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Alejandro
L. Briseno: Studies of Potential-Dependent
Metallothionein Adsorptions Using a Low-Volume
Electrochemical Quartz Crystal Microbalance Flow Cell
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This work has demonstrated that the MT adsorption onto the
mercury electrode surface is potential-dependent. A novel, low-volume
electrochemical quartz crystal microbalance flow cell was developed and combined
with a simple flow-injection system (FI-EQCM) to facilitate the study of MT
adsorption at a preset electrode potential. The adsorption process seems
to be governed by a combination of chemisorption thorough the interaction of
cysteine residues of the MT molecules with the mercury surface and the
electrostatic interaction between the positively charged MTs and the electrode
surface. When the applied potential (e.g., ?0.9 V) is more negative than the PZC
of the mercury, the amount of MT adsorption was found to be greater than that
observed under an open-circuit condition. On the other hand, a potential more
positive than the PZC (e.g., - 0.3 V) appears to cause the electrode
surface to repel the MT molecules in the solution, resulting in a smaller extent
of adsorption via mercury-cysteine thiolate formation. The magnitude of the MT
oxidation current in the DPV scan was found to be consistent with the amount of
MT adsorbates. Metal ions (i.e., Cd2+) released by the MT molecules
adsorbed at –0.9 V or stripped off the Hg film, were differentiated from the
total mass increase measured by the FI-EQCM device. The separate determination
of the metal ions released was carried out using electrochemical ICP-atomic
emission spectrometry (EC/ICP-AES). The contribution of the metal accumulation
to the net mass change was found to be rather small. Finally, the
comparison between the voltammetric data and the FI-EQCM results allowed the
average number of cysteine (sulfhydryl) groups per MT molecule involved in the
electrode reaction, n, to be determined. We found that the n value is 4.2 +/-
1.8 or 4.1 +/- 1.7, depending on the mechanism employed for the explanation of
the MT oxidation. The range of the n value is consistent with the literature
value and that we reported in a previous study. Our approach based on this
flow cell should be generic, since many metalloproteins are charged and can
interact with an electrode surface through electrostatic interactions. The
relatively small internal volume is particularly desirable, as the availability
of many biological samples is generally limited.
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