Chemistry Reference
In-Depth Information
Table 1.1
Bond strengths and bond lengths for selected biologically relevant bonds to carbon and
the corresponding bonds to silicon [
3
-
5
]
Bond
Bond energy (kJ/
mol) [kcal/mol]
Bond length (Å)
Bond
Bond energy (kJ/
mol) [kcal/mol]
Bond length (Å)
Si-C
369 [88]
1.89
C-C
334 [80]
1.54
Si-H
376 [90]
1.48
C-H
420 [100]
1.09
Si-O
531 [127]
1.63
C-O
340 [81]
1.41
Si-N
401 [96]
1.74
C-N
335 [80]
1.47
Si-S
414 [99]
2.14
C-S
313 [75]
1.80
Si-Si
308 [74]
2.34
stable for a biologically significant timeframe and it becomes more and more diffi-
cult to imagine silicon-based life existing, at least according to our definition of life.
That being said, the first steps to silicon-based life may have been taken by
researchers who have developed zeolite catalysts that behave as enzyme mimics.
These systems, which support oxidizing Fe and Pd species, have the capacity to ac-
tivate molecular oxygen and perform hydrocarbon oxidation in a manner similar to
cytochrome P450 [
6
]. Also, recent research has shown that silicon may have played
an integral role in the formation of what is now ribonucleic acid (RNA) molecules
(Chap. 2). It may not be silicon-based life, but is it a step in that direction?
1.2
Not Science Fiction After All: Plants, Diatoms, and
Sponges
As much as life as we know it is based on carbon rather than silicon, silicon does
play a very important role in the life cycles of a number of marine and terrestrial
organisms [
7
-
10
]. Of these the diatoms, sponges, and plants are perhaps the most
well known [
11
].
1.2.1
Plants
Silicon has been reported to play a number of roles in plant species, such as the
banana, [
12
] cucumber, [
13
,
14
] sorghum, [
15
] wheat, [
16
] rice, [
17
] and horsetail,
[
18
] which absorb silicon from the soil in the form of silicic acid [
19
]. Although the
beneficial role of silicon in promoting the growth and development of a number of
plant species is generally accepted, the exact physiological and metabolic mecha-
nisms with these phenomena are not well understood and in some cases ignored
altogether [
20
,
21
].
In some plant species silicon is drawn into the plant and deposited in epidermal
cells or as phytoliths in lumen cells [
19
]. In this role the silica that is deposited
