Biomedical Engineering Reference
In-Depth Information
1
0.5
0.5
0
1
2
0.5
1
3
0.5
0
0.5
E/V vs Ag/AgCl
0
0.5
0.5
0
0.5
E
/ V vs Ag/AgCl
FIGURE 6.5
CVs at MPA-modifi ed Au electrode in 5 mM phosphate buffer (pH 7.0) containing (1) Fe-
SOD (0.32 mM), (2) Mn-SOD (0.40 mM), and (3) Cu/Zn-SOD (0.20 mM). Inset: CV of the MPA-modifi ed
Au electrode in pure phosphate buffer. Potential scan rate, 100 mV s
1
. (Reprinted from [138], with permis-
sion from the American Chemical Society.)
SAM of MPA. Interestingly, the electron transfer of Fe-SOD and Mn-SOD could not be
facilitated by the SAM of cysteine even though the SAM of cysteine could be used for
promoting the electron transfer of Cu, Zn-SOD [98], as described above. This again sug-
gests the promoter-dependent nature of the electron transfer properties of the SODs.
The formal potential
E
0
of bovine erythrocyte Cu, Zn-SOD, Fe-SOD and Mn-SOD
both from
E. coli
, estimated as (
E
p
a
E
p
c
)/2, were 0.21 V, 0.14 V, and 0.23 V vs Ag/
AgCl at pH 7.0, respectively. These values correspond to 0.41 V, 0.33 V, and 0.42 V vs
NHE and are mostly within a range from ca. 0.04 to 0.403 V vs NHE reported for the
SODs in literatures [127, 130, 131]. The diversity in these documented values is prob-
ably due to the differences in enzyme preparation, electron transfer promoters used,
and experimental conditions employed.
Similar to those observed with the cysteine-modifi ed electrode in Cu, Zn-SOD
solution [98], CVs obtained at the MPA-modifi ed Au electrode in phosphate buffer
containing Fe-SOD or Mn-SOD at different potential scan rates (
v
) clearly show that
the peak currents obtained for each SOD are linear with
v
(not
v
1/2
) over the poten-
tial scan range from 10 to 1000 mVs
1
. This observation reveals that the electron
transfer of the SODs is a surface-confi ned process and not a diffusion-controlled one.
The previously observed cysteine-promoted surface-confi ned electron transfer process
of Cu, Zn-SOD has been primarily elucidated based on the formation of “a cysteine-
bridged SOD-electrode complex” oriented at an electrode-solution interface, which is
expected to suffi ciently facilitate a direct electron transfer between the metal active
site in SOD and Au electrodes. Such a model appears to be also suitable for the
SODs (i.e. Cu, Zn-SOD, Fe-SOD, and Mn-SOD) with MPA promoter. The so-called
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