Pulse and γ-radiolysis studies on aqueous solution of 1,1′-dimethyl-2-selenourea

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Radiation Physics and Chemistry 77 (2008) 125–130 www.elsevier.com/locate/radphyschem

Pulse and g-radiolysis studies on aqueous solution of 1,10 -dimethyl-2-selenourea Beena Mishraa, B. Santhosh Kumarb, K.I. Priyadarsinia, a

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b Department of Physiology, Deccan College of Medical Sciences, Hyderabad 500058, India Received 9 March 2007; accepted 27 March 2007

Abstract One-electron oxidation of 1,10 -dimethyl-2-selenourea (DMSeU) by hydroxyl radicals, one-electron-specific oxidants, was studied using pulse radiolysis technique in aqueous solution. Hydroxyl (dOH) radicals and one-electron oxidants, Nd3 , Xd 2 (X ¼ Cl, Br, and I) react with DMSeU to form a transient having an absorption spectrum with lmax at 430 nm. By following the absorbance at 430 nm as a function of solute concentration and in analogy with similar sulfur and selenium compounds, this transient is assigned to dimer radical cation. The dimer radical cations of DMSeU react with oxygen with bimolecular rate constant of 1.070.3  108 M1 s1. Steady-state g-radiolysis studies on aqueous solution of DMSeU under hydroxyl radical-induced oxidation condition indicated formation of elemental selenium as one of the by-products, which has been stabilized by the addition of poly vinyl alcohol (PVA), and characterized by dynamic light scattering technique. r 2007 Elsevier Ltd. All rights reserved. Keywords: Dimethyl selenourea; Pulse radiolysis; Dimer radical cation; Oxidation; Nanoselenium

1. Introduction Of late interest in selenium chemistry has increased significantly due to its application in medicine, nutrition (Brenneisen et al., 2005; Bock, 1994), development of novel materials (Nicolaou and Petasis, 1984), nanoparticles (Nano Red Se, www.nanoport.net; Yang-Wei et al., 2005, 2006), etc. In biology its importance is growing with the identification of 34 proteins containing selenium at their active site and also its role in cellular redox balance (Sagher et al., 2006), immune response, cancer prevention, (Fleming et al., 2001), anti-inflammatory action (Shilo et al., 2005) and radioprotection (Weiss et al., 1992). A few selenourea derivatives such as dimethyl selenourea (DMSeU), aminoethylisoselenourea, aminopropylisoselenourea, aminopropylisoselenourea, and 2-aminoselenazaline have also been reported as potent inhibitors of enzymes such as nitric oxide synthase (Southan et al., 1996), prostaglandin D synthase (Islam et al., 1991) and Corresponding author.

E-mail address: [email protected] (K.I. Priyadarsini). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.03.005

tyrosinase (Ha et al., 2005). Takahashia et al. (2005) have reported selenourea derivatives to be effective scavengers of superoxide radicals and among them, DMSeU was found to be the most effective. Compounds like selenoformamide and selenourea are important building blocks for the synthesis of biologically important selenium compounds (Renaud, 2000). Many cyclic selenoureas have also found application in carbohydrate research (Dianez et al., 1993). Recently, selenourea and its derivatives are used as precursors for the synthesis of selenium nanoparticles (Mishra et al., 2005). Dimethyl selenourea has been used to synthesize water-soluble CdSe nanoparticles (Yang-Wei et al., 2005, 2006). There are several reports on radiation chemistry of sulfur compounds (e.g. Asmus and Bonifacˇicˇ, 1999; KorzeniowskaSobczuk et al., 2002; Maity, 2002; Mohan and Mittal, 2002; Wang et al., 1999), but such studies on compounds derived from selenium, which belongs to the same group in periodic table as sulfur, are very rare. In order to apply such compounds for suitable applications, it is important to study the nature and the redox reactions involving such selenium compounds. Recently, we have reported the

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oxidation chemistry of organoselenium compounds such as selenourea, selenomethionine, methylselenocysteine, etc. (Mishra et al., 2004, 2005, 2006). In continuation of our studies on selenium oxidation chemistry, in this manuscript, the results on the one-electron oxidation of DMSeU have been reported by using pulse radiolysis technique. The formation of stabilized nanoselenium from such oxidation reaction has also been studied. The chemical structure of DMSeU is given here. CH3 H 3C

40 Gy min1 was used. The dosimetry was performed by standard Fricke dosimeter (Fielden, 1982). DLS measurements were carried out on Malvern 4800 Autosizer employing 7132 digital correlator, equipped with an Ar-ion laser and BI-90 particle sizer equipped with He–Ne laser. The intensity correlation function of scattered light is analyzed by the method of cumulants using the mean and variance of the distribution as the fitted parameter (Mishra et al., 2005). 3. Results and discussion

N Se

3.1. Pulse radiolysis studies

H 2N

DMSeU was obtained from Aldrich Chemicals, USA. All the other chemicals and reagents were of ‘Analar’ grade and used as received. Solutions were prepared in ‘nanopure’ water with conductivity of 0.1 mS cm1, obtained from a nanopure water system and freshly prepared solutions were used for each experiment. The ground-state absorption spectra were recorded on a Hitachi spectrometer, model 330. The pH of the solutions was adjusted using HClO4, Na2HPO4  2H2O, KH2PO4 and NaOH. 2.2. Pulse radiolysis studies Pulse radiolysis experiments were carried out with highenergy electron pulses (7 MeV, 500 ns) obtained from a linear electron accelerator and the details are given elsewhere (Guha et al., 1987). Aerated aqueous solution of KSCN (1  102 M) was used for determining the dose delivered per pulse using Ge475 ¼ 2.59  104 m2 J1 for the transient (SCN)d species (Buxton and Stuart., 1995; 2 Fielden, 1982). G denotes the radiation chemical yield in mol J1 and e denotes the molar extinction coefficient in m2 mol1. The dose per pulse was close to 10 to 18 Gy (1 Gy ¼ 1 J kg1). The transient species formed on pulse radiolysis were detected by optical absorption method. Radiolysis of N2-saturated neutral aqueous solution leads to the formation of three highly reactive species (dH, dOH, e aq) in addition to the less reactive or inert molecular products (H2, H2O2, and H3O+). The reaction with dOH radicals was carried out in N2O-saturated d solutions, where e aq is quantitatively converted to OH d d radicals. Specific one-electron oxidants, N3 , Cl2 , Brd 2 and Id were generated by the radiolysis of N2O-purged 2 aqueous solutions containing 0.1 M of corresponding sodium salts according to the procedure reported in references (Mishra et al., 2004; Neta et al., 1988). For steady-state g-radiolysis, 60Co g-source with a dose rate of

27

0.06

1/ΔA

2.1. Materials

b

c

24

a 21

ΔAbsorbance

2. Experimental

The optical absorption spectrum of parent DMSeU showed absorption bands at 205 nm (e ¼ 1.3  103 M1 cm1) and 250 nm (e ¼ 1.1  103 M1 cm1) and without any appreciable absorption at l4300 nm. Therefore, pulse radiolysis studies in the wavelength region of 300–600 nm, with optical absorption detection technique could be employed without any correction for the ground-state absorption. The nature of the spectrum also remained the same in the pH range 1–11 and was found to be stable and no pKa was observed in this pH region. At pH411, the solution is stable up to 30 min but showed decomposition on prolonged storage. The pulse radiolysis studies are restricted in the pH range 1–11. Fig. 1a shows transient optical absorption spectrum (lmax ¼ 430 nm) obtained on pulse radiolysis of N2Osaturated aqueous solution of DMSeU (1  104 M, pH ¼ 7). In the presence of t-butyl alcohol (0.3 M), an efficient dOH radical and weak H atom scavenger, small absorption (DO.D. ¼ 0.004 at 430 nm) was seen in 300–600 nm region. These results suggest that the contribution of Hd atom reaction with DMSeU (pH 7) is small and the absorption spectrum (Fig. 1a) is mainly due

0.04

0

3

6

9

1/[DMeSeU], mM-1

0.02

0.00 300

400 Wavelength, nm

500

600

Fig. 1. Transient absorption spectra generated by reaction of dOH radical with DMSeU (a ¼ 1  104 M, 8 ms after the pulse and b ¼ 1  103 M, 2 ms after the pulse) at pH 7. Inset (c) shows double reciprocal plot showing the variation of absorbance at 430 nm as a function of DMSeU concentration in accordance to Eq. (1) (dose: 18 Gy pulse1 500 ns).

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0.04

c log (k/k0)

0.05

Δ Absorbance

0.2

0.06

Δ Absorbance

to the reaction of dOH radicals with DMSeU. The rate constant for the reaction of dOH radicals with DMSeU was determined by competition kinetic studies using 2-propanol (IP) as the standard solute. The transient absorbance at 430 nm was determined for various solutions containing different concentration of 2-propanol (2.5  103–1  102 M) and fixed concentration of DMSeU (5  104 M). Using the rate constant for isopropanol with dOH as 1.9  109 M1 s1, the bimolecular rate constant for the reaction of dOH with DMSeU was determined to be 9.9  109 M1 s1. From this, it can be noticed that under the experimental condition of Fig. 1a, 95% of dOH radicals should have reacted with DMSeU to form the transient absorption band at 430 nm, however, the transient absorbance at 430 nm increased with increasing solute concentration (41  104 M) reaching a saturation value only at DMSeU concentration of 1  103 M. This increase in the absorbance could not be due to the direct reaction of dOH radicals with higher concentration of DMSeU, but due to the reaction of the initial transient species formed by dOH radical reaction with DMSeU with the solute, to form a new transient species, probably a dimer radical species as observed with selenourea, thiourea and its derivatives (Mishra et al., 2004; Wang et al., 1999). The absorption spectrum obtained by the reaction of dOH radicals with 1 mM DMSeU is given in Fig. 1b, which shows the same lmax but higher absorbance. Since dOH radicals are known to react with organic compounds by more than one mechanism (addition, abstraction, electron transfer), the reaction with specific one-electron oxidants like Nd3 , Cld and Id was carried 2 2 d out to establish the nature of OH radical reaction with DMSeU. Pulse radiolysis of N2O-saturated aqueous 4 solution (pH 7) of N M 3 (0.1 M) containing 1  10 DMSeU showed the formation of a transient band at 430 nm (Fig. 2a) attributed to one-electron-oxidized species of DMSeU. As observed with dOH radical reaction, the absorbance at 430 nm increased with increasing DMSeU concentration. Radicals like Cld (pH 1) and Id (pH 7) 2 2 react with DMSeU with bimolecular rate constants of 3.6  109 and 2.8  108 M1 s1, respectively, determined d by following the decay of Cld 2 and I2 at 345 and 380 nm, respectively as a function of DMSeU concentration. In all these reactions, formation of similar transient species, with absorption maximum at 430 nm, which increased with increasing DMSeU concentration up to 1 mM, was observed. The transient absorption spectrum at pH 1 produced by one-electron oxidation of DMSeU by Cld 2 is given in Fig. 2b. It can be seen that Fig. 2a and b match well with that obtained by dOH radical reaction (Fig. 1b). The reactivity of DMSeU with Id indicates that the 2 reduction potential for the oxidation of DMSeU is less than 1.0 V as the one-electron reduction potential of  Id 2 /2I couple is 1.04 V vs NHE (Wardman, 1989). In DMSeU, since selenium has high electron density and low ionization potential, the radical cations produced by

127

0.1

0.0

0.02

0.01

0.00 0.0

0.1

0.2

0.3

300

0.4

400

500

Wavelength, nm

μ0.5/1+(μ)0.5

0.03

b

a

0.02 0.01 0.00 300

400

500 Wavelength, nm

600

Fig. 2. (a) Transient absorption spectrum generated on pulse radiolysis of N2O-saturated aqueous solution containing 0.1 M NaN3 and 1  104 M DMSeU at pH 7. Inset (b) shows transient absorption spectrum generated on pulse radiolysis of aerated aqueous solution containing 0.1 M KCl and 1  104 M DMSeU at pH 1. Inset (c) shows the effect of ionic strength on the observed decay rate of the dimer radical cation at 430 nm according to Eq. (2).

the reaction of dOH radicals and one-electron oxidants are mostly centered on the selenium atom. These seleniumcentered radical cations would be highly unstable and would have acquired stability by coordinating with other selenium atom of another DMSeU molecule, forming twocentered three-electron bond between the two selenium atoms, as observed in many sulfur and selenium compounds (Asmus and Bonifacˇicˇ, 1999; KorzeniowskaSobczuk et al., 2002; Mishra et al., 2004, 2006; Mohan and Mittal, 2002; Wang et al., 1999). Since no new transient absorptions were noticed in all these reactions, the transient produced by the reaction of all these radicals with DMSeU has been assigned to the dimer radical cation, the structure of which is given in Scheme 1. Other transient species like OH adduct, radical species formed by H-abstraction or solute monomer radical cation could not be observed under the present experimental conditions. Earlier reports on thiourea and selenourea showed similar reactions and explained as due to the existence of equilibrium between the monomer and its dimer radical cation (Mishra et al., 2004; Wang et al., 1999). In analogy with that, the increase in the absorbance of the transient with DMSeU concentration is also attributed to the existence of equilibrium as shown in Scheme 1. The equilibrium constant (K) for the equilibrium process (Scheme 1) is determined by 1 1 1 ¼ , þ DA DAo DAo K½DMSeU

(1)

where DA and DAo are the respective absorbances at 430 nm at any given concentration of DMSeU (6  106–1  103 M) and saturation absorbance of DMSeU (1  103 M) at pH 7. The double-reciprocal plot of absorbance at 430 nm against

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128

H3C N Se H2N

(a)

CH3

CH3

CH3 HO

DMSeU

H3C N

H3C N Se

+

Se

H2N

K

H2N

+ ... Se

N CH3 H3C

(c)

(b)

NH2

CH3 H3C N

.+

Se H2N

(d) Scheme 1.

Here, k and ko are the observed second-order rate constants (2k/el) in presence and absence of added NaClO4 (0.02–0.5 M). The decay rate was found to increase linearly with increasing ionic strength (Fig. 2c). The slope of the plot is positive, suggesting that the electric charge z1 and z2 of the two reaction partners is the same and thus support the reaction mechanism as shown in Scheme 1. In the absence of oxygen, the transient at 430 nm decayed (decay trace given in Fig. 3a) by second-order kinetics with 2k/el value of 4.770.1  105 s1. Normally, the radical cations derived from sulfur do not show any reactivity with oxygen. However, a few exceptions are reported in the literature, for example, the dimer radical cation of thiourea and selenourea are reported to react with oxygen with a bimolecular rate constant of 1.2  107 and 8.6  107 M1 s1. Similarly in the case of DMSeU, the decay of the transient at 430 nm was observed to be faster

c 1.2

5

kobs/10 , s

-1

1.5

0.06 Δ Absorbance

DMSeU concentration gave a straight line (Fig. 1c). From the ratio of intercept and slope, the equilibrium constant (K) is evaluated to be 3.3  104 M1. The equilibrium constant for the formation of dimer radical cation on reaction with d OH radicals remained nearly the same in the pH range of 1–10. The K values for thiourea and selenourea were earlier reported to be 5.5  104 and 7.9  104 M1, respectively (Mishra et al., 2004; Wang et al., 1999). From this, it appears that selenourea showed highest equilibrium constant value for dimerization. The lower value with DMSeU can be attributed due to the presence of the two methyl groups, which may cause steric hindrance for the approach of the second DMSeU molecule thereby making the dimer radical cation formation less feasible. To confirm its radical cation nature, we studied the kinetic salt effect on the decay kinetics. The radical cations of DMSeU, decay by second-order kinetics and the effect of the ionic strength (m) on the decay rate is followed by addition of added electrolyte (NaClO4) (Laidler, 1996). The following equation is applied to understand the nature of charge on reaction partners. pffiffiffi m k (2) log ¼ 1:02z1 z2 pffiffiffi . 1þ m ko

a

0.9 0.6 0.3 0.0

0.3

0.6

0.9

1.2

[Oxygen], mM

0.03 b

0.00 0

20

40 60 Time, μs

80

100

Fig. 3. Absorption–time plot showing the decay of the transient at 430 nm formed on reaction of DMSeU (5  104 M) with dOH radical at pH 7 in the absence (a) and presence of oxygen (b). Inset shows linear plot for the variation of the observed decay rate constant at 430 nm as a function of oxygen concentration.

in the presence of oxygen (decay trace given in Fig. 3b) and showed pseudo-first-order decay. This decay rate constant (kobs) increased linearly with increasing oxygen concentration (Fig. 3c). The slope of the linear plot gave a bimolecular rate constant value of 1.0  108 M1 s1 for the reaction of DMSeU dimer radical cations with oxygen. This reactivity of the dimer radical cation with oxygen is explained on the basis of formation of a resonating structure having a carbon-centered radical, which would react with oxygen to form a peroxyl type radical (Mishra et al., 2004; Wang et al., 1999). Scheme 2 shows the possible resonating structures of the dimer radical cation of DMSeU. The presence of the structure (f), which is a carbon-centered radical, may be responsible for the reactivity of the dimer radical cation with oxygen. Also, the presence of two methyl groups on DMSeU induces +I inductive effect, due to which the electron density on carbon increases and therefore DMSeU can show more reactivity to oxygen as compared to selenourea. In addition

ARTICLE IN PRESS B. Mishra et al. / Radiation Physics and Chemistry 77 (2008) 125–130

CH3

CH3

CH3 H3C N Se

+ .

. . Se

H2N

NH2 N CH3

H3C N

+

Se

-.

+

H3C

NH2 Se

.

N Se

+

N CH3

NH2

Se

NH2

C

N

NH2

H3C

H3C

(c)

129

CH3

H3C

(e)

(f)

Scheme 2.

4. Steady-state c-radiolysis When N2O-saturated aqueous solutions of DMSeU (1  104 M) were irradiated with g radiation, with a total absorbed dose of 2.4 kGy, the colorless solution of DMSeU, turned red, which on standing for few hours turned into a precipitate and this red precipitate after 2 days was converted to gray powder. This precipitation was confirmed to be due to formation of elemental colloidal selenium. The same product was observed on reaction of DMSeU with other oxidants. When 0.1% polyvinyl alcohol (PVA) was added immediately after irradiation, the solution remained stable for several days, without any change in color due to stabilisation of initially formed nanoparticles by PVA. Fig. 4 gives absorption spectra of N2O-saturated aqueous solutions of different concentrations (1  104–2  103 M) of DMSeU before (Fig. 4a) and after irradiation (Fig. 4b–f). The absorption spectrum due to DMSeU at 250 nm, disappeared with the formation of a broad feature less spectra from 200 to 600 nm. The size of the stabilized colloidal selenium was estimated by using dynamic light scattering technique. Inset in Fig. 4 shows a representative plot of normalized intensity correlation function [g2(t)1] as a function of time for stabilized selenium particles obtained after irradiation of 5  104 M DMSeU stabilized with PVA. The solid line in the inset in Fig. 4 shows the fit to the measured data using cumulants, and the corresponding average hydrodynamic diameter was estimated to be 19275 nm, with polydispersivity index (P.I.) of 0.1. The size of the Se nanoparticle formed by oxidation of DMSeU was found to depend on the initial concentration of DMSeU. Plots b, c, d, e, and f in Fig. 4 show the absorption spectra of the Se nanoparticles with average size of 116, 165, 192, 216, and 228 nm, respectively. It can be seen that the spectrum becomes sharper and red shifted with increasing size of the Se nanoparticles in these respective solutions. The red shift in the absorption spectral features of Se nanoparticles as a function of size can be explained due to confinement effect, which is observed earlier in semiconductor nanocrystals and Se nanoparticles in particular (Burda et al., 2005; Lin and Chris Wang, 2005).

Absorbance

2.4

0.9

2.0

0.6

g2(τ-1)

to this, some monomer radical cation structures (structure (d) in Scheme 1) may also contribute towards its oxygen reactivity.

0.3

1.6

0.0

1.2

10

a

f

0.8 c

0.4 0.0 200

100 Time, μs

1000

e d

b

300

400

500 600 Wavelength, nm

700

800

900

Fig. 4. Absorption spectra of (a) 1  104 M DMSeU and Se nanoparticle formed by reaction of (b) 1  104 M, (c) 2.5  104 M and (d) 5  104 M (e) 1  103 M (f) 2  103 M DMSeU with dOH radicals produced by g-irradiation at an absorbed dose of 2.4 kGy, with the addition of 0.1% PVA as stabilizer. Inset shows variation of normalized intensity correlation function (g2(t)1) with time for Se nanoparticles prepared by dOH radical reaction with 5  104 M DMSeU. The solid line represents fit by the method of cumulants.

5. Conclusions One-electron oxidation reaction of DMSeU in aqueous solution has been studied. Hydroxyl radicals and specific one-electron oxidants react with DMSeU to produce selenium-centered radical cations, which are converted to the dimer radical cations at high concentration of DMSeU. The broad transient optical absorption band with lmax at 430 nm observed in all these oxidation reactions has been attributed to the dimer radical cation. Further confirmation of its cationic nature of the transient was done by following the kinetic salt effect, on the decay kinetics. The dimer radical cation reacts with molecular oxygen with a rate constant that is about one order of magnitude higher compared to selenourea. In all these oxidation reactions of DMSeU elemental selenium was produced as one of the products, whose size could be stabilized to nanometers using 0.1% PVA. The studies thereby provide mechanistic details for the controlling of oxidation of DMSeU as a method for generation of selenium nanoparticles, which find applications in material science and biology.

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