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Study of adenine and guanine oxidation
mechanism by Surface-Enhanced Raman
Spectroscopy.
SUPPORTING INFORMATION
D. Ibañez1, A. Santidrian
2, M. Kalbáč
2, A. Heras
1, A. Colina
1,*.
1. Department of Chemistry, Universidad de Burgos, Pza. Misael Bañuelos s/n, E-
09001 Burgos, Spain.
2. J. Heyrovsky Institute of Physical Chemistry of the AS CR, v.v.i., Dolejskova
2155/3, CZ-182 23 Prague 8, Czech Republic
Figure S1.
Figure S1. Raman spectra of adenine at 20 s and 80 s of AuNPs electrodeposition on a
SWCNT electrode. In this figure, G and D bands of the SWCNT electrode are the main
feature of the Raman spectra. For this reason, Figure 1.b is shown in the manuscript in a
narrower spectral window.
Figure S2.
a)
b)
Figure S2. SEM images of AuNPs with the optimal electrodeposition conditions on a
SWCNT electrode at (a) 80k and (b) 250k magnification.
Independent component analysis (ICA) is a mathematical method for resolving mixed
signals, separating a multivariate signal into independent non-Gaussian signals. ICA is
based on the assumption that the response is a set of statistically independent non-
Gaussian signals, allowing a blind separation of each signal, which is very useful for
unknown problems, as our case is. ICA shows limitations in analytical chemistry
respect to multivariate curve resolution, but although ICA does not provide direct
information about the composition of the solutions, it provides very valuable
information about the main components that take part in the chemical process.
The ICA bilinear model can be written as R=CST where R (E x J) is the original
Raman spectra, C (E x N) and ST (N x J) are the so-called mixing and source matrices,
respectively. N is the number of components, E is the number of applied potentials and
J is the number of points in the Raman spectrum. C gives us an idea about the weight of
a component for the different potentials and S gives us an idea about the weight of the
Raman bands for each component.
Prior to perform the ICA calculation, a selection of Raman shifts was done. The
algorithm used to select the Raman shifts was based on removing the zones of the
spectra that only contain noise and do not show any Raman intensity change. Thus, we
diminished the level of noise introduced to calculate the ICA model.
From the C and S matrices we obtain a first approach to the evolution of the Raman
bands during the oxidation of adenine and guanine.
For a better understanding of the C and S matrix, we have changed in some cases
the sign of the signal in Figure S3 and S5. In this way, the signal is not mathematical
meaningful but the chemical meaning is more intuitive.
Figure S3 provides information about the three components of adenine oxidation.
First of them (Fig. S3.a, red line) shows its highest weight at the beginning of the
experiment, at potentials near 0.00 V, and it decreases at more positive potentials.
According to the red line in Figure S3.b this component is associated with SWCNT and
adenine, specially shown in the disorder band (D-band, 1340 cm-1
) for SWCNT and the
breathing mode (737 cm-1
) and the other bands described in Table 1 for adenine. Second
component (green line in Fig. S3.a) exhibits the highest weight at +0.40 V and it can be
related to 2-oxoadenine (2-oxoA). In this case the strongest changes in the in the Raman
spectrum (Fig. S3.b) are observed between 1200-1400 cm-1
as is explained in the
Section 3.2. Third component (blue line in Fig. S3.a) displays the highest weight at
more positive potential, +0.70 V, and it is due to 2,8-dioxoadenine (2,8-dioxoA). The
most important changes in the Raman spectrum (Fig. S3.b) are observed in the 2,8-
dioxoA breathing mode rather than between 1200-1400 cm-1
as the second component.
At more positive potentials the hydrolysis takes places.
Figure S3.
a)
b)
Figure S3. Plots of (a) the weight of each component for the different potentials and
(b) the weight of Raman bands in the ICA components related to the adenine oxidation
mechanism.
Figure S4.
Figure S4. Cyclic voltammogram obtained in a 1 mM adenine and 0.01 M KCl solution
(pH = 5.85). The potential was scanned from 0.00 V to +0.90 V and back to 0.00 V at
scan rate of 25 mV·s−1
.
Figure S5 provides information about the three components of guanine oxidation.
First component (red line in Fig. S5.a) exhibits its highest weight at the beginning of the
experiment, at potentials near 0.00 V, and it decreases at more positive potentials. This
component is related to guanine as it is evident in Raman spectrum (re line in Fig. S5.b),
specially shown in the 1382 cm-1
band and in the other bands described in Table 2. In
this case, SWCNT disorder band (D-band, 1340 cm-1
) is completely overlapped with a
strong guanine bending (N1-C2-N3) band peaked at 1382 cm-1
. Second component
(green line in Fig. S5.a) shows the highest weight at +0.10 V and is related to the 8-
oxoguanine (8-oxoG). In this case the strongest changes in the in the Raman spectrum
(green line in Fig. S5.b) are observed between 1200-1400 cm-1
as is explained in the
Section 3.3. Third component (blue line in Fig. S5.a) displays the highest weight at
more positive potential, +0.30 V, and it is associated to the oxoguanine oxidized
(oxoGuox
). The most important change in Raman spectrum (blue line in Fig. S5.b) is
appreciated in the band centered at 1382 cm-1
because the molecule modifies its
orientation after the second oxidation step. At more positive potentials the hydrolysis is
produced.
Figure S5.
a)
b)
Figure S5. Plots of (a) the weight of each component for the different potentials and
(b) the weight of Raman bands in the ICA components related to the guanine oxidation
mechanism.
Figure S6.
Figure S6. Cyclic voltammogram obtained in a 1 mM guanine and 0.01 M KOH
solution (pH = 13.1). The potential was scanned from 0.00 V to +0.60 V and back to
0.00 V at scan rate of 25 mV·s−1
.