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

.