Role of the Sulfur Atom on the Reactivity of Methionine toward OH Radicals:Comparison with Norleucine

Misaela Francisco-Marquez†,‡ and Annia Galano*,†

Departamento de Quımica, UniVersidad Autonoma MetropolitanasIztapalapa, San Rafael Atlixco 186,Col. Vicentina, C. P. 09340 Iztapalapa, Mexico D. F., Mexico, and Laboratorio de QuımicaComputacional, FES-Zaragoza, UniVersidad Nacional Autonoma de Mexico (UNAM),C. P. 09230 Iztapalapa, Mexico, D. F., Mexico

ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: February 11, 2009

Density functional theory has been used to model the OH reaction with Gly-Met-Gly and Gly-Nle-Glytripeptides. The first one is predicted to be about 100 times faster than the second one. Therefore, if a methioninefragment is replaced by norleucine, the overall reactivity of the peptide toward free radicals is expected to besignificantly reduced, which is in agreement with previous experimental findings. Since the most reactivesites were found to be located in the central backbone for Nle and in the terminal fragment of the side chainfor Met, this decrease is expected to be even more critical for large-sized free radicals. The S atom seems toactivate not only those alkyl sites next to it but also those located an odd number of bonds apart. In additionthe viability of different paths explaining the formation of methionine sulfoxide has been tested, and it isproposed that this process involves the formation of R-SO radical and formaldehyde. The results from thepresent work offer an explanation to the role of sulfur atom on the reactivity of methionine toward freeradicals. They also support the preponderant role of Met35 on the development of the Alzheimer disease.

Introduction

Reactive oxygen species (ROS) are formed in vast quanti-ties in each cell, as a result of normal oxygen metabolism.The most reactive ROS is the OH radical, and it can beformedintracellularlybyaFenton-typereaction,byHaber-Weissrecombination, via water radiolysis, or by other radicalscreated from enzyme reactions.1-5 OH radicals can also beproduced by ultraviolet and ionizing radiations.6 The exposureof proteins to hydroxyl radicals, or to the combination ofthem with the superoxide anion radical, causes grossstructural modifications. Such modified proteins can undergospontaneous fragmentation or can exhibit a substantialincrease in its proteolytic susceptibility.7 The reactions ofbiological molecules with ROS are assumed to cause severaldiseases such as cancer,8-11 cardiovascular disorders,12-16 andatherosclerosis.17-20 In particular Alzheimer’s disease (AD)has been associated with the free-radical-mediated oxidativestress of the amyloid �-peptide (A�). 21-26

Even though the mechanism for A�-associated radicalproduction is still unknown, several studies have called attentionto the potential role of methionine (Met, M) in AD. Removalof Met35 or substitutions by structurally similar aminoacids likenorleucine (Nle) has been reported to inhibit the aggregationand neurotoxic properties of the peptide.27,28 In addition,examination of senile plaques have shown a high proportion ofmethionine sulfoxide,29 which is believed to be formed througha free radical intermediate.30

In a previous work from our group, the mechanism andkinetics of the OH radical reaction with methionine wasstudied,31 and it was found that hydrogen abstractions occuralmost exclusively from the γ site, which can be attributed to

the vicinity of the sulfur atom. It was a comparative study onthe intrinsic site reactivity of this amino acid, and it wasperformed for the free form of Met in gas phase. However, thechemical behavior of an amino acid is expected to be influencedby the peptidic environment, as well as by the solvent.Accordingly, for a better understanding in the oxidative stressprocesses involving this amino acid, especially when it is inthe A� peptide, a more detailed study is needed including theeffects of the environment. Therefore it is the aim of this workto study the reactivity of Met toward OH radicals in solutionusing the tripeptide Gly-Met-Gly to mimic the peptidic environ-ment. A comparative study with the Gly-Nle-Gly has beenperformed, in an attempt to provide an explanation to thefindings reported by Pike et al.27 and Varadarajan et al.28 Therate constants and percent contributions of the different pathsof reaction to the overall reaction, for both peptides, are studiedhere for the first time.

Computational Details

All the calculations have been performed within densityfunctional theory (DFT) frame. This methodology has beenpreviously used to model free radical reactions with diverseamino acids.32-35 Electronic structure calculations have beenperformed with the Gaussian 0336 package of programs. Fullgeometry optimizations and frequency calculations were carriedout for all the stationary points using the BHandHLYP hybridHartree-Fock (HF) density functional and the 6-311G(d,p) basisset with an Onsager37 self-consistent reaction field (ε ) 78.39).The energies of all the stationary points were improved bysingle-point calculations at BHandHLYP/6-311++G(d,p) levelof theory, using polarizable continuum model, specifically theintegral-equation-formalism (IEF-PCM).38 Since we are inter-ested in H abstraction reactions, the solute cavity was computed

* Corresponding author. E-mail: agalano@prodigy.net.mx.† Universidad Autonoma Metropolitana-Iztapalapa.‡ UNAM.

J. Phys. Chem. B 2009, 113, 4947–4952 4947

10.1021/jp900118f CCC: $40.75 2009 American Chemical SocietyPublished on Web 03/19/2009

using atomic radii from the universal force field (RADII )UFF), which assigns individual spheres to H atoms (explicithydrogens).

The BHandHLYP functional was chosen on the basis of itsproven effectiveness.39-50 Zang et al.39 have shown that forproton-transfer reactions in multiple hydrogen bonded systems,only the hybrid BHandHLYP method (from a large set of DFTmethods, including B3LYP) is capable of predicting the structureand energetic information of both the minimum energy and thetransition structures, at a comparable accuracy with the MP2level. In addition, Rice et al.44 have shown that, for H + OCSpotential energy surface, BHandHLYP geometrical parametersand energy barriers are in close agreement with QCISD(T)predictions. For example, the difference between these twomethods for zero-point energy (ZPE) corrected energy barrier(HS + CO) goes from 0.1 to 0.8 kcal/mol. In addition, Maityhas shown that BHandHLYP properly describes two-center,three-electron-bonded systems under restricted open shellformalism.49,50 Even though B3LYP is probably the most widelyused functional, it has not been chosen for the present studysince it was found to underestimate barrier heights by an averageof 4.4 kcal/mol for a database of 76 barrier heights.51,52 Thisissue makes B3LYP a nonadequate method of choice for studiesinvolving kinetic calculation.

Thermodynamic corrections at 298 K were included in thecalculation of relative energies. Restricted calculations were usedfor closed shell systems and unrestricted ones for open shellsystems. Local minima and transition states were identified bythe number of imaginary frequencies (NIMAG ) 0 or 1,respectively). In addition, the vibrational modes with imaginaryfrequencies were inspected using the GaussView53 program, andit was confirmed that they do connect the correspondingreactants and products. For selected path of reaction, intrinsicreaction coordinate (IRC) calculations were carried out and itwas corroborated that the transition states properly connectreactants and products.

The rate constants (k) were calculated using conventionaltransition-state theory (TST)54-56 and 1 M standard state as

where kB and h are the Boltzman and Planck constants, ∆Gq isthe Gibbs free energy of activation, and σ represents the reactionpath degeneracy, accounting for the number of equivalentreaction paths.

Results and Discussion

The conformational analysis of peptides itself constitutes acomputational challenge that escapes the purpose of the presentwork. In fact there are works exclusively devoted to suchpurpose.57 Here the backbones of both tripeptides have beenmodeled in the � sheet conformation, to mimic the A�-peptideaggregates. The amino acids fragments are all L isomers, whichcorrespond to the natural occurring ones. The conformation ofthe side chains for the central amino acids in the tripeptidescorrespond to those of minimal energy. For Nle the side chainis in the zigzag conformation, which is characteristic of n-alkylgroups. For Met a conformational analysis involving theCε-Cδ-Cγ-C� dihedral angle (Figure 1) was performed, andit was found that the minimum energy corresponds to a valueof 82.2°; therefore, this conformation was used for the side chainof Met. The same criteria have been used for constructing the

starting geometry of every modeled species, followed by fulloptimizations. However, since the potential energy surface forthe rotation of the side chain in Met is very flat, the optimizationsof some of the transition-state structures lead to a conformationwith a Cε-Cδ-Cγ-C� dihedral angle very different from thatin the free peptide.

The optimized structures of the studied peptides are shownin Figure 1, including the names for each modeled site ofreaction. The only difference between Met and Nle is that inthe latter the sulfur atom has been replaced by a methylenegroup. This is expected to be an important structural differenceif the reactivity of methionine is ruled by the S center. It seemsimportant to notice that the role of the S atoms does not involvedirect addition of the radical to this site. It has been proventhat such reaction represents only a minor path in the reactionof thioethers with OH.58,59 Accordingly only H abstractionreactions have been modeled in the present study.

The site reactivity of the studied species toward the OHradical is expected to be directly related with the location ofthe highest occupied molecular orbital (HOMO). The sitecontributing the most to the HOMO density is expected to bethe most reactive one. The HOMO density surfaces obtainedfor the Gly-Met-Gly and Gly-Nle-Gly peptides, in solution, arealso shown in Figure 1. It is interesting to notice that for thelatter one the HOMO density is mainly located on the backbone,while for the Gly-Met-Gly peptide it is mainly located on theside chain, with largest contributions in the vicinity of the Satom. This supports the hypothesis that the S atom plays animportant role on the reactivity of methionine. According tothe HOMO density, R and � sites are expected to be the mostreactive ones in Gly-Nle-Gly. For Gly-Met-Gly, on the otherhand, the most reactive sites are expected to be γ and ε.

The fully optimized geometries of the transition-state (TS)structures corresponding to the OH + Gly-Met-Gly reaction areshown in Figure 2. All of them present hydrogen bondintramolecular interactions, with the exception of Met-TSε. Forthe transition states involved in H abstractions from R and γsites, this interaction takes place between the H atom in theOH radical and the O atoms in one of the amide groups, withd(H · · ·O) ) 2.24 and 2.10 Å, respectively. In Met-TS�, itinvolves the H atom in the OH radical and the sulfur atom inthe methionine side chain, with d(H · · ·O) ) 2.56 Å.

The transition states corresponding to OH hydrogen abstrac-tion reactions from Gly-Nle-Gly also present H bond intramo-lecular interactions between the OH radical and some of the

k ) σkBT

he-(∆Gq)/RT (1)

Figure 1. Optimized structures of the studied tripeptides, in aqueoussolution, and HOMO density surfaces, computed with an isodensityvalue of 0.02 au.

4948 J. Phys. Chem. B, Vol. 113, No. 14, 2009 Francisco-Marquez and Galano

polar groups in the peptide (Figure 3). In this case, there aretwo of them that do not show such interaction: Nle-TSδ andNle-TSε. For those TS structures that do present this interaction,it always involves the H atom in the OH radical and one of theO atoms in the peptidic backbone. The interaction distances werefound to be as follows: 2.22, 2.06, and 2.12 Å for Nle-TSR,Nle-TS�, and Nle-TSγ, respectively. All of them are attractiveinteractions that are expected to lower the energy of the species.For all the TSs the nature of the breaking and forming bonds isequivalent (C-H and O-H, respectively), and so is the natureof the intramolecular interaction (H · · ·O). Accordingly, it canbe hypothesized that the stronger the intramolecular interactionsthe lower the reaction barrier. Therefore, and on the basis of

this geometrical parameter, H abstractions from the � site ofNle are expected to have the lowest barrier of reaction.

The optimized structures of the transition states correspondingto Met-Gly and Gly-Nle-Gly in the gas phase have been includedfor comparison as Supporting Information (Figures S1 and S2,respectively). In general the braking bonds are larger and theforming bonds are shorter in gas-phase TS structures, comparedto those in aqueous solution; i.e., the TSs are earlier in aqueoussolution. Therefore, according to Hammond postulate,60 thepresence of the solvent increases the feasibility of the studiedreactions.

The enthalpies of reaction, as well as the Gibbs free energiesof reaction and barriers, at 298.15 K, are reported in Table 1for the modeled channels corresponding to OH radical reactionswith Gly-Met-Gly and Gly-Nle-Gly tripeptides, in aqueoussolution. All processes were found to be exothermic (∆H < 0)and exergonic (∆G < 0) with the largest energy releaseassociated with the H abstractions from R sites. In all the casesthe energy barriers corresponding to Gly-Met-Gly are lower thanthose involving Gly-Nle-Gly. This indicates that the reactivityof the peptide is reduced when a Met fragment is replaced byNle. In addition, the presence of the S atom seems to affect notonly those sites next to it but also those located an odd numberof bonds apart. This long-range effect lowers the barrier of theH abstraction from the R site by ∼2.5 kcal/mol, compared tothe equivalent site in Gly-Nle-Gly. The � site, on the other hand,is placed an even number of bonds (two) apart from the S atomand shows the opposite effect; i.e., its barrier increases by about1.7 kcal/mol, also compared to that of Gly-Nle-Gly.

For the Gly-Nle-Gly peptide the lowest barrier correspondsto channel R, which is in agreement with the predictions madeon the basis of the distribution of the HOMO electron density.For the Gly-Met-Gly system, on the other hand, the lowestbarrier was found to correspond to H abstractions from theterminal methyl group, also in agreement with the HOMOdensity of this peptide. However, this is a surprising result andcannot be explained on the basis of the intrinsic reactivity ofthe different sites. A similar behavior is also found for Gly-Nle-Gly when comparing the barriers of the ε channel with thoseof channels �, γ, and δ. Terminal methyl groups (primary) arenot expected to be more reactive than secondary ones. The onlyplausible explanation for that behavior is that the high polarityof the solvent favors such a process. To test that hypothesis,the calculations were also performed in the gas phase, usingthe neutral form of the peptides instead of the zwitterions, andthe results have also been included in Table 1. These resultsexhibit the expected tendency; i.e., the terminal methyl siteshave the highest barriers and consequently are expected to bethe least reactive in the gas phase. This is consistent with the

Figure 2. Fully optimized geometries of the Gly-Met-Gly + OHtransition states, in aqueous solution.

Figure 3. Fully optimized geometries of the Gly-Nle-Gly + OHtransition states, in aqueous solution.

TABLE 1: Heats of Reaction and Barriers at 298.15 K, inTerms of Enthalpy and Gibbs Free Energies, and Enthalpiesof Reaction (All in Kilocalories per Mole)

∆Hsol ∆Gsolq ∆Gsol ∆Ggas

q ∆Ggas

Gly-Met-GlyR -39.01 3.76 -39.94 7.46 -28.12

� -23.45 10.03 -25.15 11.33 -12.40γ -28.61 4.68 -29.61 8.39 -19.35ε -28.16 1.47 -29.35 11.66 -16.14

Gly-Nle-GlyR -30.55 6.38 -31.97 7.42 -27.02� -19.29 8.29 -21.57 11.89 -13.95γ -21.70 10.23 -23.69 9.64 -15.17δ -19.13 10.68 -21.60 12.98 -14.19ε -20.26 7.69 -21.86 13.46 -11.72

Reactivity of Methionine toward OH Radicals J. Phys. Chem. B, Vol. 113, No. 14, 2009 4949

expected intrinsic reactivity of the different sites of reactions.Accordingly, it seems that high polar environments significantlyincrease the reactivity of the terminal group of the side chainof these peptides toward OH radicals. This finding seemsrelevant since in protein and peptides terminal groups of theside chains are the most exposed ones. Therefore they are themost accessible sites of reactions, and their reactivity becomesmore important as the size of the attacking radical increases.

In addition to the thermochemical study, discussed above,the kinetic data on the modeled processes have also beencalculated. The rate constants (k), at 298.15 K, are reported inTable 2. They were first calculated using the conventional TSTfor each channel of reaction. However, some of the k valuescalculated this way were found to be near or even higher thanthe diffusion-limited rate constant. Accordingly, the apparentrate constant (kapp), which is expected to reproduce theexperimental observations, cannot be directly obtained from TSTcalculations. In thepresentworkwehaveusedtheCollins-Kimball(CK) theory for that purpose:61

where kact. is the activation rate constant (obtained from TSTcalculations) and kD is the diffusion-controlled rate constant (kD

∼ 1 × 1010 L mol-1 s-1).After calculating the rate constants for each channel, using

the CK theory with kact. ) kTST, the overall rate constant thatmeasures the rate of OH disappearance has been estimated bysumming up the rate coefficients calculated for all the differentpathways. This approach implies that once a specific pathwayhas started, it proceeds to completion, independently of the otherpathways; i.e., there is no mixing or crossover between differentpathways. In the TST calculations the reaction path degeneracy(σ) has been obtained by imaging all identical atoms to belabeled and by counting the number of different but equivalentarrangements that can be made by rotating (but not reflecting)the molecule.62 Accordingly σR ) 1, σ� ) σγ ) 2, and σε ) 3for Gly-Met-Gly, and σR ) 1, σ� ) σγ ) σδ) 2, and σε ) 3 forGly-Nle-Gly.

According to our results (Table 2) the reaction of OH withGly-Met-Gly is expected to be about 100 times faster than thatinvolving Gly-Nle-Gly, since the rate constant of the first processis 2 orders of magnitude higher than that of the second one.For the peptide involving methionine, three different channelsmake significant contributions to the overall reaction. They arethose involving H abstractions from ε, R, and γ sites, and theircontributions were found to be 54.4, 28.4, and 17.2%, respec-tively. The channel of reaction involving H abstractions fromthe � site was found to contribute to the overall reaction in lessthan 0.01%. These results dramatically differ from thoseobtained without taking into account the influence of the solvent

and of the peptidic environment.31 Both factors have a stronginfluence on the reactivity of the different sites in the side chainof methionine. For the Gly-Nle-Gly peptide the contributionsof channels γ and δ to the overall reaction were found to benegligible (0.2 and 0.1%, respectively). The main channels ofreaction, in order of importance, were found to be R, ε, and �with percent contributions to the overall reaction of 70.6, 23.4,and 5.7%, respectively. Accordingly, compared to Met, not onlythe overall reactivity of Nle toward OH radicals is lower butalso the most reactive site in this case changes. In Nle the Rsite shows the higher reactivity. It placed in the peptidicbackbone, which makes this site much less accessible than theterminal methyl group in Met. Therefore, if a methioninefragment is replaced by norleucine, the overall reactivity of thepeptide toward free radicals is expected to be significantlyreduced. This decrease is expected to be even more critical forlarge-sized free radicals.

The results discussed above explain the experimental findingsthat removal of Met35 or substitutions by Nle inhibit theaggregation and neurotoxic properties of the A� peptide.27,28

They also offer an explanation to the role of sulfur atom on thereactivity of methionine toward free radicals. This work alsosupports the preponderant role of Met35 on the developmentof Alzheimer’s disease. It should be noticed, however, that theoxidative stress affecting the A� peptide is a very complexprocess, while this work only addresses the direct reactions ofthe peptide with free radicals.

Additional PCM single-point calculations have been carriedout at MP2 and B3LYP levels, for testing how sensitive theresults are to the employed methods. The three most significantchannels of reaction have been computed for each peptide,namely, channels R, γ, and ε for Gly-Met-Gly and channels R,�, and ε for Gly-Nle-Gly. To make the comparison fair, in allthe cases the calculations were performed with the same basisset: 6-311++G(d,p). All the tested methods produce similarGibbs free energies of reaction, and predict channels R as themost exergonic ones, for both peptides (Table S1). The averagedifference among them is less than 1 kcal/mol. The reactionbarriers, on the other hand, are more sensitive to the method ofchoice (Table S2). The average difference between MP2 andBHandHLYP results is only ∼0.42 kcal/mol, including all thecomputed barriers. However, B3LYP barriers are lower by anaverage of 4.9 and 5.4 kcal/mol than those obtained withBHandHLYP and MP2 methods, respectively. This underesti-mation on the B3LYP barrier heights is in correspondence withthose previously reported by Truhlar and co-workers.51,52

However, in despite of the discussed differences, all the testedmethods predict channel ε and R, for Gly-Met-Gly and Gly-Nle-Gly, respectively, as those with the lowest barrier, sup-porting the relative site reactivity proposed above. All the testedmethods also predict that the barriers for the reaction paths ofGly-Met-Gly are systematically lower than those of Gly-Nle-Gly, supporting the higher reactivity of the first one.

The overall rate coefficients, at room temperature, computedwith the different tested methods are reported in Table S3 ofthe Supporting Information. Logically, there are large discrep-ancies between those obtained using the B3LYP functional andany of the other ones, given that B3LYP barriers are quitedifferent from those obtained at MP2 or BHandHLYP methods.Since the rate coefficients are very close to the diffusion limitfor the OH reaction with Gly-Met-Gly, but not so close for Gly-Nle-Gly; the latter is much more sensitive to the barrierunderestimation. The good agreement among all the overall ratecoefficients, for the Gly-Met-Gly peptide, is only apparent. There

TABLE 2: Rate Constants (L mol-1 s-1) at 298.15 K, inAqueous Solution, Obtained from Collins-Kimball Theorywith kact.) kTST

Gly-Met-Gly Gly-Nle-Gly

kR 5.20 × 109 1.29 × 108

k� 5.50 × 105 1.04 × 107

kγ 3.14 × 109 3.94 × 105

kδ 1.83 × 105

kε 9.94 × 109 4.29 × 107

koverall 1.83 × 1010 1.84 × 108

kapp )kDkact.

kD + kact.(2)

4950 J. Phys. Chem. B, Vol. 113, No. 14, 2009 Francisco-Marquez and Galano

is significant difference in TST rate constants that vanishes bythe use of the CK equation. We call attention on the fact that adifference of 1 kcal/mol in the barrier height represents adifference of about 1 order of magnitude in k. When the reactionrates are very close to the diffusion limit, this differencedisappears since the diffusion starts playing the preponderantrole in the kinetics, instead of the reaction barrier. However,for reactions significantly slower than that, such as that involvingGly-Nle-Gly, the barrier height rules the kinetics and, accord-ingly, the B3LYP rate constants become highly overestimated.This causes that B3LYP results lead to a kGly-Met-Gly/kGly-Nle-Gly

ratio dramatically lower than those obtained from MP2 andBHandHLYP calculations. The latter two, on the other hand,show an excellent agreement, supporting that the reaction ofOH with Gly-Met-Gly is expected to be about 100 times fasterthan that involving Gly-Nle-Gly. The values of the kGly-Met-Gly/kGly-Nle-Gly ratio were found to be equal to 99.7 and 98.8 forBHandHLYP and MP2 methods, respectively.

Sulfoxide Formation. It has been previously reported thatsenile plaques show a high proportion of methionine sulfoxide,28

which is believed to be formed through a free radical intermedi-ate.30 Even though the elucidation of the sulfoxide formationescapes the main purpose of the present work, some paths ofreaction leading to that kind of products have been explored.For that purpose energies of reaction, in terms of enthalpy andGibbs free energy, have been computed using a reduced model(R ) CH3, Scheme 1). The simplest way of rationalizing thesulfoxide formation is by imagining the direct reaction betweenthe S atom in the thioether site with an O atom (path A, Scheme1). Another possibility, based on the results obtained for thefirst oxidation step of methionine (its reaction with the OHradical), is the reaction of methionine ε radical with an O atom(path B) or even with another hydroxyl radical (path C). Thesethree processes were found to be exothermic and exergonic withenergy releases larger than 50 kcal/mol (Table 3). However,since the physiological concentrations of O atoms and OHradicals are very low, it seems more likely that the sulfoxideformation involves an O2 molecule instead, due to its relativeabundance.

Any attempt to find the products corresponding to directadditions of O2 to the S atom, in the neutral thioether or in itsε radical, invariably evolved through the separation of thefragments. Accordingly, such paths have been ruled out. Onthe other hand, it seems that the O2 molecule readily reacts with

the radical end of the ε radical to form a stable adduct (path D)that may eventually lead to the formation of formaldehyde andCH3SO radical. The same products might be directly formedfrom the O2 + ε radical reaction (path E). Since the Gibbs freeenergy of reaction involved in the transformation of the D adductinto formaldehyde and CH3SO radical is very large (-54.37kcal/mol), it seems reasonable to assume that these are theproducts most likely to be formed from the methionine ε radical.The R-SO radicals are then expected to react with neighbormolecules to yield sulfoxide compounds. According to theresults in Table 3 and taking into account the scarce amount ofatomic oxygen in biological systems, compared to that of O2,the R-SO radical formation through paths D or E would explainthe sulfoxide formation reported in ref 29. Our hypothesis is inline with the suggestion of Miller et al.30 that radical intermedi-ates are expected to be involved in the formation of methioninesulfoxides.

Conclusions

The reaction of OH with Gly-Met-Gly was found to be about100 times faster than that involving Gly-Nle-Gly. Therefore, ifa methionine fragment is replaced by norleucine the overallreactivity of the peptide toward free radicals is expected to besignificantly reduced, which is in agreement with previousexperimental findings.27,28 This decrease is expected to be evenmore critical for large-sized free radicals because while the mostreactive site in norleucine is the R center in the backbone, inmethionine it is located in the side chain. The polarity of theenvironment seems to have a significant influence on thereactivity increase of the terminal group of the side chains ofthese peptides. In addition the viability of a different pathexplaining the formation of methionine sulfoxide has beentested, using a simplified model, and it is proposed that thisprocess involves theformationofR-SOradicalandformaldehyde.

Acknowledgment. The authors thank project SEPCONACYTGrant SEP-2004-C01-46167 for financial support. A.G. alsothanks Project CONACYT 46124 PROMEP and Laboratoriode Visualizacion y Computo Paralelo at UAMsIztapalapa forthe access to its computer facilities. M.F.-M. thanks theDireccion General de Servicios de Computo Academico (DG-SCA) at Universidad Nacional Autonoma de Mexico, andInstituto de Ciencia y Tecnologıa del D.F., for a postdoctoralresearch fellowship.

Supporting Information Available: Fully optimized ge-ometries of the transition states, for OH reactions with Gly-Met-Gly and Gly-NLe-Gly, in gas phase and comparisonsamong heats of reactions, energy barriers, and overall ratecoefficients, computed using MP2, B3LYP, and BHandHLYPmethods.This material is available free of charge via the Internetat http://pubs.acs.org.

SCHEME 1 TABLE 3: Heats of Reaction of Potential Reaction PathsLeading to Sulfoxide Formation, in Terms of Enthalpy andGibbs Free Energies, at 298.15 K and in Kilocalories perMole

∆Hsol ∆Gsol

A -63.00 -53.29B -55.10 -45.58C -58.95 -46.95D -22.85 -11.49

-43.05 -54.37E -65.90 -65.85

Reactivity of Methionine toward OH Radicals J. Phys. Chem. B, Vol. 113, No. 14, 2009 4951

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