Spectrophotometric and electrochemical determination of the formation constants of the complexes Curcumin–Fe(III)–water and Curcumin–Fe(II)–water

Spectrochimica Acta Part A 60 (2004) 1105–1113

Spectrophotometric and electrochemical determination of the formation constants of the complexes Curcumin–Fe(III)–water and Curcumin–Fe(II)–water Marganta Bernabé-Pineda a , Maria Teresa Ram´ırez-Silva a , Mario Alberto Romero-Romo b , Enrique González-Vergara c , Alberto Rojas-Hernández a,∗ a

Departamento de Qu´ımica, Área de Qu´ımica, Anal´ıtica, Universidad Autónoma Metropolitana-Iztapalapa, San Rafel Altixco 186, Apdo Postal 55-534, C.P. 09340, México, D.F. México b Departamento de Materiales, Universidad Autónoma Metropolitana Azcapotzalco, Av. San Pablo # 180, Col. Reynosa-Tamaulipas, C.P. 02200, México, D.F. México c Centro de Qu´ımica, Benemérita Universidad Autónoma de Puebla, Puebla, México Received 9 July 2003; received in revised form 31 July 2003; accepted 31 July 2003

Abstract The formation of complexes among the Curcumin, Fe(III) and Fe(II) was studied in aqueous media within the 5–11 pH range by means of UV-Vis spectrophotometry and cyclic voltammetry. When the reaction between the Curcumin and the ions present in basic media took place, the resulting spectra of the systems Curcumin–Fe(III) and Curcumin–Fe(II) presented a similar behaviour. The cyclic voltammograms in basic media indicated that a chemical reaction has taken place between the Curcumin and Fe(III) before that of the formation of complexes. Data processing with SQUAD permitted to calculate the formation constants of the complexes Curcumin–Fe(III), corresponding to the species FeCur (log β1 1 0 = 22.25 ± 0.03) and FeCur(OH)− (log β1 1-1 = 12.14 ± 0.03), while for the complexes Curcumin–Fe(II) the corresponding formation constants of the species FeCur− (log β1 1 0 = 9.20±0.04), FeHCur (log β1 1 1 = 19.76±0.03), FeH2 Cur+ (log β1 1 2 = 28.11±0.02). © 2003 Elsevier B.V. All rights reserved. Keywords: Curcumin; Iron; Spectrophotometry; CPE; Cyclic voltammetry; SQUAD

1. Introduction The study of metal elements (Cu, Fe and Mn) with certain chelates [1] having antioxidant properties have shown the relevance of the role associated with several biological damages (gastritis, intestinal ulcers, etc.). Diverse plants having some phenol components have been studied far in the past because of their significant antioxidant power [1]. The Curcumin (H3 Cur) is the main pigment of the curcuma longa that displays quite a powerful antioxidant activity [2–5], which takes place mainly when diverse relevant biological free radicals are produced during physiological processes [6], besides which, it can participate in the oxidative cascade and inhibit the oxidation of certain metal ions [7]. ∗ Corresponding author. Tel.: +52-55-58044670; fax: +52-55-58044666. E-mail address: [email protected] (A. Rojas-Hern´andez).

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00344-5

Iron is one of the ions studied with the Curcumin which has gained quite a relevant position, given its great importance in biological processes such as in oxygen transfer and the DNA synthesis, to mention but a couple of the most important ones. The aforementioned underlines the fact that iron plays an important role in human consumption, such that the deferoxiamine turns out to be the only chelating agent used for clinical purposes, mainly because of its low gastrointestinal absorption. Thus, it is important to seek for other alternative chelating agents that can be used for the purpose [7,8]. Several studies have been carried out with the Curcumin– Fe(III) in non-aqueous media that refer to the formation of complexes [5,7,8]; although the experimental conditions for each of the studies have been different, namely, those of Kunchandy [8], which showed the capacity of the Curcumin to reduce the Fe(III). Although Tonnesen and Greenhill [5] questioned their experimental conditions, the author did not discard the possibility that such a reaction could take

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place mainly because of the ability of the Curcumin to form complexes with either Fe(III) or Fe(II). The studies that determined the equilibrium constants for the complexes Curcumin–Fe(III) were carried out by Borsari et al. [7] in a 1:1 methanol–water medium. In view of the antecedents, the aim of the present work is the assessment of the formation constants of the Curcumin–Fe(III) and Curcumin–Fe(II) in aqueous media and to give evidence of the formation of the said complexes by means of electrochemical studies.

2. Experimental 2.1. Reagents and equipment All solutions were prepared using deionised water Type I (18.2 M) free from organic matter, produced with a US Filter Purelab Plus UV. The potentiometer calibration was performed with a phosphate buffer solution Beckman (pH = 7.0 ± 0.01 at 25 ◦ C). During the experimental campaign a protective nitrogen atmosphere was provided to the cells used for every determination. Analytical grade Curcumin (Merck), NaOH lentils (Merck) were used as reagents: the Curcumin solutions were prepared in 5 × 10−3 sodium hydroxide solutions to keep an initial basic pH. The pH readings were obtained throughout the experimentation using a LPH 430T pH-Meter Tacussel Radiometer—Tacussel LPH430T (pH = ±0.001) used in conjunction with an electrode Radiometer Analytical pHC3006-9, and the absorbance measurements were effected with a UV-Vis Perkin-Elmer Lambda 20, and quartz cells having an 1 cm optical path. A BAS-100W potentiostat was used to conduct the cyclic voltammetry experiments using a classical three-electrode cell, with saturated calomel

(SCE) acting as reference, Pt as auxiliary and a carbon paste electrode (CPE) as working substrate [9].

3. Results and discussion 3.1. Spectrophotometric study of the Curcumin–Fe(II)–H2 O system The UV-Vis spectra corresponding to the Curcumin–Fe(II) were obtained in an aqueous system starting at a 10.50 but in order to adjust such value to the experimental requirements, HCl was added to the solutions accordingly. Fig. 1 shows typical spectra of the systems having Curcumin [2.44 × 10−5 M]–Fe(II) [1.93 × 10−5 M] within the 10.50–5.54 pH range. As shown in Fig. 1, the Curcumin–Fe(II) presented an absorption maximum at a 469 nm wavelength: however, when the pH decreased there occurred a hypsochromic shift and two absorbance maxima that appeared at pH values lower than 8.14. When assessment of the system began at acid pH (3.51) and evolved later to more basic pH values, the spectral behaviour remained unchanged. Once the UV-Vis behaviour of the Curcumin–Fe(II) was recorded, it became necessary to propose different models for the possible Curcumin–Fe(II) complexes to feed them into the SQUAD refining software [10–13], the results of which are summarised in Table 1. The best model refined is the one proposed in Table 1, which take into consideration three complexes of the Cur–Fe(II) species, the constants of the Curcumin [14] and the Fe(OH)+ hydrocomplex formation constant [15]. This is confirmed by the results shown in Fig. 2, which presents the plots of the absorptivity coefficients for each of the species and the corresponding error bars.

Fig. 1. Typical UV-Vis spectra for the system constituted by Curcumin [2.44 × 10−5 M]–Fe(II) [1.99 × 10−5 M] as a function of pH.

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Table 1 Results obtained from the refinement using SQUAD for the formation of Curcumin–Fe(II) complexes in aqueous media Model proposed

(log β ± σ) refined

HCur2−a H2 Cur−a H3 Cura Fe(OH)+a FeCur− FeHCur FeH2 Cur+

10.51a 9.88a 8.38a −9.3b log ␤1 1 0 = 9.20 ± 0.04 log ␤1 1 1 = 19.76 ± 0.03 log ␤1 1 2 = 28.11 ± 0.02

Sum of squares (U)

σA

4.52 × 10−2

7.32 × 10−3

Twenty two spectra in the 560 and 238 nm range with 7 nm increments were fed into the software. a Fixed constants [14]. b Fixed constants [15].

As observed from the plots in Fig. 2, the errors ascribed to each data point are smaller than 3% compared to other models proposed; this way, the model which was best refined by SQUAD is that which considered three species formed in the Curcumin–Fe(II)–H2 O. Once the spectrophotometry behaviour has been characterised in the said system, an experimental study was required for the Curcumin–Fe(III)–H2 O, to enable completion of the studies on the two metal ions Fe(III)/Fe(II) with Curcumin. 3.2. Spectrophotometric study of the Curcumin–Fe(III)–H2 O Several solutions having the Curcumin–Fe(III) were initially studied at pH 10.54, which were modified by HCl additions and the UV-Vis absorption spectra were subsequently obtained; Fig. 3 presents some typical spectra

Fig. 3. UV-Vis absorption spectra corresponding to the Curcumin [1.14 × 10−5 M]–Fe(III) [5.77 × 10−6 M], as a function of pH.

corresponding to the said system within the 10.54–5.69 pH range. The Curcumin–Fe(III) system exhibited an absorbance maximum at a 469 nm wavelength at pH 10.542 (see Fig. 3) with the maximum decreasing as the pH changed to smaller values: the absorbance maxima was displaced to shorter wavelength values as the pH decreased, with the system displaying a hypsochromic shift. When the system reached pH values smaller than 7.7 the spectra displayed two absorbance maxima at 420 and 378 nm wavelength, respectively. This behaviour is similar to that obtained for the Curcumin–Fe(II) system (see Fig. 1); however, for the system with Fe(III), Fig. 4 indicates the presence of spectra at initial pH values of 5.06. There was the presence of an absorption maximum at

Fig. 2. Molar absorptivity coefficients for the Curcumin–Fe(II) system in aqueous solution.

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3.3. Electrochemical study: cyclic voltammetry

Fig. 4. UV-Vis absorption spectra of the Curcumin [1.53×10−5 M]–Fe(III) [1.59 × 10−5 M] as a function of pH.

409 nm wavelength, and as the pH increased there occurred an absorption decrease, apart from that the system exhibited an isosbestic point at 469 nm. The system with Fe(III) did not present the same behaviour when the study began at acid pH as compared to the opposite when it started at basic pH; however, looking at the latter it was the same as when it included the Fe(II), which suggests that the Curcumin may be reducing the Fe(III) [5,8] in basic conditions through a redox reaction before formation of the complexes. This may well explain the apparent behaviour of the spectra for both systems. Thus, before calculating the corresponding formation constants of the Curcumin–Fe(III) complexes, it became necessary to study the electrochemical behaviour of both systems to give support to the Fe(III) reduction hypothesis by the Curcumin in basic medium.

3.3.1. Electrochemical behaviour of the Curcumin Before characterising the electrochemical behaviour of the Fe systems containing Curcumin, it was necessary first to proceed with the characterisation of the electrochemical processes pertaining to the Curcumin, Fe(III) and Fe(II) in acid and basic media separately. Fig. 5 presents the cyclic voltammograms of the Curcumin [2.44 × 10-5 M] at pH 3 on a CPE. Fig. 5(a) shows the voltammogram corresponding to a Curcumin solution [2.44 × 10−5 M] at pH 3, where the potential scan took place in the anodic direction: three oxidation processes became apparent. The first peak appeared at EPa (I) = 497 mV versus SCE which is characteristic of the cathecol system [16]; the second oxidation process had a peak associated at EPa (II) = 682 mV versus SCE that can be attributed to the phenol group of the compound [16], while the third oxidation peak appeared not so well defined at EPa (III) = 994 mV versus SCE. When the potential scan was reversed a reduction process became evident at EPc (I ) = 268 mV versus SCE. Fig. 5(b) shows the shape and characteristics of the voltammogram when the potential scan was started in the cathodic direction, which revealed that there were no reduction processes; when the potential scan was reversed there appeared only one oxidation process at a EPa (II) = 685 mV versus SCE. Fig. 6 shows the electrochemical behaviour of the Curcumin [1.68 × 10−5 M] at pH 10: the voltammograms in Fig. 6(a) started in the anodic direction and reveal two oxidation processes. For the first, the peak at EPa (I) = −32.9 mV versus SCE was not well defined, while for the second process at EPa (II) = 305 mV versus SCE was rather shaped like a plateau. When the scan was reversed, a reduction process became apparent at EPc (I ) = −115 mV versus SCE. In Fig. 6(b) it is possible to see the features when the

Fig. 5. Cyclic voltammograms of the Curcumin [2.44 × 10−5 M] in acid medium at pH 3, at a scan rate of 100 mV s−1 . (a) In anodic direction; (b) in the cathodic direction.

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Fig. 6. Cyclic voltammogram of the Curcumin [1.68 × 10−5 M] at pH 10 with a potential scan rate of 100 mV s−1 . (a) In anodic direction; (b) in cathodic direction.

potential scan started in the cathodic direction, where only there was one peak associated to a reduction process at EPc (I ) = −125 mV versus SCE, though when the scan was reversed, one oxidation process became evident forming a shoulder in the voltammogram trace at EPa (II) = 315 mV versus SCE. All electrochemical processes labelled as EPa (I), EPa (II), EPa (III) and EPc (I ) undertook a displacement of their potential values as a function of pH, as pointed out in the Table 2. Table 2 summarises the results concerning the potential values of the voltammetry characterisation of the Curcumin as the various scans were produced in acid or basic media, which reveal that a potential shift had occurred as a function of pH. This is, the oxidation peak EPa (I) appearing at pH 3 displays a shift of 529 mV toward more negative potentials when the potential scan started at pH 10, while the process EPa (II) at pH 3 is shifted 370 mV toward more negative potentials compared to that when the scan started at pH 10. Further, the oxidation process EPa (III) is only present at the start of the potential scan in the anodic direction at pH 3. Regarding EPc (I ) this only appeared at pH 3 when the reversal of the potential scan occurs, while at pH 10 it was present during both, the anodic and the cathodic scans, but in the process the potential at pH 3

undertook a shift of 383 mV with respect to pH 10 toward more negative values (see Table 2), apart from the fact that it displayed a better definition when the system was at pH 3 (see Fig. 5(a)). 3.3.2. Electrochemical behaviour of the Fe(III)/Fe(II) The cyclic voltammetry study for the solutions with Fe(II) and Fe(III) permitted to record the oxidation and reduction peaks for both the species, however, it is relevant to mention that in the Fe(II) case the solution was kept under constant deareation by bubbling nitrogen through the solutions. Cyclic voltammetry studies of the Fe(III) [5.27×10−5 M] at pH 3 were carried out. When the scan rate was initiated in the anodic direction there were no oxidation process taking place, however, upon reversal of the scan there appeared a reduction process producing a plateau-shaped trace at EPc (II ) = −275 mV versus SCE, which is ascribed to reduction of Fe(III). When the scan was done in the cathodic direction, there is a reduction process of Fe(III) to Fe(II): when the potential scan was reversed there became evident an oxidation process at EPa (IV) = 870 mV versus SCE ascribed to Fe(II) oxidation formed during the previous scan. On the other hand, cyclic voltammetry studies corresponding to Fe(II) [1.39 × 10−5 M] at pH 4 were also undertaken.

Table 2 Potentials of the electrochemical processes associated to the Curcumin in basic and acid media EPa (I) (mV vs. SCE)

EPa (II) (mV vs. SCE)

EPa (III) (mV vs. SCE)

EPc (I ) (mV vs. SCE)

Acid media Anodic scan Cathodic scan

497 –a

682 685

994 –a

268 –a

Basic media Anodic scan Cathodic scan

−32.9 –a

305 315

–a –a

−115 −125

a

Without peak.

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Table 3 Potential values of the electrochemical processes of the Fe(II) y Fe(III) in acid media EPa (IV) (mV vs. SCE)

EPc (II ) (mV vs. SCE)

Fe(II) Anodic scan Cathodic scan

870 840

−295 –a

Fe(III) Anodic scan Cathodic scan

–a 870

−269 −275

a

There was no peak.

When the potential scan started in the anodic direction: there appeared an oxidation peak at EPa (IV) = 870 mV versus SCE ascribed to oxidation of Fe(II) but reversing the potential scan there is a reduction process at EPc (II ) = −295 mV versus SCE attributed to Fe(III) reduction: when the potential scan started in the cathodic direction, which revealed that there were no reduction processes taking place, but upon reversal of the potential scan there appeared an oxidation peak at EPa (IV) = 840 mV versus SCE ascribed to Fe(II) oxidation. The potential values obtained for the oxidation and reduction processes of the Fe(II) and Fe(III), respectively, are summarised in Table 3. Table 3 shows the potential value assigned to oxidation of Fe(II) (EPa (IV)) which is the same in both scans for the species as evidenced when the scan of Fe(III) started in the cathodic direction, while the potential peak EPc (II ) was present for both scans of the Fe(III) but it appeared only with the Fe(II) when the scan started in the anodic direction. With these results characterised for the Fe(II) and Fe(III), the cyclic voltammograms were then recorded for the interactions with the Curcumin.

3.3.3. Electrochemical behaviour of the Curcumin–Fe(II) and Curcumin–Fe(III) systems in acid media The results presented in Fig. 7 correspond to the system Curcumin [2.44×10−5 M]–Fe(III) [1.99×10−5 M] pH 3.0. When the potential scan was started in the anodic direction (not shown in figure), there are three oxidation processes the first of which was not well resolved as it resembles a shoulder at EPa (I) = 557 mV versus SCE, while the second one was recorded at EPa (II) = 687 mV versus SCE and the third also with a shoulder-like shape at EPa (III) = 997 mV versus SCE; upon reversal of the potential scan in the cathodic direction there appeared a reduction peak at EPa (I ) = 303 mV versus SCE. Fig. 7(a) presents the cyclic voltammogram of the system composed by Curcumin [2.44 × 10−5 M]–Fe(III) [1.99 × 10−5 M] with the potential scan starting in the cathodic direction: the voltammogram bore only two not very well defined reduction processes. Thus, it became necessary to produce a zoom of the plotted data corresponding to that reduction zone, as shown in Fig. 7(b). Two reduction processes became apparent: the first was located at EPa (I ) = 265 mV versus SCE, while the second peak at EPa (II ) = −30 mV versus SCE. As the potential scan was reversed, there resulted three oxidation processes, the first being located at EPa (I) = 315 mV versus SCE, the second at EPa (II) = 670 mV versus SCE and the third at EPa (III) = 985 mV versus SCE. The voltammogram for the system made up of Curcumin [1.67 × 10−5 M]–Fe(II) [1.56 × 10−5 M] at pH 3 is shown in Fig. 8. Fig. 8 gives the results of the cyclic voltammetry for the system made up of Curcumin [1.67 × 10−5 M]–Fe(II) [1.56 × 10−5 M] as the scan started in the anodic direction three oxidation peaks appear located at EPa (I) = 512 mV versus SCE, the second at EPa (II) = 702 mV versus SCE

Fig. 7. Cyclic voltammograms of the system at pH 3 composed by Curcumin [2.44 × 10−5 M]–Fe(III) [1.99 × 10−5 M] with a potential scan rate of 100 mV s−1 . (a) Cathodic scan; (b) zoom of the cathodic scan.

M. Bernab´e-Pineda et al. / Spectrochimica Acta Part A 60 (2004) 1105–1113

Fig. 8. Cyclic voltammograms of the system having Curcumin [1.67×10−5 M]–Fe(II) [1.56×10−5 M] at pH 3 with a potential scan rate of 100 mV s−1 (in the anodic direction).

and the third at EPa (III) = 1002 mV versus SCE; when the scan was reverted only one reduction process became apparent at EPa (I ) = 343 mV versus SCE. When the potential scan started in the cathodic direction, that did not reveal a reduction process, though as the scan was reversed, two oxidation peaks can be noted at EPa (II) = 710 mV versus SCE and a second not very well defined formation at EPa (III) = 1005 mV versus SCE. It becomes straightforward that the presence of Fe(III) and Fe(II) in the solutions containing Curcumin have modified the electrochemical behaviour of the processes EPa (I), EPa (II), EPa (III) and EPc (I ) as the examination of their associated potentials shown in Table 4 evidences. Table 4 shows the potential values obtained for the systems Curcumin–Fe(III) and Curcumin–Fe(II) at pH 3: it becomes plain that the values display a shift respect to those obtained for the Curcumin at pH 3 (see Table 2). The presence of Fe(III) or Fe(II) in the system modified the peak shape and the value of the process made evident at EPa (I), having the largest shift of 60 mV in the presence of Fe(II) toward more positive potentials. When Fe(III) was present, similar modifying effects appear to have taken place regardless of the direction of the potential scan, whereas when Fe(II) was present the change only occurred scanning

1111

in the anodic direction. The process occurring at EPa (II) was modified in a similar manner as for EPa (I), except that the largest shift of 17 mV toward more positive potentials (see Table 3) was observed when the Fe(II) was present in the systems studied. The process at EPa (III) underwent a change in the value of the potential towards a more negative potential when Fe(III) was present, while the Fe(II) induced a shift of 11 mV toward more positive values. The peak EPc (II ) was ascribed to reduction of the complex formed by the Curcumin with Fe(III), although, in consideration of the potential value associated with the Fe(III) (see Table 3) a shift of 239 mV toward more positive values occurred when the said cation interacted with the Curcumin. However, the said process does not take place when the Fe(II) was present, which supports the hypothesis that the formation of a complex Curcumin–Fe(III) is plausible. Regarding EPa (II) and EPa (III) they appear to be modified mainly in the presence of Fe(II), thus indicating that some chemical interaction had taken place between the Curcumin and Fe(II). In consideration of the shape of the peaks EPc (I ) and EPa (I), this suggests that the Curcumin undergoes an adsorption process, which was affected by the presence of Fe(III) and Fe(II) (see Table 4) as well as by the media pH (see Table 2). 3.3.4. Electrochemical behaviour of the systems Curcumin–Fe(III) and Curcumin–Fe(II) in basic media The voltammogram obtained with the system Curcumin [1.68 × 10−5 M]–Fe(III) [1.54 × 10−5 M] in basic media, with the potential scan in the anodic direction: two oxidation processes took place, the first at EPa (I) = 58 mV versus SCE and the second at EPa (II) = 283 mV versus SCE. When the scan was reversed there was a reduction process at EPc (I ) = −113 mV versus SCE. The voltammogram when the scan was initiated in the cathodic direction presents only one reduction process located at EPc (I ) = −115 mV versus SCE, although when the scan was inverted, one oxidation process with a shoulder-like shape appeared at EPa (II) = 295 mV versus SCE. The voltammograms of the system Curcumin [1.85 × 10−5 M]–Fe(II) [1.59×10−5 M] at pH 10 obtained when the potential scan started in the anodic direction, revealed two oxidation processes with the first taking place at EPa (I) =

Table 4 Potentials associated with the electrochemical processes of the systems Curcumin–Fe(III) and Curcumin–Fe(II) in acid media EPa (I) (mV vs. SCE)

EPa (II) (mV vs. SCE)

Curcumin–Fe(III) acid media Anodic direction Cathodic direction

557 315

687 670

Curcumin–Fe(II) acid media Anodic direction Cathodic direction

512 –a

702 710

a

No peak present.

EPc (I ) (mV vs. SCE)

EPc (II ) (mV vs. SCE)

977 985

303 265

–a −30

1002 1005

343 –a

–a –a

EPa (III) (mV vs. SCE)

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Table 5 Potentials associated with the electrochemical processes of the systems Curcumin–Fe(III) and Curcumine–Fe(II) in basic media EPa (II) (mV vs. SCE)

EPa (III) (mV vs. SCE)

EPc (I ) (mV vs. SCE)

EPc (II ) (mV vs. SCE)

Curcumin–Fe(III) basic media Anodic scan 58 Cathodic scan –a

283 295

–a –a

−113 −115

–a –a

Curcumin–Fe(II) basic media Anodic scan 55 Cathodic scan –a

280 280

–a –a

−115 −120

–a –a

EPa (I) (mV vs. SCE)

a

No peak present.

55 mV versus SCE and the second at EPa (II) = 280 mV versus SCE; when the scan was reversed there was a reduction process at EPc (I ) = −115 mV versus SCE. The voltammogram for the cathodic scan, which only revealed the presence of one reduction peak at EPc (I ) = −120 mV versus SCE; when the scan was inverted, one oxidation process with a shoulder-like shape appeared at EPa (II) = 280 mV versus SCE. The potential of the reduction process at EPc (I ) did not seem to be affected by the direction in which the scan started, as summarised by the values shown in Table 5. The potential values associated with the electrochemical processes taking place during cyclic voltammetry in the systems containing Curcumin–Fe(III) and Curcumin–Fe(II): namely, the oxidation processes located at EPa (I) and EPa (II) have been shifted toward more negative values respect to the values obtained for the Curcumin (see Table 2), which indicates that it is interacting with the ions in solution. The potential of the reduction process at EPc (I ) did not undergo changes with respect to the Curcumin alone (see Table 2), thus supporting the hypothesis that the Curcumin absorbed onto the electrode and that the process did not appear to be affected by the presence of Fe(III) and Fe(II) as it retained the same potential value (see Table 2). Considering the potential values obtain in the voltammetry study (Table 5) it can be inferred that the electrochemical behaviour for both Curcumin–Fe(III) and Curcumin–Fe(II) is the same at basic pH. Thus, this study supports the hypothesis that the Fe(III) reduction by the Curcumin was favoured in the said media. The latter would permit to explain the equivalent spectrophotometric behaviour displayed by the system Curcumin–Fe(III) with that of the Curcumin–Fe(II). In order to assess the formation constants of the complexes involving Curcumin–Fe(III), it is therefore required to provide the data of the UV-Vis spectra obtained in acid media. 3.4. Assessment of the formation constants of the system Curcumin–Fe(III)–H2 O As indicated elsewhere the spectrophotometric data of the system composed by Curcumin–Fe(III) in acid media will be used to suggest different models to be used as input to

Table 6 Model refinement by means of SQUAD for the formation of CurcuminFe(III) complexes in aqueous media Model proposed

(log β ± σ) refined

HCur2−a H2 Cur−a H3 Cura Fe(OH)2+a Fe(OH)2 +a FeCur FeCur(OH)−

10.51a 9.88a 8.38a −3.5b −6.37b log ␤110 = 22.25 ± 0.03 log ␤11-1 = 12.14 ± 0.03 3.91 × 10−3

Sum of the squares (U)

σA

2.53 × 10−3

Fifteen spectra in the 560 and 238 nm range with 7 nm increments were fed into the software. a Fixed constants [14]. b Fixed constants [15].

SQUAD, which should give the best model refined; Table 6 gives the results produced by model refinement through data processing. The acidity constants of the Curcumin remain fixed in the model [14], as well as those of the Fe(OH)2+ and Fe(OH)+ 2 complexes [15]; the constants refined by SQUAD agreed with those reported by Borsari et al. [7], as indicated in Table 7. The equilibria proposed by Borsari et al. [7] did not consider water taking part in them, but the chemical species proposed analogous to those obtained in the present work considering the following: FeH2 Cur(OH)2 = 2H2 O + FeCur FeH2 Cur(OH)3 = 2H2 O + FeCurOH The values obtained using SQUAD clearly agree with those reported by Borsari et al. [7]. Both systems compared were studied starting in acid media and modifying the pH Table 7 Results reported by Borsari et al. [7] Complexes

log β

[FeH2 Cur]2+ [FeH2 Cur(OH)2 ] [FeH2 Cur(OH)3 ]−

29.13 22.06 12.51

M. Bernab´e-Pineda et al. / Spectrochimica Acta Part A 60 (2004) 1105–1113 Table 8 Comparison between the results obtained by SQUAD with those from the work of Borsari et al. [7] Complexes

log β (calculated with SQUAD in aqueous media)

log β (reported by Borsari et al. [7] in methanol–water 1:1)

FeCur FeCur(OH)−

log ␤1 1 0 = 22.25 ± 0.03 log ␤1 1-1 = 12.14 ± 0.03

22.06 12.51

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that the Curcumin is reducing the Fe(III); this implies that a chemical reaction takes place before the formation of a complex. The formation constants of the Curcumin–Fe(III) were refined and correspond to the following species FeCur (log β1 1 0 = 22.25 ± 0.03) and FeCur(OH)− (log β1 1-1 = 12.14 ± 0.03). The latter are consistent with the data reported by Borsari et al. [7], although the comparison refers to two different media, the present in aqueous solutions and their methanol–water media. The formation constants corresponding to the Curcumin–Fe(II) system were calculated for the species FeCur− (log β1 1 0 = 9.20 ± 0.04), FeHCur (log β1 1 1 = 19.76 ± 0.03) and FeH2 Cur+ (log β1 1 2 = 28.11 ± 0.03).

Acknowledgements We want to acknowledge CONACyT for scholarship given to MBP to follow doctoral studies and partial financial support through 31119-E Project.

References

Fig. 9. Molar absorptivity coefficients for the Curcumin–Fe(III) system in aqueous solution.

by NaOH additions, though the system used by Borsari et al. was based on methanol–water 1:1 (Table 8) [7]. The molar absorptivity coefficients given by SQUAD for the Curcumin–Fe(III) system are presented in Fig. 9. Fig. 9 presents the plots of the molar absorptivity coefficients for both Curcumin–Fe(III) complexes as a function of wavelength. It is relevant to add that each data point is plotted with its error bar included, which due to the fact that they were smaller than 10% it becomes difficult to appreciate them: thus, it becomes plain that fitting the model proposed to the experimental data was indeed acceptable. 4. Conclusions The Curcumin is capable of forming complexes with both Fe(III) and Fe(II) ions in acid conditions as evidenced by the spectrophotometric behaviour for the systems: the use of data processing with SQUAD made it possible to assess the species present in the both. When the Curcumin reacts with Fe(III) in basic conditions, its spectrophotometric behaviour is equivalent to that exhibited by the Fe(II), which suggests

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