
sodium hydroxide but robust when the catalyst is sodium silicate.
The possibility that silicate mediation can stabilize the products of the formose
reaction may revive its role as the mode for prebiotic synthesis of sugars. Silicate
minerals are widely available, although the basic conditions are less available in
Nature. On earth, such conditions occur naturally under conditions of extreme evap-
oration and concentration, as occurs in the Dead Sea, the Great Salt Lake, and the
lakes of the Atacama Desert of Chile [
16
]. A similar rationale had been proposed
with borate minerals [
17
,
18
]. Although borates may be kinetically more effective
than silicates, their much lower availability makes them a less likely stabilizing
agent under prebiotic conditions [
19
]. Moreover, Grew et al. have pointed out that
the development of borate minerals in the Earth's crust may have occurred too late
to be useful for prebiotic processes: “concentrations of B either on land or in the sea
sufficient to play a role in ribose stabilization are thus unlikely” [
20
].
Addressing a different issue, Vázquez-Mayagoitia et al. carried out
ab initio
cal-
culations at the B3LYP level on silicate complexes of the C5 sugars arabinose,
lyxose, ribose, and xylose[
21
]. They studied five different stereochemical versions
for each sugar, such as
4
. They found that the ribose silicates were more stable than
the other sugar silicates, “to the extent that the least stable of these is even more
stable than the most stable stereoisomer of the other 2:1 sugar-silicate complexes.”
They suggested that formose reactions in the presence of sodium silicate should
form ribose products preferentially over the other C5 sugars.
2.4
Summary
In summary, polyhydroxy compounds readily form complexes with sodium sili-
cate at room temperature. Stereochemical requirements limit the reaction to sugars
that exist in the furanoid form, have an unsubstituted anomeric hydroxy group, and
