Nanoscale Mechano–Electronic Behavior of a
Metalloprotein as a Variable of Metal Content
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Abstract
In
this work, we have explored an approach to finding a correlation
between the mechanical response of a metalloprotein against a range
of applied force (by force curve analysis) and its electrical response
under pressure stimulation (by current sensing atomic force spectroscopy)
at
the nanoscale. Iron-storage protein ferritin has been chosen as an
experimental model system because it naturally contains a semiconducting
iron core. This core consists of a large number of iron atoms and
is therefore expected to exert a clear influence on the overall mechanical
response of the protein structure. Four different ferritins (apoferritin,
Fe(III)-ferritins containing ∼750 and ∼1400 iron atoms,
and holoferritin containing ∼2600 iron atoms) were chosen in
order to identify any relation between the mechano–electronic
behavior of the ferritins and their metal content. We report the measurement
of Young’s modulus values of the ferritin proteins as applicable
in a nanoscale environment, for the first time, and show that these
values are directly linked to the iron content of the individual ferritin
type. The greater the iron content, the greater the Young’s
modulus and in general the slower the rate of deformation against
the application of force. When compressed, all the four ferritins
exhibited increased electronic conductivity. A correlation between
the iron content of the ferritins and the current values observed
at certain bias voltages could be made at higher bias values (beyond
0.7 V), but no such discrimination among the four compressed ferritins
could be made at the lower voltages. We propose that only at higher
voltages can the iron atoms that reside deeper inside the core of
the ferritins be accessed. The iron atoms that could be situated at
the inner wall of the protein shell appear to make a general contribution
to the electronic conductivity of the four ferritin systems