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| Figure 1. A d6-piano stool complex for Iridium. |
I have recently read an article from Chem. Commun. on the chemistry-biology interface [1]. It should be fascinating to both researchers in chemistry and biology alike, as the research is about the application of an artificial metalloenzyme in catalysis.
Modular in nature, the metalloenzyme consists of a host
protein and the synthetic metal complex. In order to incorporate the metal
catalyst to the protein host, a well-established way, for which the researchers
have adopted, is the biotin-streptavidin technology.
The metal complex the researchers were using belonged to the
class of d6-piano stool complex, and in this particular case, the metal was
iridium (Figure 1). This type of piano-stool complexes seemed to be stable in biological environment, thus providing a benefit for the development of bio-catalysis. In order to make
their catalyst more active in a biological environment, they have devised a
‘shielding strategy’, where they coated silicon nanoparticles as a protective
layer on the metalloenzyme (Figure 2).
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| Figure 2. The design of the catalyst architecture. |
The synthesis of the iridium complex involved the reaction
of a protected TFA salt of the precursor di-amine with [Cp*IrCl2]2
in the presence of triethylamine at room temperature, resulting in the iridium
catalyst with satisfactory yield (Figure 3).
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| Figure 3. Synthesis of the iridium complex. |
The metalloenzyme, henceforth known as an ATHase, was covalently
anchored onto the silicon nanoparticles (Figure 2). After self-assembly and the
poly-condensation of silanes, a protective layer could be formed and embedded
the ATHase inside the 'silica core'. The nanoparticles were visualized by
scanning electron microscopy.
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| Figure 4. Catalysis of imine reduction by ATHase. |
The ATHase was used to catalyze an imine reduction reaction (Figure 4).
Depending on the specific mutant employed, it could give rise to a specific
enantiomer as the product. In a nutshell, the conversion, enantioselectivity
and turnover number of the silica-protected ATHase were impressive. What was
notable was that, when the researchers compared the data of the catalysis of
the metalloenzyme with or without silica-protection, the results were more or
less comparable, signifying that the extra silica protection has not led to a
negative impact to the catalysis. The silica-protected catalyst could be
recycled and re-used, with almost the same enantiomeric excess in subsequent
runs.
In terms of the context of the research aim, the authors would
like to explore the applications of their ATHase in biological scenarios. Thus,
they used the ATHase to carry out a reaction well known to anyone in
biochemistry : the conversion of NAD+ to NADH. They have used
absorbance spectroscopy (340 nm) – another obvious technique for biochemists –
to monitor the conversion. They observed that the silica-protected ATHase had a
somewhat better turnover number as compared to the non-protected version.
Another interesting aspect was that the researchers
attempted to test their ATHase in vivo,
which meant they did the reduction in a cellular ‘soup’. Using cell lysate of
E. coli and other cellular media, satisfactory turnover numbers could be
achieved, and the results were indeed better than the homogeneous
metalloenzyme. They rationalized that, since the homogeneous metalloenzymes did
not possess any protective layers, the catalytic activity was lost in the
presence of cellular debris in the cellular ‘soup’. Thus, the silica ‘shell’
was able to protect the ATHase in the cellular media, while at the same time
allowing the ATHase to carry out its mission. Even an imine reduction in urine
(!!) could be achieved (TON= 4500)!
The methodology has the added advantages that it could be
conducted in physiological conditions – room temperature, pH 7, making it a
stunning approach for bio-catalysis.
Impressive!
by Ed Law
20/8/2016
Reference:
1. Immobilization of an artificial imine reductase within
silica nanoparticles improves its performance
Martina Hestericová, M. Rita Correro, Markus Lenz, Philippe
F.-X. Corvini, Patrick Shahgaldian and Thomas R. Ward
Chem. Commun., 2016, asap
DOI: 10.1039/c6cc04604e
