Environmental Engineering Reference
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The second shell mechanism was supported by the loss of activity when Ile142
was exchanged against alanine in the site-directed mutagenesis experiments
[
22
]. However, in all DFT calculations performed so far, a first shell mechanism
with a direct binding of acetylene to the W ion gave much lower energy barriers
[
34
,
35
,
37
]. Antony and Bayse [
34
] calculated the substitution of the water
molecule by acetylene to form a
2
-acetylene complex to be favorable by a
G
of -10 kcal/mol. Therefore, a reaction mechanism starting with the formation of a
η
η
ʔ
2
-acetylene complex followed by a nucleophilic attack by a water molecule was
proposed, yielding either a
η
2
-complex of a vinyl alcohol or a
β
-
hydroxovinylidene
(Figure
6
)[
34
]. Vincent et al. [
37
] calculated the energetic barriers for the inter-
mediates of the reaction mechanism proposed by Seiffert et al. [
21
] and Antony and
Bayse [
34
]. The nucleophilic attack of a water ligand on the C
ʱ
of acetylene to form
a vinyl alcohol via a vinyl anion and deprotonation of Asp13 [
21
] was calculated to
be exothermic in total (-21.4 kcal/mol) but the barrier of 43.9 kcal/mol was quite
high [
37
]. The results for the nucleophilic attack of a water molecule on a
2
-
acetylene complex were quite similar, the overall reaction was slightly exothermic
(-1.9 kcal/mol) but the barrier was also quite high (41.0 kcal/mol) [
37
]. The nearly
identical barriers for both mechanisms could be explained by the similarities
between both pathways, involving a nucleophilic attack of a water molecule, a
cyclic intermediate structure and a proton shuttle from Asp13 [
37
].
Therefore, Vincent et al. [
37
] proposed a new reaction mechanism with overall
lower barriers. The mechanism starts with a
η
2
-acetylene complex that forms
η
an end-on bound vinylidene complex (W
CH
2
) by deprotonation of Asp13.
The attack of a water molecule activated by a hydrogen bond to Asp13 leads to
the formation of a carbene complex that will isomerize to form acetaldehyde
(Figure
6
)[
37
]. The barriers for the formation of the end-on bound vinylidene
complex and the formation of the carbene complex were calculated to be 28.1 kcal/
mol and 34.0 kcal/mol respectively. The isomerization to acetaldehyde via a
tungsten hydride complex requires the breaking of a W-C bond with a barrier of
29 kcal/mol and the decomposition of
¼
C
¼
the product
(3 kcal/mol barrier)
(Figure
7
)[
37
].
Liao et al. [
35
] postulated another reaction mechanism based on DFT calcula-
tions. Notably in this model, Asp13 is in a deprotonated state because the model
includes three hydrogen bonds of Asp13 to Cys12, Trp179, and the H
2
O ligand of
the W ion, lowering the calculated p
K
a
of Asp13 to 6.3. In the first step of this
mechanism acetylene forms a
2
-complex with the W ion by displacing the water
ligand. The displaced water molecule is activated for a nucleophilic attack on the
η
η
2
-acetylene complex by a proton transfer to the ionized Asp13. The resulting vinyl
anion will be protonated by Asp13, yielding a vinyl alcohol. The tautomerization
needed to form acetaldehyde from the vinyl alcohol, can either occur spontaneously
after release of the alcohol from the active site or will be assisted by the W ion and
Asp13 [
35
]. The assisted tautomerization starts with a proton transfer from the OH
group of the vinyl alcohol to Asp13, yielding an enolate that binds with the oxygen
atom instead of a carbon atom to the W ion. The proton is then delivered back to C2
yielding the product acetaldehyde (Figure
6
)[
35
]. The displacement of the water
molecule by acetylene in the first reaction step was calculated to be exothermic
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