Oxidation Reactions of 2-Thiouracil: A Theoretical and Pulse

Article pubs.acs.org/JPCA

Oxidation Reactions of 2‑Thiouracil: A Theoretical and Pulse Radiolysis Study K. P. Prasanthkumar,† C. H. Suresh,*,‡ and C. T. Aravindakumar*,§,∥ †

School of Chemical Sciences, Mahatma Gandhi University, Kottayam 686560, India Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum 695019, India § School of Environmental Sciences and ∥Inter University Instrumentation Centre, Mahatma Gandhi University, Kottayam 686560, India ‡

S Supporting Information *

ABSTRACT: The reaction of hydroxyl radical (•OH) with the nucleic acid base analogue 2-thiouracil (1) has been studied by pulse radiolysis experiments and DFT. The generic intermediate radicals feasible for the •OH reactions with 1, namely, one electron oxidation product (1•+), •OH-adducts (3•, 4•, and 5•), and H-abstracted radicals (6• and 7•), were characterized by interpreting their electronic and structural properties along with calculated energetics and UV−vis spectra. Pulse radiolysis experiments showed that the transient formed in the reaction of • OH with 1 in water at pH 6.5 has λmax at 430 nm. A bimolecular rate constant, k2 of 9.6 × 109 M−1 s−1, is determined for this reaction via competition kinetics with 2-propanol. The experiments suggested that the transient species could be a dimer radical cation 2•+, formed by the reaction of 1 with the radical cation 1•+. For this reaction, an equilibrium constant of 4.7 × 103 M−1 was determined. The transient formed in the reaction of 1 with pulse radiolytically produced Br2•− at pH 6.5 as well as Cl2•− at pH 1 has also produced λmax at 430 nm and suggested the formation of 2•+. The calculated UV−vis spectra of the transient species (1•+, 3•, 4•, 5•, 6•, and 7•) showed no resemblance to the experimental spectra, while that of 2•+ (λmax = 420 nm) agreed well with the experimental value and thus confirmed the formation of 2•+. The 420 nm peak was due to σ → σ* electronic excitation centered on a 2-center−3-electron (2c−3e) sulfur− sulfur bond [−S∴S−]. 2•+ is the first reported example of a dimer radical cation in a pyrimidine heterocyclic system. Further, 5-C and 6-C substituted (substituents are −F, −Cl, −NH2, −N(CH3)2, −OCH3, −CF3, −CH3, −CH2CH3, n-propyl, phenyl, and benzyl) and 5,6-disubstituted 2-thiouracil systems have been characterized by DFT and found that the reaction (1 + 1•+ → 2•+) is exergonic (1.12−13.63 kcal/mol) for many of them.

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INTRODUCTION 2-Thiouracil (1), a minor component of natural tRNA is chemically the thio analogue of the RNA base uracil (Scheme 1).1 Derivatives of 1 have been used as melanoma (tumor derived from melanin forming cells) seekers as they bind covalently to the growing melanin polymer through sulfur. Furthermore, derivatives of 1 have been used for early diagnosis as well as chemotherapy of disseminated melanoma.2−6

Compound 1 and its several derivatives, for example, 6methyl-2-thiouracil, 6-n-propyl-2-thiouracil, and 6-phenyl-2thiouracil are effective for the treatment of hyperthyroidism.7 Bhabak and Mugesh have shown that the antioxidant activity of 6-n-propyl-2-thiouracil and 6-methyl-2-thiouracil can be attributed to the inhibition of peroxynitrite-mediated nitration of tyrosine residues in proteins.8 Compound 1 is also reported as a selective inhibitor of neuronal nitric oxide synthase,9 an enzyme that catalyzes the NADPH-dependent formation of nitric oxide from L-arginine and O2. Among the several tautomeric structures possible for 1, the experimental and theoretical studies suggest that the oxo-thione form (Scheme 1) is the most stable tautomer.10−15 The one electron oxidation of thioethers, thioamines, thioacids, methionine, and its derivatives by oxidants such as • OH has been studied exhaustively by pulse radiolysis with

Scheme 1. Structures of Uracil, 2-Thiouracil (1), and Thiourea

Received: April 20, 2012 Revised: October 14, 2012 Published: October 15, 2012 © 2012 American Chemical Society

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dx.doi.org/10.1021/jp303808r | J. Phys. Chem. A 2012, 116, 10712−10720

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optical absorption or conductance detection.16−19 The •OHinduced formation of intra- and intermolecular 2c−3e bond [−S∴X−]+ is well-established in radiation chemistry for X = N, O, S, Cl, Br, or I. The symbol ∴ is used to represent a 2c−3e bond16 to indicate that two electrons are located in a σ bonding orbital and one in a σ* antibonding orbital.20 In aryl sulfur compounds, the •OH induced one electron oxidation gives either a stable monomer radical cation or a neutral radical produced via hydroxylation/proton elimination. The possibility of an aryl sulfur compound reacting with an (aryl sulfur)•+ to yield an (aryl sulfur dimer)•+ with 2c−3e [−S∴S−] interaction is remote because the unpaired electron on (aryl sulfur)•+ is unreactive due to delocalization with the aromatic ring. Further, aromatic ring show high reactivity toward •OH.21−24 Sonntag and co-workers have reported the formation of a [−S∴S−] bonded transient dimer radical cation in the reaction between • OH and thiourea with rate constant k2 = 1.2 × 1010 M−1 s−1 and λmax = 400 nm.25 The [−S∴S−] bonding was confirmed via the reversible combination of a primary radical cation to another thiourea molecule.25 To the best of our knowledge, no work has been published on the reaction of 1 with one electron oxidants, and we expect that, due to the structural similarity between 1 and thiourea around the SC double bond (Scheme 1, colored region), the reactivity of 1 may show some similarity with thiourea. Apart from this reason, 1 is a worthy molecule to study using pulse radiolysis because little is known about its fate in living systems in the presence of a large amount of reactive oxygen species (ROS) such as • OH generated from endogenous cellular metabolism and external ionizing radiations. Therefore, its in vitro study may throw some light into the chemical basis of such interactions. The main aim of the article is to study the one electron oxidation of 1 with •OH. A comparison of transients derived from the reactions of other specific one electron oxidants such as Br2•− and Cl2•− will also be made. Pulse radiolysis technique in conjunction with optical absorption detection will be used to observe the transient intermediates involved in the one electron oxidations. Further, DFT calculations will be used to substantiate the experimental results by structural, electronic, and UV−vis absorptions spectra characterization of various transient species.

The Br2•− and Cl2•− were generated by the pulse radiolysis of 0.01 M aqueous solutions of KBr/KCl, according to the following reactions (X = Br/Cl) X− + •OH → X• + OH−

(3)

X• + X− → X 2•−

(4)

The pulse radiolysis studies were conducted using a 7 MeV linear accelerator at the National Centre for Free Radical Research (NCFRR) Pune, and the details are described elsewhere.27 Dose available per pulse was determined using thiocyanate dosimetry.28 Theoretical Studies. DFT calculations were carried out by means of the BHandHLYP method29 using the 6-311+G(d,p) basis set. The optimizations of the structures at the BHandHLYP/6-311+G(d,p) level in gas phase were followed by frequency calculations at the same level of theory to obtain the thermal correction to free energy (ΔGcorr_gas) and to verify that the reported minimum energy structures have no imaginary frequency, while the transition states have one imaginary frequency. The transition states were located by means of Synchronous Transit-Guided Quasi-Newton (QST3) search method.30,31 Further, the aqueous phase optimizations were done by using the polarizable continuum model (PCM)32 and by incorporating the keywords cav, dis, and rep. The electronic energy in solution is thus the sum of all corrections and also the nonelectrostatic contributions to the solvation free energy (Esolv + Gnes). The free energy values in solution phase were calculated by adding ΔGcorr_gas to Esolv + Gnes.33 The charge and spin density distributions in transient systems were considered by using the Mulliken scheme. The electronic excitations in transient species were predicted by timedependent density functional theory (TD-DFT)34 calculations at the BHandHLYP/6-311+G(d,p) level in solution phase. TDDFT is widely used in radiation research to identify the transient species.24,35−38 Electronic structure calculations were performed by using the Gaussian09 suite of programs.39

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RESULTS AND DISCUSSIONS Pulse Radiolysis Studies. The transient absorption spectrum for the •OH reaction with 1 in water at pH 6.5 is characterized by λmax at 430 nm (Figure 1). Further, the timeresolved spectra has not exhibited any change in λmax except a decrease in intensity at higher time scales. For N2O-saturated solutions, the effective radiation chemical yield of •OH

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EXPERIMENTAL AND THEORETICAL METHODS Materials. 2-Thiouracil (minimum 99%) was purchased from Sigma and used without further purification. Solutions for all pulse radiolysis experiments were prepared in water purified with a Millipore Milli Q system. All other chemicals were of the purest commercially available grade and used as received. The pH adjustments were done by the addition of either HClO4 or NaOH. Pulse Radiolysis Studies. The impact of high energy electrons or γ-radiations on water leads to the formation of radical/ionic/molecular products as described in eq 1. The radiation chemical yields or G values of all species in units of 10−7 M J−1 are shown in parentheses.26 H 2O → H•(0.6) + eaq −(2.8) + •OH(2.8) + H+(3.4) + OH−(0.6) + H 2(0.5) + H 2O2 (0.7)

(1)

Figure 1. Transient absorption spectrum recorded at (■) 5 μs after pulse irradiation of N2O-saturated aqueous solution containing 1 (1 mM) at pH 6.5 (Dose = 13.7 Gy/pulse). Insets: (i) Time vs absorbance of the transient at 430 nm; (ii) competition kinetics of • OH reactions with 2-propanol (0.05 M) and 1 (0.5 to 5 mM).

In N2O-saturated solutions, the hydrated electron (eaq−) gets converted into •OH (eq 2) and doubles the yield of •OH. N2O + eaq − → N2 + •OH + OH−

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(G(•OH)) converting a given substrate (S) into substratederived radicals can be calculated on the basis of eq 5 reported by Schuler and co-workers,40 where k(S) represents the pseudofirst-order rate constant of •OH with S and 4.7 × 108 s−1 is the track recombination frequency. G(•OH) = 5.2 + 3

[k(S)/4.7 × 108]1/2 1 + [k(S)/4.7 × 108]1/2

•

(5)

•

The G( OH) for the reaction of OH with 1 is thus determined as 5.46 × 10−7 M J−1 using a pseudo-first-order rate constant of 4.16 × 106 s−1 at 1 mM concentration of 1. It means that ca. 98% of the •OH produced (total yield is considered as 5.6 × 10−7 M J−1 based on eq 2) during the pulse irradiation is reacting with 1 leading to transients. The k2 value for •OH reaction with 1 was determined by competition kinetics using 2-propanol. Competition kinetics studies were performed at a constant concentration of 2-propanol (0.05 M) containing various concentrations of 2-thiouracil (0.5 to 5 mM). On the basis of the two competing reactions (eqs 6 and 7), the expression given in eq 8 can be derived, where A0 and A are the absorbances noted at 430 nm in the absence and presence of 2-propanol. Using the literature value41 of k(2propanol + •OH) as 1.9 × 109 M−1 s−1, the bimolecular rate constant (k(2-thiouracil + •OH)) obtained from the slope of the plot (inset (ii) in Figure 1) is 9.6 × 109 M−1 s−1. •

OH + (CH3)2 CHOH → (CH3)2 •COH + H 2O

•

OH + 2‐thiouracil → transients

Figure 2. (A0/A) − 1 vs [2-thiouracil]−1 plot based on eq 9. Absorbance values were noted at 430 nm during the pulse radiolysis of aqueous solutions of 2-thiouracil (0.1 to 5 mM) at pH 6.5. Inset: Transient absorption at 430 nm with varying [2-thiouracil].

The inverse of the slope of (A0/A) − 1 vs [2-thiouracil]−1 plot (Figure 2) gave the K value as 4.7 × 103 M−1, which is smaller than the reported25 K values for thiourea (5.5 × 105 M−1), tetramethylthiourea (7.6 × 104 M−1), and selenourea (7.9 × 104 M−1).42 However, the K value of 2-thiouracil is comparable to the reported K value of n-allyl thiourea (4.9 × 103 M−1),43 methionine (1.9 × 103 M−1),44 and its methyl ester (3.1× 103 M−1)45 and selenomethionine (9.2 × 103 M−1).44 Therefore, it can be well assumed that a dimer radical of the type 2•+ is the major intermediate herein. More experimental evidence for 2•+ formations are obtained by studying Br2•− and Cl2•− reactions of 1. The Br2•− reaction with 1 is studied at pH 6.5, and the ensuing spectrum (Figure 3) is characterized with a λmax at 430 nm and is found to be very

(6) (7)

•

A0 k(2‐propanol + OH) [2‐propanol] =1+ × • A k(2‐thiouracil + OH) [2‐thiouracil]

(8)

•

Generally the reaction of OH with pyrimidines produces OH adducts.26 However, the electrophilic attack of •OH with 1 could also be possible with the sulfur atom. It is found that the observed λmax at 430 nm is very close to the value 400 nm reported by Sonntag and co-workers25 for a transient dimer radical cation in the reaction between •OH and thiourea. Hence, the possibility of the reaction given in Scheme 2 is Scheme 2. Dimer Radical Cation Formation from 1•+ and Neutral 1

Figure 3. Transient absorption spectrum after subtracting the contribution of Br2•− in pulse irradiation of an N2O-saturated aqueous solution containing KBr (0.01 M) and 1 mM 2-thiouracil at 5 μs after the pulse (pH 6.5, Dose = 11.8 Gy/pulse). Inset: (i) Transient absorption at 430 nm against [2-thiouracil]; (ii) plot of kobs vs [2thiouracil] measured at 360 nm.

comparable to the •OH reaction spectrum (Figure 1). Furthermore, the absorbance of the transient increases with increasing concentration of 1 (inset (i) in Figure 3) as noted in the •OH reaction. The k2 value is determined from the slope of the kobs vs [2-thiouracil] plot at the absorption maximum of Br2•− (λmax = 360 nm) and is 2.1 × 109 M−1 s−1. The reaction of Cl2•− with 1 is investigated in aerated aqueous solutions at pH 1. The transient spectrum is characterized by λmax at 430 nm (Figure 4). The spectrum is nearly identical in shape to that resulting from •OH and Br2•− (both at pH 6.5) reactions, apart from better G × ε values. The

investigated by studying concentration-dependent transient absorption intensities at 430 nm. This experiment revealed that the transient absorption increases with an increase in the concentration of 1 (Figure 2). The equilibrium constant K for the reversible reaction depicted in Scheme 2 is calculated by means of eq 9 as used by Sonntag and co-workers25 where A0 is the transient absorbance from pulse radiolysis of 2 mM solution of 1, and A is the transient absorbance at a given concentration of 1. A 0 /A = 1 + K −1[2‐thiouracil]−1

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of transient 1•+ by water molecules. Similarly, transients 6 and 7 can be obtained by the deprotonation reactions of 1•+. The dominant reactions of •OH with pyrimidine nucleobases are the addition to the C5C6 double bonds (for example, in cytosine, uracil, and thymine) and, to a lesser extent, the Habstraction from the methyl group of thymine.26 The Habstractions as well as one electron oxidation reaction are not competitive as •OH additions in natural nucleobases. The presence of sulfur in 1 can rationalize our proposition of one electron oxidation in the present case. The H-abstractions from C5 and C6 atoms are also possible; however, the C−H bond energy is comparatively higher than the N−H bond energy. Depicted in Figure 5a is the solution phase optimized geometry of 1 at the BHandHLYP/6-311+G(d,p) level of

Figure 4. Transient absorption spectrum after subtracting the contribution of Cl2•− in pulse irradiation of an aqueous solution containing KCl (0.01 M) and 1 mM 2-thiouracil at 5 μs after pulse at pH 1 (Dose = 19.3 Gy/pulse). Inset: (i) dependence of absorbance of transient with [2-thiouracil] at 430 nm; (ii) plot of kobs vs [2thiouracil] measured at 345 nm.

k2 value is determined at the characteristic absorption maximum of Cl2•− (λmax = 345 nm). The slope of the kobs vs [2-thiouracil] plot (inset (i) in Figure 4) at 345 nm gave a k2 of 3.2 × 109 M−1 s−1. The very similar nature of absorption spectra obtained for • OH, Br2•−, and Cl2•− reactions with 1 suggests the formation of identical transient species. However, if one closely looks at the spectrum resulting from •OH, there is a small shoulder around 340 nm (Figure 1), though not very significant. Theoretical Studies. All the plausible reactions that •OH can induce in 1 are depicted in Scheme 3. The possible intermediate radicals are (i) 1•+ (radical cation) by the one electron oxidation, (ii) 3• (C5_OH• adduct), 4• (C6_OH• adduct), and 5• (S8_OH• adduct) via adduct formations, and (iii) 6• ((N1-H)•, the H7-abstracted radical) and 7• ((N3-H)•, the H9-abstracted radical) by H-abstraction reactions. Transients 3•, 4•, and 5• can also be obtained by the hydroxylation

Figure 5. (a) Optimized geometry of 1 in solution phase; bond lengths are in Å. (b,c) HOMO of 1 and uracil, respectively (isocontour value = 0.05 a.u.).

theory along with its highest occupied molecular orbital (HOMO) (Figure 5b). For a comparison, the HOMO of uracil (Figure 5c) at the same level of theory is also presented. A planar geometry for 1 is located where C5C6 is the shortest bond (1.334 Å) in the ring. The HOMO of 1 is more localized on the SC double bond. A recent theoretical study46 demonstrated that the •OH interaction with C5 atom (main donor for the HOMO) of uracil is a kinetically driven process, which accounts for the transient produced during the pulse radiolysis.47 Accordingly, the principal interaction of •OH

Scheme 3. Possible Reactions That •OH Can Induce in 2-Thiouracil

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Figure 6. Solution phase optimized geometries of transients with selected bond lengths in Å units. Maximum spin density located on each is given in parentheses.

Figure 7. Solution phase optimized geometries of H-abstraction pathways. Bond lengths are in Å. Maximum spin density is given in red color in parentheses. The ΔGrel soln (in kcal/mol) values are presented near the notations in blue.

ionization. The spin density of 1•+ is fully localized over the Satom, and the positive charge is located more on C2 (0.408 a.u.). The C2S8 and C2N3 bonds are elongated by 0.074 and 0.062 Å, respectively, while the C2N1 bond is contracted by 0.039 Å for 1•+ in comparison to 1. The optimized structures of the •OH adducts are also presented in Figure 6 with selected geometrical parameters. The •OH additions are barrierless on C5, C6, and S8 positions of 1, and they occur without intervention of any reactant complexes. The odd spin density in 3• and 4• are localized on adjacent C5 and C6 atoms. Slight puckering of the ring occurs at the addition sites C5 and C6. The spin density of 5• is more dispersed with the maximum values located on the C2 and C5 atoms. The relative free energy in solution, ΔGrel soln (relative with

should be with the S8-atom of the C2S8 double bond (Scheme 1). Hence, positions C5, C6, and S8 can be considered as the nucleophilic centers in 1 for an electrophilic reagent such as • OH. The ionization potential (IP) of 1 (the difference in energy between 1 and its radical cation 1•+) at the BHandHLYP/6311+G(d,p) level in solution phase is 6.39 eV, and this value is slightly higher than the calculated IP value 6.0 eV for thiourea at the same level of theory. Previously reported theoretical IP value of thiourea was 6.12 eV computed at the BHandHLYP/631+G(d,p) level in an aqueous medium.42 The solution phase optimized geometry of 1•+ is depicted in Figure 6. The planarity of 1 is retained during the formation of 1•+; hence, its formation can be considered as a result of rapid vertical 10716



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respect to the sum of •OH and 1), for 3•, 4•, and 5• are, respectively, −6.85, −10.53, and 12.01 kcal/mol. Hence, thermodynamically, the formation and independent existence of 3• and 4• are likely, while 5• is very unlikely. The H-abstraction from N1/N3 atom results in the formation of H-abstracted radicals 6•/7• and a water molecule. The structures of 6• and 7• are depicted in Figure 6. Both showed the unpaired spin density fully localized on the S8 atom. This spin localization on the exocyclic S-atom can be attributed to the aromatization of the ring moiety, which avoids the electron on the sulfur atom for delocalization. The 6• is more stable than 7• by ΔGsoln of 1.29 kcal/mol. The H-abstraction reactions can take place from adducts 1···•OH(1) (for H7 abstraction) and 1···•OH(2) (for H9 abstraction). The product complexes 6•···H2O and 7•···H2O are formed from the reactant complexes 1···•OH(1) and 1···•OH(2) via the transition states TS1 and TS2 (eqs 10 and 11). The optimized structures in the H-abstraction pathways are depicted in Figure 7 along with their relative free energy values. 1···•OH(1) → TS1 → 6•···H 2O

(10)

1···•OH(2) → TS2 → 7•···H 2O

(11)

Table 1. λmax Values and Oscillator Strengths of Transients transient

λmax (nm)

f

1•+ 3• 4• 5• 6• 7•

228 337 227 340 242 245

0.234 0.252 0.352 0.183 0.013 0.318

Figure 9. TD-DFT calculated absorption spectrum of 2•+ on account of σ to σ* electron excitation.

The free energy of activation for the H7 abstraction is 2.53 kcal/mol, while for the H9 abstraction is 3.79 kcal/mol. Hence, the H7 abstraction is thermodynamically more favorable than H9 abstraction. The highly localized spin density on the S atom in 1•+ suggests strong possibility for the formation of 2•+ (Scheme 2). The optimized geometry of 2•+ is depicted in Figure 8. The S−

Hence, we can confirm that the transient species observed in the reaction of •OH with 1 is the dimer-based radical cation 2•+. The λmax at 420 nm is originated from the characteristic σ to σ* electronic transition of the 2c−3e [−S∴S−] bond. Further validation of the theoretical results for 2•+ has been obtained by theoretical studies on dimer radical cations of thiourea, tetramethylthiourea, and selenourea. The one electron oxidation of these compounds by earlier pulse radiolysis studies have established the formation of dimer radical cations.25,42 The computed Mulliken spin densities, X−X bond distances, ΔGsoln, and λmax for these systems and 2-thiouracil are compiled in Table 2. Among the sulfur based dimer radical cations, the S−S bond distance is maximum for thiourea dimer and minimum for 2•+. The unpaired spin is evenly distributed over the interacting S or Se atoms of all dimer radical cations. The λmax are in good agreement with the experimental values. The ΔGsoln values suggest that dimer formation is exergonic for all the systems

Figure 8. Optimized geometry of 2•+; bond lengths are in Å units. Spin densities are given in parentheses.

Table 2. Mulliken Spin Densities, X−X Bond Distances, ΔGsoln, λmax, and Oscillator Strength ( f) of Dimer Radical Cations of Thiourea, Tetramethylthiourea, Selenourea, and 2-Thiouracil

S bond length of 2•+ is 2.797 Å long, and other bond lengths are equal for the 2-thiouracil units, suggesting a symmetrical interaction of the two entities. The unpaired electron spin density is evenly distributed over both the S atoms. The dimer formation is barrierless as well as exergonic by 2.36 kcal/mol. TD-DFT Calculations. The λmax values and the corresponding oscillator strengths ( f) for the monomer-based transients, viz. 1•+, 3•, 4•, 5•, 6•, and 7•, calculated using the TD-DFT BHandHLYP/6-311+G(d,p) level calculations in solution phase, are compiled in Table 1. The λmax values of 1•+, 4•, 6•, and 7• fall in the middle of the UV region, while those for 3• and 5• lie close to the visible region. All these λmax values are very different from the observed value 430 nm of the transient. Hence, it is obvious that none of the transients (considered in Scheme 3) seems to be responsible for the experimentally observed transient absorption spectrum. The TD-DFT calculated UV−vis spectrum of 2•+ is shown in Figure 9 which, shows that 2•+ absorbs at 420 nm and that the value is in good agreement with the pulse radiolysis experiment.

dimer radical cation thioureaa spin density (a.u.) on X X−X bond length (Å) ΔGsoln (kcal/mol) λmax (nm) f exptl λmax (nm)

0.502 (0.470d) 1.071c 2.806 (2.808d) −4.20 415 0.396 400e

tetramethylthioureaa 0.547

selenoureab 2-thiouracila

2.82

0.520 (0.472d) 1.030c 2.999 (2.975d) −6.84

−2.36

467 0.339 450e

415 (415d) 0.386 400d

420 0.404 430

1.121 2.802

c

0.613 1.163c 2.797

a

X = S. bX = Se. cSpin density on X of monomer radical cation. dFrom ref 42. eFrom ref 25.

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Table 3. Mulliken Spin Densities, S−S Distance, ΔGsoln, λmax, and Oscillator Strength ( f) of 33 Substituted Dimer Radical Cations spin density on S of substituted

substituent position 5

6

1•+

2•+

S−S bond length (Å)

ΔGsoln (kcal/mol)

λmax (nm)

f

NH2 F OCH3 Cl N(CH3)2 CF3 CH3 CH2CH3 CH2CH2CH3 C6H5 CH2C6H5 H H H H H H H H H H H NH2 F OCH3 Cl N(CH3)2 CF3 CH3 CH2CH2CH3 CF3 CH3 CH2CH3

H H H H H H H H H H H NH2 F OCH3 Cl N(CH3)2 CF3 CH3 CH2CH3 CH2CH2CH3 C6H5 CH2C6H5 NH2 F OCH3 Cl N(CH3)2 CF3 CH3 CH2CH2CH3 N(CH3)2 N(CH3)2 N(CH3)2

0.258 1.161 0.334 0.754 1.164 1.139 0.751 0.752 0.749 0.215 0.753 1.181 1.172 1.176 1.167 1.168 1.199 1.159 1.147 1.143 1.165 1.169 0.097 1.171 0.141 0.746 0.053 1.176 1.172 0.680 1.188 0.055 1.203

0.607 0.610 0.610 0.606 0.630 0.634 0.635 0.640 0.650 0.638 0.659 0.631 0.622 0.629 0.605 0.646 0.613 0.627 0.622 0.630 0.624 0.640 0.622 0.619 0.623 0.602 0.643 0.624 0.628 0.667 0.646 0.630 0.644

2.794 2.796 2.797 2.801 2.798 2.800 2.797 2.797 2.800 2.798 2.801 2.795 2.799 2.795 2.792 2.795 2.791 2.797 2.793 2.798 2.797 2.796 2.794 2.791 2.793 2.795 2.796 2.793 2.795 2.796 2.796 2.792 2.791

12.80 −3.17 3.84 −2.55 10.71 −1.98 −3.77 −4.10 −4.02 −13.56 −2.98 −3.13 −2.35 −2.70 −2.18 −4.20 −2.03 −5.38 −3.93 −3.72 −8.60 −1.75 23.37 −3.69 8.68 −2.82 27.38 −1.12 −3.51 −3.68 −1.74 −3.39 −13.63

420 420 422 425 426 428 420 421 423 424 424 420 422 419 419 420 420 420 420 421 423 420 421 419 419 422 426 423 419 420 421 419 420

0.434 0.403 0.423 0.437 0.413 0.438 0.415 0.426 0.423 0.496 0.401 0.417 0.413 0.430 0.443 0.452 0.426 0.424 0.438 0.444 0.522 0.476 0.425 0.418 0.443 0.465 0.450 0.461 0.432 0.460 0.471 0.460 0.454

mations of all the 6-C substituted dimer radical cations are exergonic. In general, compared to 5-C substituted systems, the 6-C substituted systems show higher spin density on S both in 1•+ and 2•+. However, in 6-C substituted CF3, n-alkyl, phenyl, and benzyl substituted 2•+, the spin density is slightly lower compared to 5-C substituted counterparts. For 5,6-double substitution, eight symmetrically substituted (NH2, F, OCH3, Cl, N(CH3)2, CF3, CH3, and CH2CH2CH3) and three mixed combinations (5-CF3 and 6-N(CH3)2; 5-CH3 and 6-N(CH3)2; and 5-CH2CH3 and 6-N(CH3)2) are studied. Symmetrical double substitution with NH2 and N(CH3)2 gives highly unstable dimer radical cations. Among the 33 substituted 2thiouracils, the most stable dimer with mono substitution is found for 5-C6H5, while the most stable dimer with double substitution is obtained for the mixed combination 5-CH2CH3 and 6-N(CH3)2.

except tetramethylthiourea. Experimentally, it has been suggested that dimer formation is less probable in tetramethylthiourea than thiourea on the basis of K values.25 Several substituted 2-thiouracils have been reported to be clinically useful systems as antioxidants and anticancer agents.8,48−53 Since our theoretical data provide a fairly accurate description of the transient produced due to reaction with ROS, it is felt that theoretical characterization of dimer radical cations of substituted 2-thiouracils will be useful for future experiments. Compiled in Table 3 are the spin density on S, S−S bond length, ΔGsoln, λmax, and corresponding oscillator strength for 33 dimer radical cations of substituted 2-thiouracils, and among them, 11 are 5-C substituted, 11 are 6-C substituted, and the rest are substituted at both 5- and 6-C positions. The selected substituents are −F, −Cl, −NH2, −N(CH3)2, −OCH3, −CF3, −CH3, −CH2CH3, n-propyl, phenyl, and benzyl. The S−S bond length and spin density on S of all substituted 2•+ show consistent values. Their λmax is also very close to that of 2•+. With electron donating substitutions at the 5-positon (NH2, N(CH3)2, and OCH3) and electron donating substitutions at the 5- and 6-positons (NH2, N(CH3)2, and OCH3), the dimer radical cation formations are unlikely since the reactions are endergonic (2.65−27.38 kcal/mol). For-

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CONCLUSIONS The present study has established that the reaction of •OH with 1 is quite distinct from that of the natural base uracil. Identical transient absorption spectra are observed in the reactions of •OH, Br2•−, and Cl2•− with 1 having λmax at 430 nm. Theoretical calculations described that, due to the presence 10718



Article

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of sulfur, one electron oxidation is very conducive for 1 compared to uracil. Therefore, major reaction of •OH with 1 is conceived as a one electron oxidation rather than •OH-adduct formation as in uracil. The transient observed is a dimer radical cation 2•+, characterized by a [−S∴S−]+ bond. The 2•+ formation occurs via the combination of an initially formed radical cation 1•+ with 1. Computational calculations at the BHandHLYP/6-311+G(d,p) level of theory proved that the excitation of an electron from the σ-bonding molecular orbital to the unoccupied σ* antibonding molecular orbital is responsible for the observed experimental spectrum with λmax = 430 nm. Using 2-thiourcil as paradigmatic, the dimer radical cation formation in a number of 2-thiouracils has been verified. The antioxidant properties of many of these molecules are not known. In this context, there is scope for further investigation of such molecules for the synthesis and applications in developing new antioxidants/anticancer agents. This work also demonstrated the efficiency of quantum chemical calculations in the scrutiny of various possibilities beyond the experimental data.

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ASSOCIATED CONTENT

S Supporting Information *

Thermodynamic parameters of all the systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(C.H.S.) Tel: +91-471-2515264. Fax: +91-471-2491712. Email: [email protected]. (C.T.A.) Tel: +91-481-2732120. Fax: +91-481-2731009. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank NCFRR, Pune, for pulse radiolysis experiments. C.T.A. is thankful to BRNS, Mumbai, and DST, New Delhi (Purse programme), for financial support. We thank CSIR Centre for Mathematical Modeling and Computer Simulation (C-MMACS) and National Chemical Laboratory (NCL) for providing computational facilities.

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Oxidation Reactions of 2-Thiouracil: A Theoretical and Pulse

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