of 6
S1
Electron Transfer Rates in DNA Films as a Function of Tether Length
T. Gregory Drummond, Michael G. Hill, and Jacqueline K. Barton
Division of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena, California 91125
Supporting Information
Table S-1.
Tabulated electrochemical data for the series of tether lengths investigated.
Figure S-1.
Variation of cathodic and anodic peak potentials as a function of scan rate
(in V/s) for the series of tethers investigated. Black squares:
n
= 4; red circles:
n
= 5; blue
up triangles:
n
= 6; dark yellow down triangles:
n
= 7; purple diamonds:
n
= 8; green left
triangles:
n
= 9. Laviron formalism was used to generate fit lines for each data set using
an
α
value of 0.6. Background-subtracted cyclic voltammograms were generated by
numerically interpolating the charging currents to points just before and after the onset of
the respective cathodic and anodic Faradaic responses. The resulting backgrounds were
then subtracted from the raw data to yield traces like those shown in Figure 2 of the text.
We note that the “trumpet plots” shown above are based on uncorrected voltammograms;
use of background-subtracted data instead gave peak splittings (and resulting k
s
values)
within experimental error of those shown.
Figure S-2.
Representative cyclic voltammograms for two DM-DNA films (
n
= 5 top,
and
n
= 9, bottom) recorded in 5mM phosphate, 50mM NaCl, pH 7.5 at a series of scan
rates. As the scan rate increases, the peak potentials shift slowly for the shorter tether but
much more quickly for the longer tether due to the distance-dependence of charge
transfer kinetics across the
σ
-bonded tether.
Figure S-3.
Demonstration of background subtraction to yield corrected cyclic
voltammograms. Analysis of raw and background-subtracted data yielded very similar
values for
Δ
E
p
and k
s
.
Figure S-4.
Potential dependence of the DNA film height, as measured by atomic force
microscopy (AFM) under electrochemical control (for comparison, the open-circuit
height is 45 Å). The inset shows a plot of the maximum film height (measured at ~100
mV vs. the quasi-reference electrode) measured for duplexes possessing 15, 18, and 20
bases. These data show a slope of 3.2 Å/bp, close to the predicted value of 3.4 Å/bp.
The intercept, ~7 Å, is somewhat smaller than the ~ 16 Å expected for a fully extended
alkylthiol linker. See reference 18a for experimental details.
S2
Table S-1.
# Methylenes
E
0
' E
pc
E
pa
k
s
Γ
DNA-DM
(
n
)
a
(mV)
b
(mV)
c
(mV)
d
(s
-1
)
e
(pmol/cm
2
)
f
4 -600(±6) -605(±5) -596(±5) 733(±45) 39(±5)
5 -598(±6) -603(±5) -593(±5) 176(±30) 37(±6)
6 -607(±6) -613(±5) -601(±5) 89(±14) 38(±6)
7 -605(±6) -612(±5) -599(±5) 32(±8) 36(±7)
8 -606(±7) -613(±5) -599(±5) 12(±4) 33(±8)
9 -607(±7) -616(±5) -598(±5) 4.4(±1) 33(±8)
Averages -604(±7) -610(±6) -598(±5)
36(±9)
a) Number of me
thylene units in
σ
-bonded alkydiamine portion of tether. b) Apparent formal potential
of daunomycin in the DNA-modified electrode. c) Cathodic peak potentials from cyclic
voltammograms at low scan rates; all potentials stated
versus saturated AgCl/Ag reference. d) Anodic
peak potentials from cyclic voltammetry. e) Apar
ent rates of electron tran
sfer in DNA-DM films
determined by Laviron method using the value
α
= 0.6, which gave good fits to the experimental data.
f) Surface excess of DNA-DM conjugate in monolayer, measured routinely by adsorption charge and
phosphate binding assay.
15
S3
Figure S-1.
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
-0.35
-0.40
-0.45
-0.50
-0.55
-0.60
-0.65
-0.70
-0.75
-0.80
Peak Potentials (V)
log (scan rate)
S4
Figure S-2.
-200
-300
-400
-500
-600
-700
-800
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
n
= 5 tether
increasing
n
increasing
n
0.010 V/s
0.032 V/s
0.10 V/s
0.32 V/s
1.0 V/s
3.2 V/s
10 V/s
Current (A)
Potential (mV)
-200
-300
-400
-500
-600
-700
-800
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
n
= 9 tether
increasing
n
increasing
n
Current/Scan Rate (s)
Potential (mV)
0.010 V/s
0.032 V/s
0.10 V/s
0.32 V/s
1.0 V/s
3.2 V/s
S5
Figure S-3.
-200
-300
-400
-500
-600
-700
-2x10
-7
-1x10
-7
0
1x10
-7
2x10
-7
3x10
-7
4x10
-7
Experimental CV
Background CV
Background-subtracted CV
Current (A)
Potential (mV, Ag/AgCl)
S6
Figure S-4.
10
20
30
40
50
60
0
200
400
600
Applied Potential (mV vs. Ag wire)
0
10
20
30
40
50
60
70
80
0
5
10
15
20
25
Maximum Film Height (Å)
# Base Pairs in Duplex
Film Height (15-mer, Å)