Supplementary material for
Field-controlled electron transfer and reaction kinetics of the biological catalytic system of
microperoxidase-11 and hydrogen peroxide
Yongki Choi and Siu-Tung Yau*
Department of Electrical and Computer Engineering, Cleveland State University, Cleveland, OH
44115, and Department of Physics and Astronomy, University of California Irvine, Irvine, CA
92697-4576
* E-mail: s.yau@csuohio.edu
A. The effect of reversing the polarity of VG on the redox properties of immobilized FePP
To provide additional support for the model of modulation of the tunnel barrier, we have
performed cyclic voltammetry with an FePP-immobilized HOPG electrode with VG increasing in
the negative polarity as shown in Figure S1 (a). A negative VG reverses the direction of the
induced field and therefore increases the effective height of the tunnel barrier1. The black CV,
obtained with VG = 0 V, shows a pair of redox peaks, indicating the molecule’s reversible
electron transfer (redox) reaction, in which Fe3+ is reversibly reduced to Fe2+ as previously
reported2. When VG is changed to -0.1 V, the corresponding red CV shows a slight upward shift
with decreased redox peak currents. The blue CV corresponds to a more negative VG of -0.4 V.
It shows further decreases in the redox peak currents. Figure S1 (b) shows detailed decreases in
the anodic peak and cathodic peak currents described in Figure S1 (a). The dashed baselines are
used to indicate the changes in the peak currents. The inset of Figure S1 (a) shows the
dependence of the peak currents on VG. The peak currents of the redox peaks of FePP decrease
as the magnitude of the reversed voltage increases, indicating an increased effective height of the
tunnel barrier.
1
Figure S1 The effect of VG on the redox properties of FePP immobilized on an electrode. (a) The
CVs were obtained in 10 mM sodium borate buffer (Na2B4O7∙10H2O, pH 10) with VG. The green
dotted lines are used to indicate the baselines of the peaks and the peak heights. The inset shows
the dependences of the peak currents on VG. Ip,o and Ip,r are the peak currents of the oxidation
peak and the reduction peak, respectively. The peak currents of the redox peaks of FePP decrease
as the magnitude of the reversed VG increases, indicating an increased effective height of the
tunnel barrier. (b) Detailed increases in the anodic peak and cathodic peak currents in (a). The
green dashed lines are used to indicate the baselines for estimating the peak heights, which are
indicated by the solid lines with double arrows.
2
B. Characterization of MP-11-immobilzied electrode
Figure S2 shows the CVs obtained with a MP-11-immobilized electrode at various potential scan rates
in the absence of H2O2. The well-defined redox peaks indicate a formal potential of E0 = -0.31 V (vs.
Ag/AgCl) for the redox couple Fe3+/Fe2+ contained in MP-11 as observed in the literature3. The inset of
Figure S2 shows that the redox peak currents has a linear dependence on the scan rate, indicating that
the MP-11 molecules that were communicating electrically with the electrode were immobilized on the
electrode4. The number of electrons exchanged, n, between the MP-11molecule and the electrode can
be estimated using the Laviron equation, Ip,r = (nFQv) / (4RT), where Ip,r is reduction peak current, v is
the scan rate, Q is the charge of an electron, and F is faraday constant4. Therefore, for the redox reaction
of immobilized MP-11, the CVs in Figure S2 yield a value of n = 1.09, which is consistent with the
single electron transfer process of Fe3+/Fe2+.
The electrode’s surface coverage by the MP-11
molecule, can be estimated by  = (4Ip,rRT) / (n2F2Av)5. Measuring the reduction peak height from
the CV in Figure S2 obtained at a scan rate of 100 mV/s and using an area of 1 mm2 for the electrode
surface area A,  was estimated to be 4.7266  10-11 mol/cm2.
Since the size of MP-11 is
approximately 2.5 nm  1.3 nm  1.5 nm6, the value of  corresponds to a monolayer.
Figure S2 CVs of MP-11 immobilized on the HOPG electrode obtained at different potential scan rates
in 100 mM phosphate buffer (42mM NaH2PO4∙H2O, 58mM Na2HPO4∙7H2O, pH 7.0). The inset shows
the dependences of Ip,o and Ip,r on the potential scan rate.
C. The Lineweaver-Burk (LB) plot
The reduction peak current, Ip,r, is given in terms of kcat and K’m by the LB equation7,
1 / Ip,r = {Km / (nFAkcat [H2O2]) + 1 / (nFAkcat )}
where, n , F , A and  are discussed and estimated in the previous section. The equation shows that 1 /
Ip,r has a linear dependence on 1 / [H2O2] as shown in Figure S3. Therefore, kcat and K’m for VG can be
determined from the slope and the vertical intercept of the plot in Figure S3. The black dot and its
fitting line with the fitting coefficient R2 = 0.98 are obtained in the absence of VG. The Red dot and its
fitting line with R2 = 0.99 are obtained in the presence of VG = 0.7 V. The black line shows higher
values of slope and intercept than that of red line so that the black line has higher K’m and lower kcat
values.
Figure S3 Plot of 1 / Ip,r vs. 1 / [H2O2]. The black dot and its fitting line and red dot and its fitting line
are obtained in the absence of VG and in the presence of VG, respectively.
D. References
1.
Snow, E. S.; Campbell, P. M.; Rendell, R. W.; Buot, F. A.; Park, D.; Marrian, C. R. K.; Magno,
R., Appl. Phys. Lett. 1998, 72, 3071-3073.
2.
Shlgehara, K.; Anson, F. C., J. Phys. Chem. 1982, 86, 2776-2783.
3.
10.
Liu, Y.; Wang, M.; Zhao, F.; Guo, Z.; Chen, H.; Dong, S., J. Electroanal. Chem. 2005, 581, 1-
4.
Laviron, E., J. Electroanal. Chem. 1979, 101, 19-28.
5.
Bard, A. J.; Faulkner, L. R., Electrochemical Methods. 2 ed.; John Wiley & Sons: New Jersey,
2001.
6.
Luo, Y.; Brayer, G. V., J. Mol. Biol. 1990, 214, 585-595.
7.
Kumar, A. S.; Zen, J.-M., Electroanalysis 2002, 14, 671-678.