Reactions of CH3SH and CH3SSCH3 with Gas-Phase Hydrated

Article pubs.acs.org/JPCA

Reactions of CH3SH and CH3SSCH3 with Gas-Phase Hydrated Radical Anions (H2O)n•−, CO2•−(H2O)n, and O2•−(H2O)n Robert F. Höckendorf,†,‡ Qiang Hao,§ Zheng Sun,⊥,∥ Brigitte S. Fox-Beyer,⊥ Yali Cao,†,‡,¶ O. Petru Balaj,†,⊥ Vladimir E. Bondybey,⊥ Chi-Kit Siu,*,§,⊥ and Martin K. Beyer*,†,‡,⊥ †

Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstrasse 40, 24098 Kiel, Germany Institut für Chemie, Sekr. C4, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany § Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong ⊥ Department Chemie, Lehrstuhl 2 für Physikalische Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany ∥ Department of Chemistry and Materials Sciences, Hebei Normal University, No. 20 South Second Ring East Road, 050024 Shijiazhuang, P. R. China ‡

S Supporting Information *

ABSTRACT: The chemistry of (H2O)n•−, CO2•−(H2O)n, and O2•−(H2O)n with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. With hydrated electrons and hydrated carbon dioxide radical anions, two reactions with relevance for biological radiation damage were observed, cleavage of the disulfide bond of CH3SSCH3 and activation of the thiol group of CH3SH. No reactions were observed with CH3SCH3. The hydrated superoxide radical anion, usually viewed as major source of oxidative stress, did not react with any of the compounds. Nanocalorimetry and quantum chemical calculations give a consistent picture of the reaction mechanism. The results indicate that the conversion of e− and CO2•− to O2•− deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H2O)n•− with CH3SH as well as CO2•−(H2O)n with CH3SSCH3, the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters. dissociation (ECD)17 of proteins in mass spectrometry and was modeled theoretically by Simons and co-workers.18 In the condensed phase, the one-electron reduction of proteins in reaction with CO2•− leading to disulfide radical anions and thiyl radicals was studied by pulse radiolysis.19,20 A quantum mechanics/molecular mechanics (QM/MM) approach was applied toward electron addition on the disulfide bond in thioredoxin.13,14 The reactions of hydrated electrons with small sulfides and disulfides have also been studied by pulse radiolysis,21−24 with RSH decomposing into R• and SH−, while RSSR form radical anions by electron attachment. The cleavage of the S−S bond, leading to thiolate and thiyl radical, is only secondary.24 To learn more about the inherent chemical reactivity of radical anions toward sulfur compounds, we investigate in the present work the reactions of (H2O)n•−, CO2•−(H2O)n, and O2•−(H2O)n, n < 70, with small sulfur-containing model molecules, dimethylsulfide (DMS), methyl mercaptan (MeSH), and dimethyl disulfide (DMDS) in the gas phase with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrom-

1. INTRODUCTION Ionizing radiation generates electrons, leading to formation of free radicals in biological cells,1,2 which in turn are involved in cell damage.2,3 Well known for its role in biological radiation damage is the O2•− superoxide radical anion.4 The hydrated electron5 and hydrated CO2•− carbon dioxide radical anion are common radicals in biological cells and known precursors for the hydrated superoxide radical anion.6 Thiol groups contribute to the tertiary structure of proteins via formation of cystine residues. As experimentally and theoretically tractable model systems, various small sulfur compounds have been studied experimentally and theoretically in the gas phase.7−14 Electron attachment to disulfide bonds was studied both theoretically and experimentally.11−13 Reactions of dimethyl disulfide (DMDS) with a series of anions were studied by Grabowski and co-workers.7,8 From electron-transfer reactions, the adiabatic electron affinity of DMDS was bracketed to lie between 0.024 and 0.44 eV.8 Carles et al. place the adiabatic electron affinity of DMDS at around 0.1 eV based on a Franck−Condon analysis of Rydberg electron-transfer spectroscopy and MP2 calculations.9 Dissociative electron attachments to cysteine15 and cystine16 are the gas-phase models closest to radiation damage in proteins. One-electron reduction of the cystine residue is important in electron capture © 2012 American Chemical Society

Received: March 2, 2012 Revised: March 21, 2012 Published: March 21, 2012 3824

dx.doi.org/10.1021/jp302076f | J. Phys. Chem. A 2012, 116, 3824−3835

The Journal of Physical Chemistry A

Article

this work for comparison. The numerical integrations were performed with the default FineGrid for B3LYP and the UltraFineGrid for M06.51

etry. Density functional theory (DFT) calculations for selected reactions with n ≤ 6 elucidate the reaction mechanism. For (H2O)n•− hydrated electrons in the gas phase,25 bulklike chemistry is frequently observed.26 In the cluster, hydrated electrons with n > 30 are stable for several seconds.27 Under the collision-free conditions of a FT-ICR mass spectrometer, their lifetime is limited only by blackbody infrared radiative dissociation (BIRD).28−36 For hydrated electrons with n < 30, electron detachment competes with water loss.27,37 Also the CO2•− radical anion requires stabilization by solvation,38 while due to the positive electron affinity of O2, no solvation is needed to stabilize O2•−.

4. RESULTS AND DISCUSSION Reactions Kinetics and Product Formation. No reactions were observed in the experiments with DMS as neutral reactant with any of the radical anions under study. However, both (H2O)n•− and CO2•−(H2O)n showed efficient product formation with MeSH and DMDS, while O2•−(H2O)n are largely unreactive. Hydrated Electron (H2O)n•−. A mass spectrum taken at 0 s reaction delay for the reaction of (H2O)n•− with MeSH at a pressure of 3 × 10−8 mbar is shown in Figure 1. Since ions from

2. EXPERIMENTAL DETAILS The experiments were performed on a modified Bruker/ Spectrospin CMS47X FT-ICR mass spectrometer, equipped with a 4.7 T superconducting magnet, infinity cell, and APEX III data station.39 Hydrated electrons are generated by laser vaporization40 of a solid zinc target and supersonic expansion of the plasma in a 50 μs long helium/water gas pulse.41 CO2•−(H2O)n or O2•−(H2O)n were generated by addition of small amounts of CO2 or O2 to the expansion gas mixture. The ionic clusters are transferred via an electrostatic lens system through four differential pumping stages into the UHV region of the mass spectrometer and stored in the ICR cell. The base pressure of the UHV is below 5 × 10−10 mbar. Reaction gases MeSH (Fluka, ≥99.0%), DMDS (Janssen Chimica, 99%), and DMS (Janssen Chimica, 99%) are introduced through leak valves at pressures of 5 × 10−9− 5 × 10−8 mbar, depending on the reaction efficiency. Liquid reactants were degassed by several freeze−pump−thaw cycles. The vacuum and gas inlet system are regularly baked out to avoid memory effects. The leak valve is kept open during bake-outs and purged with highpurity nitrogen gas. Reactions are monitored by recording mass spectra of the ions in the ICR cell as a function of time. The reaction kinetics is analyzed by summing the intensities of reactant and product clusters over all cluster sizes. In reactions of hydrated electrons, this works only for reaction delays where the intensity of clusters with n < 30 is negligible, since electron detachment cannot be accounted for with sufficient accuracy. Nanocalorimetry42−44 is used to extract thermochemical information from the data by counting the number of evaporated solvent molecules during the reaction.45

Figure 1. Mass spectrum taken at 0 s reaction delay for the reaction of (H2O)n•− (solid line) with MeSH forming MeS−(H2O)m (dash-dotted line) at a pressure of p = 3 × 10−8 mbar.

20 laser shots are stored in the ICR cell to increase signal-tonoise ratio, at nominal 0 s the radical anions have been exposed for up to 2 s to the reaction gas. The selective elimination of a hydrogen atom, reaction 1, is the only reaction observed. This is in line with the dissociative attachment of cysteine studied by Illenberger and co-workers,15 where H radical abstraction is the dominant product channel. In electron ionization of mercaptans, [M−H]−, H−, S•−, and SH− are the dominant negative ions formed.52 Formation of CH3•, the dominant reaction in pulse radiolysis,21 is not observed.

3. COMPUTATIONAL METHODS The solvation structures of small clusters of hydrated dimethyl disulfide radical anions, (DMDS•−)(H2O)n, with n ≤ 6, have been modeled with density functional theory (DFT) calculations using the B3LYP46 and M0647 functionals, performed by the Gaussian 09 quantum chemical package.48 Our benchmark calculations demonstrate that M06/6-311+ +G(d,p) with 6-311+G(3df) basis set for sulfur atoms is a superior level for describing the two-center-three-electron (2c3e) bond of DMDS•− (vida inf ra) probably because of the fact that the M06 functional47 was developed with a reduced self-interaction error (or delocalization error).49 On the other hand, the M06 level with the affordable 6-311+ +G(d,p) basis set to calculate the ionic hydration energy of water clusters might underperform as compared with the B3LYP/6-311++G(d,p) level.50 Therefore, both these two functionals with 6-311+G(3df) for sulfur atoms and 6-311+ +G(d,p) for all other atoms (denoted as BS) were employed in

(H 2O)n•− + CH3SH → CH3S−(H 2O)n − x + H• + x H 2O (1)

Figure 2a shows the kinetic data and the pseudo-first-order fit of the reaction of hydrated electrons with MeSH, with the intensities summed over all cluster sizes. Reaction 1 proceeds with kabs(1) = 0.87 × 10−9 cm3 s−1. Average dipole orientation (ADO) theory predicts kADO = 1.2 × 10−9 cm3 s−1; see Table 1.53 For the weakly polarizable (H2O)n•− clusters, the collision rate is expected to lie between the hard-sphere ADO (HSA) and surface charge capture (SCC) model,54 which are for n = 50 calculated as kHSA = 1.7 × 10−9 cm3 s−1 and kSCC = 3.3 × 10−9 cm3 s−1, respectively.44 This suggests a reaction efficiency of Φ = 25−50%. DMDS reacts with hydrated electrons with high efficiency to yield the hydrated dimethyl disulfide radical anion with a branching ratio of 89%, while 11% of the reactions result in 3825



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Figure 2. (a) Pseudo-first-order kinetics for the reaction of (H2O)n•− (▲) with MeSH forming CH3S−(H2O)n (●). b) Nanocalorimetric fit to the average cluster size ⟨n⟩. (c) Difference in the average cluster size between reactant and product Δ⟨n⟩. The dashed vertical line marks the switch in fitted data from the average cluster size (b) to the difference in average cluster size. The dashed line in (a) denotes the noise level.

Figure 3. (a) Pseudo-first-order kinetics for the reaction of (H2O)n•− ( ▲ ) with DMDS forming CH 3 SSCH 3 •− (H 2 O) n ( ◊ ) and CH3S−(H2O)n (●). (b) Nanocalorimetric fit to the average cluster size ⟨n⟩. (c) Difference in the average cluster size between reactant and product Δ⟨n⟩. The dashed vertical line marks the switch in fitted data from the average cluster size (b) to the difference in average cluster size. The dashed line in (a) denotes the noise level.

Table 1. Experimentally Observed Rate Constant kabs and Theoretical Collision Rates Constants According to ADO, HSA, and SCC Models, kADO, kHSA, kSCC, Respectively, in 10−9 cm3 s−1, and the Number of Water Molecules Evaporated during the Reaction ΔNvap Determined from Nanocalorimetrya

Quantum chemical calculations pointed out that deprotonated formic acid HCOO− is formed selectively via a radical-type hydrogen atom transfer (HAT) from the thiol group of methyl mercaptan, reaction 4.44

reaction

kabs

kADO

kHSA

kSCC

ΔNvap

(1) (H2O)n•− + MeSH → CH3S−(H2O)m + H• (2) (H2O)n•− + DMDS → DMDS•−(H2O)m (3) (H2O)n•− + DMDS → CH3S−(H2O)m + CH3S• (4) CO2•−(H2O)n + MeSH → HCOO−(H2O)m + CH3S• (5) CO2•−(H2O)n + DMDS → [CH3S,CO2]−(H2O)m + CH3S•

0.87

1.2

1.7

3.3

1.1 ± 0.2

1.46

1.2

1.2

2.4

2.8 ± 0.2

0.18

1.2

1.2

2.4

4.5 ± 0.2

0.85b

1.2

1.7

3.3

−1 ± 0.2b

0.72

1.2

1.2

2.4

0.2 ± 0.2

CO2•−(H 2O)n + CH3SH → HCO2−(H 2O)n − p− + CH3S• + pH 2O

In the reaction of with DMDS, cleavage of the disulfide bond resulting in [CH3S,CO2]−(H2O)y and the loss of the most weakly bound unit, a neutral thiyl radical CH3S•, was observed, reaction 5. CH3SSCH3 + CO2•−(H 2O)n → [CH3S, CO2 ]− (H 2O)n − y + CH3S• + y H 2O

The evaporated water molecules are left out on the right hand side of the reaction equations for clarity. bkabs and ΔNvap values from ref 44.

formation of hydrated methane thiolate and elimination of a thiyl radical, reactions 2 and 3, respectively. CH3SSCH3 + (H 2O)n•− (2)

CH3SSCH3 + (H 2O)n•− → CH3S−(H 2O)n − z + CH3S• + z H 2O

(3) −9

−1

Reaction 2 proceeds with kabs(2) = 1.46 × 10 cm s and reaction 3 with kabs(3) = 0.18 × 10−9 cm3 s−1; see Table 1 and Figure 3a. Assuming a hard-sphere radius of 4 Å for DMDS, the collision rates for n = 50 are calculated as kHSA = 1.2 × 10−9 cm3 s−1 and kSCC = 2.4 × 10−9 cm3 s −1,54 resulting in a reaction efficiency Φ = 70−140%. This suggests that DMDS reacts with hydrated electrons more or less with collision rate. Hydrated Carbon Dioxide Radical Anion CO2•−(H2O)n. The formation of HCO 2 − (H 2 O) n in the collision of CO2•−(H2O)n with CH3SH has been reported previously.44 3

(5)

Kinetic analysis, as displayed in Figure 4a, yields a rate kabs = 7.2 × 10−10 cm3 s−1, which with the HSA and SCC rates reported above corresponds to an efficiency Φ = 30−60%. [CH3S,CO2]−(H2O)y is fragmenting to an ion with nominal mass m/z = 91, which is the water-free complex [CH3S,CO2]−. CID experiments with this ion did not yield any information on the structure, since the only effect seen by ion activation was signal loss. This indicates that electron detachment is the lowest-energy dissociation channel. Mechanistically, formation of CH3SCO2− via attack of CO2•− at the disulfide bond seems straightforward. Hydrated Superoxide Radical Anion O2•−(H2O)n. The hydrated superoxide radical anion is largely unreactive toward the studied sulfur compounds. Only for methyl mercaptan reacting with very small clusters, ligand exchange resulting in formation of (O2•−)(CH3SH)(H2O)2−3 was observed after long reaction delays. No reaction was observed for the superoxide radical anion with DMDS and DMS. Nanocalorimetric Analysis. Nanocalorimetry43 is used to determine the number of water molecules ΔNvap evaporated due to the exothermicity of the reaction. In Figure 2b the average cluster size ⟨n⟩ is plotted, while in 2c the difference in average cluster size between reactant and product clusters is

a

→ CH3SSCH3•−(H 2O)n − y + y H 2O

(4)

CO2•−(H2O)n

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ΔEnc(7) = ΔEnc(3) − ΔEnc(1) = −147 ± 19 kJ mol−1 (8) 57

From literature thermochemistry, reactions 9−12, it is possible to derive the reaction enthalpies ΔH(13) = −94.1 ± 8.8 kJ mol−1 by applying Hess’ law.

The direct comparison with literature thermochemistry fails, since the difference in thermochemistry between eqs 8 and 13 amounts to 56 ± 21 kJ mol−1. Differences between nanocalorimetry and expected thermochemistry in this range have been observed before.43,44 In each case, this was interpreted as a failure of one or more of the inherent assumptions made in the nanocalorimetry analysis: The reaction has to be fully ergodic, the internal energy of the cluster must not affect the reaction rate, and the rate constant must be independent of cluster size in the studied size regime.43 A nonergodic component, as suggested for the core exchange reaction of CO2•−(H2O)n with O2,43 is unlikely in the present case. However, a dependence of the reaction rate on the internal energy of the cluster, as discussed in the hydrogen atom transfer from MeSH to CO2•−(H2O)n, seems entirely plausible. Water clusters are constantly heated by blackbody radiation in the room temperature environment of the ICR cell and react by evaporative cooling.28−36 The width of the internal energy distribution of the studied clusters is substantial.36 The energy spread between internally “hot” and “cold” clusters is on the order of 43 kJ mol−1, the energy needed to evaporate a water molecule.55 Hot clusters are just below the threshold for evaporation, while cold clusters have recently lost these 43 kJ mol−1 by water evaporation. Like with CO2•−(H2O)n, MeSH reacts significantly below the collision rate with (H2O)n•−. This suggests that MeSH selectively reacts with clusters of relatively low internal energy, since it evaporates from internally hot clusters before the reaction can take place. The product clusters are thus colder than expected, resulting in an underestimated exothermicity of reaction 1 from nanocalorimetry. Conversely, reactions 2 and 3 compete after uptake of DMDS in the (H2O)n•− cluster. The low branching ratio of 11% of reaction 3 and the fact that, once DMDS•− is stabilized in the cluster, reaction 3 no longer occurs indicate that elimination of CH3S• faces a considerable barrier. Reaction 3 will thus happen selectively with hot reactant clusters, resulting in an overestimated exothermicity of reaction 3. Looking at the difference in average cluster size plotted in Figure 3c suggests that also the third assumption, independence of the rate from cluster size, might be violated. The difference between reactant and product cluster size for reaction 3 does not exhibit the typical initial falloff regime which is caused by the 2 s fill cycle of the ICR cell. At the same time, the initial intensity of the products of reaction 3 is lower than expected. This indicates that S−S bond cleavage, reaction 3, preferentially occurs for smaller clusters. As a consequence, the average product cluster size is smaller than expected for a sizeindependent rate, and nanocalorimetry yields an unrealistically high number of water molecules evaporating.

Figure 4. (a) Pseudo-first-order kinetics for the reaction of CO2•−(H2O)n (▲) with DMDS forming [CH3S,CO2]−(H2O)n (●). (b) Nanocalorimetric fit to the average cluster size ⟨n⟩. (c) Difference in the average cluster size between reactant and product Δ⟨n⟩. The dashed vertical line marks the switch in fitted data from the average cluster size (b) to the difference in average cluster size. The dashed line in (a) denotes the noise level.

shown for the reaction of MeSH with hydrated electrons, reaction 1. To avoid artifacts from a possible drift of the cluster size distribution with time, the fit algorithm refers to the cluster size difference for reaction delays above 1 s. Experimental data and nanocalorimetric fit agree very well. To improve the stability of the fit, the result parameter is scanned in the relevant range, and the remaining parameters are adjusted with a genetic algorithm to minimize the error function. This oneparameter scan results in a minimum error for ΔNvap(1) = 1.1 ± 0.2. In Figures 3b, c the corresponding nanocalorimetric analysis for the reaction of DMDS with hydrated electrons, reactions 2 and 3, is shown. To avoid convergence problems of the genetic algorithm used in the fit, the 2D-grid of the result parameters ΔNvap(2) and ΔNvap(3) is scanned; i.e. for each pair of result parameters, the remaining fit parameters are optimized to minimize the error function. A well-definied minimum was obtained for ΔNvap(2) = 2.8 ± 0.2 and ΔNvap(3) = 4.5 ± 0.2. The evaporation enthalpy of one water molecule55,56 in the studied size regime ΔEvap = 43.3 ± 3.1 kJ mol−1 is used to convert these numbers into thermochemical values. To arrive at absolute thermochemisty, thermal corrections43 are applied to account for the temperature difference of neutral reactant and product species. As discussed before, eq 6 enables us to calculate absolute thermochemistry ΔEnc,43 with ΔEtherm(1, 3) = 3 ± 1 kJ mol−1 and ΔEtherm(2) = 8 ± 1 kJ mol−1. This results in ΔEnc(1) = −45 ± 10 kJ mol−1, ΔEnc(2) = −113 ± 13 kJ mol−1, and ΔEnc(3) = −192 ± 16 kJ mol−1. ΔEnc = −ΔNvapΔEvap + ΔEtherm

(6)

In the literature, no thermochemical data are available which could be compared directly with the reaction enthalpies derived for reactions 1−3. The consistency of the results, however, can be tested with reaction 7. Subtraction of reaction 1 from 3 yields reaction 7. CH3SSCH3 + H• → CH3SH + CH3S•

(7)

Thermochemistry from nanocalorimetry yields 3827



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Table 2. S−S Bond Distances (in Å) and Dissociation Energies (in eV) of Neutral Dimethyl Disulfide and Radical Anion (DMDS•−) and Vertical Detachment Energies (VDE, in eV) and Adiabatic Electron Affinities (Ead, in eV) of DMDSa CH3SSCH3·‑

CH3SSCH3 dissociation/eV

electron affinity

dissociation/eV

VDE/eV ΔE

EA(adiabatic)/eV ΔE

ΔEzpc

S−S/Å

De

D0

S−S/Å

De

D0

B3LYP/6-311++G(d,p) B3LYP/H,C:6-w311++G(d,p) S:6311+G(3df) M06/6-311++G(d,p) M06/H,C:6-311++G(d,p) S:6-311+G(3df)

2.093 2.053

2.35 2.58

2.18 2.41

2.927 2.880

1.09 1.10

1.02 1.02

2.32 2.16

0.59 0.33

0.67 0.41

2.079 2.040

2.67 2.93

1.14 1.16

2.13 1.95

0.35 0.08

0.43 0.15

B3LYP/6-31++G(d,p)b BH&HLYP/6-31++G(d,p)b MP2/6-31++G(d,p)b QCISD/6-31++(d,p)b CASSCF/ANO1b G3b MP2/6-311+G(2d,2p)c G2e experiment

2.082 2.059 2.057 2.069 2.088

2.51 2.855 1.22 2.78 2.808 1.23 Previous Calculations 2.923 2.849 2.785 2.818 2.843

2.071 2.038f

2.68

2.736 2.80 2.82g

2.83c,d

1.16

1.69 1.00 0.94c

1.75c; 1.70c,d

0.41 0.18 0.00 0.24 0.20 0.17 0.12 0.07 0.23 ± 0.21g; 0.08c,d

a f

Values were evaluated at various levels of theory. bValues from ref 58. cValues from ref 9. dFitting of NIPES data, ref 9. eData from ref 11. Experimental data from ref 67. gValues from ref 61.

and 6-311++G(d,p) basis set for H and C and 6-311+G(3df) basis set for S (BS), the S−S distance and the dissociation energy are 2.040 Å and 2.78 eV, respectively, in excellent agreement with the experimental values (Table 2). Looking at the thermochemistry of our benchmark reaction 7 above, we arrive at ΔH°0(7) = −83.0 kJ mol−1 at the M06/BS level and ΔH°0(7) = −113.4 kJ mol−1 at the B3LYP/BS level of theory. The room temperature experimental value of ΔH298K(13) = −94.4 ± 8.8 kJ mol−1 lies between the two theoretical values, with the M06/BS result a bit closer to the experimental value. Unsolvated DMDS•−. Electron attachment to DMDS forms the well-known DMDS•− (dmds_0a C2) in which the extra electron is occupied at the antibonding σ* orbital of the S−S linkage, forming a 2c3e bond62 as shown in Figure 5, and the

The concomitant violation of the two assumptions, size independence and internal energy independence, for reaction 3 makes sense. Smaller clusters have a smaller heat capacity, so uptake of DMDS and formation of DMDS•− will heat a small cluster more than a larger cluster. As a consequence, the probability that sufficient energy is accumulated in the reaction coordinate to cleave the S−S bond is higher in smaller clusters. In summary, the three discussed effects all go in the right direction and together can account for the observed discrepancy of nanocalorimetry and literature thermochemistry. At the same time, we reach a consistent picture and get qualitative insight into the reaction dynamics of large water clusters. Regarding the reactions of the hydrated carbon dioxide radical anion, the previously established value of ΔNvap(4) = −1.0 ± 0.3 corresponds to ΔEnc(4) = +46 ± 13 kJ mol−1.44 In parts b and c of Figure 4, the nanocalorimetric analysis of the experimental data on the DMDS bond cleavage by CO2•−(H2O)n, reaction 5, is shown. The fit finds a welldefined error minimum at ΔNvap(5) = 0.2 ± 0.2 evaporated water molecules. This corresponds to ΔEnc(5) = −6 ± 9 kJ mol−1, thermoneutral to weakly exothermic. Computational Results. Neutral DMDS Benchmark Calculations. Table 2 summarizes the S−S distances and dissociation energies of neutral DMDS and DMDS radical anion (DMDS•−) and the electron affinities of DMDS. The S− S bond distance of 2.093 Å as calculated at the B3LYP/6-311+ +G(d,p) level is slightly longer than the literature values obtained from other levels of theory of 2.059−2.088 Å9,58 and the experimental value of 2.038 Å,59 which is consistent with previously reported results that DFT overestimates the distance of the disulfide bond.58,60 The distance is improved to 2.053 Å when three d-functions and one f-function are included in the basis set for the sulfur atoms (6-311+G(3df)). The homolytic dissociation energy of 2.18 eV as calculated at the B3LYP/6311++G(d,p) level is considerably underestimated by 0.64 eV, as compared with the experimental value of 2.82 eV.61 The extra polarization functions for the sulfur atoms also improve the dissociation energy to 2.41 eV. With the M06 functional

Figure 5. Geometries and spin density distribution of dimethyl disulfide radical anion (DMDS•−) were evaluated using the M06 functional and 6-311++G(d,p) basis set for hydrogen and carbon atoms and 6-311+G(3df) basis set for sulfur atom (BS). S−S distances are in Å. Relative energies ΔH°0/kJ mol−1 (and free energies at 298 K ΔG°298/kJ mol−1 in parentheses) were calculated at the B3LYP/BS (upper values) and M06/BS levels (lower values).

S−S bond is significantly elongated (Table 2). The S−S distance obtained at the B3LYP/6-311++G(d,p) level is again overestimated with the effect being more pronounced than that on the neutral counterpart. The value of 2.808 Å obtained at the M06 level with extra polarization functions for S is close to the value of 2.83 Å obtained from a best fit of data from a negative ion photoelectron spectroscopy (NIPES) experiment.9 Dissociation energies of the 2c3e bond as calculated by various 3828



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Figure 6. Geometries and spin density distribution of DMDS•−(H2O)n, n = 1−6, were evaluated using the M06/BS (see caption of Figure 5 for BS). S−S distances are in Å. Relative energies ΔH°0/kJ mol−1 (and free energies at 298 K ΔG°298/kJ mol−1 in parentheses) were calculated at the B3LYP/BS (upper values) and M06/BS levels (lower values).

0.08 eV.9 A predissociation geometry of DMDS•− (dmds_0b C2h)63 with the S−S distance of 4.267 Å (B3LYP) and 4.228 Å (M06) is also shown in Figure 5 and is 1.6 kJ mol−1 (B3LYP) and 6.2 kJ mol−1 (M06) in energy higher than dmds_0a. Hydrated DMDS•−(H2O)n, n = 1−6. Some selected geometries of DMDS•−(H2O)n, n = 1−6, are shown in Figure 6. The lowest-lying structure for n = 1 is dmds_1a C2, in which the 2c3e S−S bond is symmetrically solvated by the water molecule via two hydrogen bonds. The predissociation geometry for n = 1 (dmds_1b C2) is higher than dmds_1a in energy by 9.5 kJ mol−1 (B3LYP) and 8.5 kJ mol−1 (M06). For n = 2, the DMDS•− is “interiorly” solvated between two

levels of theory show a relatively small deviation of less than 0.2 eV (as compared with around 0.6 eV for the homolytic S−S bond cleavage of DMDS); these values of 1.02−1.16 eV are in reasonable agreement with the fitted NIPES value of 0.94 eV.9 Electron affinities in terms of the vertical detachment energy (VDE) and adiabatic electron affinity (EAad) were also calculated and are shown in Table 2. The 3df polarization functions for S are again essential; the M06 VDE value of 1.95 eV is comparable with the corresponding NIPES value of 1.75 eV,9 and the M06 EAad value of 0.15 eV is also in agreement with the bracketing value of 0.23 ± 0.21 eV,8 the high-level G2 and G3 values of 0.17 eV,58,11 and the fitted NIPES value of 3829



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evaporation energies of DMDS•−(H2O)n, for n = 1−6, were also calculated. Some geometries of (H 2 O) n •− and CH3S−(H2O)n, including the lowest-energy geometries available in the literature,64 were optimized with the present levels of theory (B3LYP/BS and M06/BS). These geometries are shown in the Supporting Information. The lowest-energy geometries of these clusters are level-dependent. Therefore, the reaction energies as summarized in Figure 7 were calculated based on the lowest-energy geometries obtained at the respective levels.

water molecules in the lowest-lying geometry (dmds_2a C2h), which is 8.4 kJ mol−1 (B3LYP) and 18.1 kJ mol−1 (M06) lower than the “surface-solvated” geometry dmds_2b. The predissociation geometry (dmds_2c) becomes even less energetically favorable by 19.9 kJ mol−1 (B3LYP) and 19.7 kJ mol−1 (M06). The solvation geometries of cluster sizes n = 3−6 were systematically built based on these interior-solvated, surfacesolvated, or predissociation structures. For n = 3, the interior-solvated geometry dmds_3a with the DMDS•− being solvated by a water dimer and a water monomer is again lower in energy than the surface-solvated geometry dmds_3b with the DMDS•− being solvated by a water trimer by 7.9 kJ mol−1 (B3LYP) and 4.2 kJ mol−1 (M06). The relative energies are again further increased for the predissociation geometries (dmds_3c and dmds_3d). Wellseparating the S−S bond results in a high-lying dmds_3e with the S−S distance and relative energies of 7.620 Å and 49.7 kJ mol−1 (B3LYP) and 6.716 Å and 64.6 kJ mol−1 (M06). Eight geometries are shown in Figure 6 for n = 4, including three interior-solvated structures (dmds_4a, dmds_4c, dmds_4e), three surface-solvated structures (dmds_4b, dmds_4d, dmds_4f), and two predissociation structures (dmds_4g and dmds_4h). With B3LYP, the lowest-lying geometry is dmds_4a with the DMDS•− being solvated by two water dimers. The next lowest-lying geometry is dmds_4b, which is slightly higher than dmds_4a in energy by 1.0 kJ mol−1 calculated at the B3LYP level. In this geometry, the four water molecules form a cyclic tetramer, and a dangling O−H bond from each of these water molecules solvates the DMDS•−. Interestingly, dmds_4b becomes favorable at the M06 level and is 12.9 kJ mol−1 lower than dmds_4a. dmds_4d and dmds_4e, both containing a cyclic trimer geometry, are lower than dmds_4a by 5.3 and 9.2 kJ mol−1, respectively, at the M06 level. For smaller n, the predissociation geometries are high in energy. For n = 5, the interior-solvated geometry becomes less energetically favorable at both B3LYP and M06 levels. The lowest-energy geometry is dmds_5a, which resembles the geometries of dmds_1a and dmds_4b. dmds_5c is the lowestenergy geometry containing a cyclic water pentamer and is 5.6 kJ mol−1 (B3LYP) and 7.3 kJ mol−1 (M06) higher than dmds_5a. Inserting the water cluster into the 2c3e S−S results in a high-lying predissociation geometry (dmds_5e) with a relative energy of 47.0 kJ mol−1 (B3LYP) and 64.3 kJ mol−1 (M06). Further adding water molecules leads to the formation of fused four-membered rings. dmds_6a, dmds_6b, and dmd_6c are all low-lying geometries that resemble “cubic” structures. In dmds_6a and dmd_6b, the two sulfur atoms of DMDS•− replace two water molecules from one edge of the cubic water structure, while two water molecules from the opposite corners of one face of the cubic water structure are replaced by the two sulfur atoms of DMDS•− in dmds_6c. In summary, as n increases, DMDS•−(H2O)n tend to form the surface-solvated geometries. This can be attributed to the separation of the hydrophobic methyl groups from the water cluster. Well-separating the S−S bond results in the predissociation geometries that are energetically unfavorable. Reaction Energies. Reaction energies of the hydrated electron clusters (H2O)n•− with MeSH (reaction 1), DMDS to form DMDS•−(H2O)n (reaction 2), and the subsequent dissociation of the 2c3e S−S bond to form CH3S−(H2O)n + CH3S• (reaction 3), together with the successive water

Figure 7. Reaction energies of clusters with n = 0−6 evaluated at the (a) B3LYP/BS and (b) M06/BS levels calculated using the lowestenergy geometries obtained at corresponding levels.

The reaction energy of reaction 1 decreases with size for the small clusters n < 4, indicating that there is an increasing hydration stabilization of the anionic CH3S− as the cluster size increases. In addition, it seems to converge with the size reaching around n = 4−5. For the cluster size n = 5, this reaction is calculated to be endothermic by 36.5 kJ mol−1 at the B3LYP level or more or less thermoneutral with a slightly negative reaction energy of −2.9 kJ mol−1 at the M06 level. Although the adiabatic electron affinity of DMDS is only slightly positive, the DMDS•− produced via the electron transfer from a hydrated electron cluster to the neutral DMDS is largely stabilized by solvation. The reaction energy of DMDS with (H2O)n•− is not sensitive to the cluster size n = 1−6, and this trend is expected to continue for larger clusters as the DMDS•− is most likely being solvated on the cluster surface due to the hydrophobicity of the methyl groups. The average values (and their standard deviations) of reaction 2 for the cluster size range of n = 1−6 as calculated at the B3LYP and M06 levels are, respectively, −144 kJ mol−1 (6 kJ mol−1) and −171 kJ mol−1 (4 kJ mol−1) (Figure 7). This amount of energy released is sufficient to evaporate around three water molecules from the resulting DMDS•−(H2O)n clusters which have the average water-binding energies (and standard deviations) of 44 kJ mol−1 (5 kJ mol−1) and 51 kJ mol−1 (8 kJ mol−1), respectively, evaluated at the B3LYP and M06 levels. The average values of the reaction 3 are −70 kJ mol−1 (7 kJ mol−1) (B3LYP) and −78 kJ mol−1 (11 kJ mol−1) (M06). The larger standard deviations are because the dissociation energy decreases with increasing cluster size, which can be attributed to the increasing hydration stabilization of the anionic CH3S− in the larger clusters. The average binding energies (and their standard deviations) of CH3S• in DMDS•−(H2O)n (i.e., (3) − (2): DMDS•−(H2O)n → CH3S−(H2O)n + CH3S•) are 3830



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Figure 8. Geometries and spin density distribution of CO2•−(H2O)n, (CH3S•)CH3SCO2−(H2O)n, and CH3SCO2−(H2O)n, for n = 0−5, were evaluated using the M06 functional and 6-311++G(d,p) basis set for hydrogen and carbon atoms and 6-311+G(3df) basis set for sulfur atom (BS). S−S distances are in Å. Reaction energies ΔH°0/kJ mol−1 were calculated at the B3LYP/BS level (upper values) and M06/BS level (lower values).

calculated to be 74 kJ mol−1 (11 mol−1) at the B3LYP level and 93 kJ mol−1 (12 mol−1) at the M06 level. Losing CH3S• from DMDS•−(H2O)n is both thermodynamically and statistically less competitive compared with the water evaporation due to its larger binding energy and many more water molecules being

available in the clusters. Effective formation of CH3S−(H2O)n is expected to be observed only when the DMDS reacts with the relatively hot (H2O)n•− from which more neutral molecules, including the CH3S•, are being evaporated. 3831



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Reactions of Hydrated Carbon Dioxide Radical Anion CO2•−(H2O)n with DMDS. The experiments show that in the reaction between CO2•−(H2O)n and DMDS a neutral CH3S• is lost and [CH3S,CO2]−(H2O)y is formed, of which subsequent solvent evaporation results in the unsolvated [CH3S,CO2]−. This is presumably a covalently bound ion CH3SCO2− as no further fragmentation was observed. Geometries of the reaction were examined with DFT calculations and are shown in Figure 8. Addition of a DMDS onto CO2•−(H2O)n with n = 0−5 is exothermic and forms a (CH3S•)CH3SCO2−(H2O)n complex which is generated by an attack of CO2•− to an S atom of DMDS forming an S−C bond with distances of 1.91−2.09 Å (B3LYP) or 1.89−2.07 Å (M06). Spin density distribution indicates that the radical is largely localized at the σ* orbital along the S−S bond with distances of 2.88−2.94 Å at the B3LYP/BS level (and 2.85−2.93 Å at the M06/BS level) resembling the S−S distances in CH3SSCH3•−(H2O)n, for n = 0−6 as shown in Figure 6. Evaporation of CH3S• from the complexes yields CH3SCO2−(H2O)n with overall reaction enthalpies being exothermic by 40−46 kJ mol−1 (B3LYP) or 39−47 kJ mol−1 (M06). It is interesting to note that generation of adducts CH3SSCH3•−(CO2)(H2O)n are theoretically possible as revealed from the DFT examinations. In these complexes, the radical originally located at CO2•− is transferred to DMDS resulting in the CH3SSCH3•−(CO2)(H2O)n with the shortest S...CO2 distance increased to 3.14−3.36 Å (B3LYP) or 2.87− 3.16 Å (M06). In these complexes, the bent CO2•− becomes linear CO2 and the C−S bond is elongated. Evaporation of this weakly bound CO2 could give CH3SSCH3•−(H2O)n.. The reaction enthalpies of such an ionic-core exchange reaction for n = 0−5 are exothermic by 50−67 kJ mol−1 (B3LYP) or 63−77 kJ mol−1 (M06), more exothermic than the reaction of CH3S• loss as summarized in Table 3. There is no experimental

Table 4. Experimental and Theoretical Thermochemistry for the Studied Reactions in kJ mol−1a

(1) (2) (3) (4) (5)

0 1 2 3 4 5

(a) CH3S• + CH3SCO2−(H2O)n −45.9 −42.1 −41.5 −41.6 −39.8 −42.9

(−47.2) (−40.5) (−38.6) (−39.0) (−39.8) (−44.2)

(H2O)n•− + MeSH → CH3S−(H2O)m + H• (H2O)n•− + DMDS → DMDS•−(H2O)m (H2O)n•− + DMDS → CH3S−(H2O)m + CH3S• CO2•−(H2O)n + MeSH → HCOO−(H2O)m + CH3S• CO2•−(H2O)n + DMDS → [CH3S,CO2]−(H2O)m + CH3S•

−45 ± 10

36.5

−2.9

−113 ± 13

−143.2

−170.3

−192 ± 16

−76.9

−85.9

+46 ± 13

−12.5b

−6.3b

−6 ± 9

−42.9

−44.2

The evaporated water molecules are left out on the right hand side of the reaction equations for clarity. The theoretical values are shown for the clusters with size of n = 5. bThe ΔH°0 (B3LYP) and ΔH°0 (M06) values are from ref 44.

the reaction efficiency. The calculated values give a more realistic idea of the thermochemistry of this reaction. The other two reactions of hydrated electrons are more exothermic in the calculations by 30−60 kJ mol−1, depending on the functional used. Here the small cluster sizes of the hydrated electrons in the calculations are most likely the reason. Significant additional solvent stabilization will occur at cluster sizes n > 6. The calculated thermochemistry of reactions 2 and 3 support the interpretation of the experiments: Cleavage of the S−S bond is endothermic, and release of a CH3S radical requires a high internal energy of the cluster. For statistical reasons, it is more favorable in smaller clusters, indicating a significant cluster size dependence of disulfide bond cleavage. In the reactions of CO2•− with MeSH and DMDS, experimental and calculated thermochemistries differ by 36− 58 kJ mol−1, with the calculations being more exothermic. In our previous publication, we discussed that the reaction efficiency may be sensitive to the internal energy content of the cluster.44 Also with DMDS, re-evaporation will be more probable with hot clusters, leading to a selective reactivity of cold clusters, which goes along with a smaller number of water molecules evaporating. Since the reaction efficiency is between 30 and 60%, many collisions are not reactive. A slight cluster size dependence may contribute to this effect, if larger clusters react more efficiently than smaller ones. Considering that experiments with cluster sizes around n = 20−60 are compared with calculations at n = 5, the systematic deviation may at least in part due to the different size regimes. Comparison with Pulse Radiolysis. The reactions of gasphase clusters presented here exhibit similarities as well as discrepancies with pulse radiolysis studies. While hydrated electrons react with thiol groups to form SH− in the condensed phase, hydrogen elimination is observed in the gas phase with MeSH, reaction 1.21−24 Without solvation, the pulse radiolysis products CH3•(g) + SH−(g) lie 99 ± 9 kJ mol−1 lower in energy than CH3S−(g) + H•(g).57 Since solvation will stabilize the compact SH− ion more than CH3S−, thermochemistry alone cannot explain the observed reaction with gas-phase hydrated electrons. The selective cleavage of the S−H bond by gas-phase hydrated electrons therefore must have dynamic reasons. In dissociative electron attachment in the condensed phase,65 the extra electron occupies, at least transiently, the lowest unoccupied molecular orbital (LUMO) of the target molecule. In MeSH, the LUMO is a linear combination of p

(b) CO2 + CH3SSCH3•−(H2O)n −67.0 −62.5 −62.6 −57.9 −50.2 −52.5

ΔH°0 (M06)

ΔEnc

a

Table 3. Reaction Enthalpies (ΔH°0/kJ mol−1) of (a) Formation of S−C Bond Together with a CH3S• Loss and (b) Ionic-Core Exchange in the Reaction between CH3SSCH3 and CO2•−(H2O)na n

ΔH°0 (B3LYP)

reaction

(−76.8) (−72.4) (−81.9) (−64.9) (−63.1) (−62.8)

a

Values are evaluated at the B3LYP/BS level (and at the M06/BS level).

evidence for such ionic-core exchange being observed probably because the electron transfer from CO2•− to DMDS is accompanied by substantial geometry changes both for the ionic cores and their solvation structures. These results indicate that the reactions of CO2•− toward a disulfide S−S linkage preferentially form the C−S bond rather than the electron transfer. Comparison of Experimental and Computational Thermochemistry. Table 4 summarizes the nanocalorimetric and computational thermochemistry of the studied reactions. The largest discrepancy occurs for reaction 3, which, as discussed above already on the basis of the experimental results, is due to the significant size and internal energy dependence of 3832



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Figure 9. Potential energy surface of the reaction between CH3SSCH3 and CO2•−(H2O)5. Reaction energies ΔH°0/kJ mol−1 were calculated at the B3LYP/BS level (upper values) and M06/BS level (lower values). Only the structures of the anionic clusters, optimized at the M06/BS level, are shown. The charge-transfer process was not observed in the present gas-phase FT-ICR experiments, indicating that potentially high barrier exists probably due to rearrangement of solvent and charge transfer from bent CO2•− with unfavorable Franck−Condon overlap.

5, facing a smaller barrier than disulfide bond cleavage in RSSR−, the reaction reported in pulse radiolysis.19,20 The products are the same, but the pathway of forming them may be faster, making CO2•− potentially more damaging to proteins than the hydrated electron.

orbitals of the S atom with some contributions from C and H atomic orbitals, leading to an overall antibonding character along both the S−C and S−H bonds.66 While this qualitatively explains the occurrence of bond cleavage, it does not explain why the S−C bond would break in the condensed phase, while the S−H bond breaks in reaction of gas-phase clusters. One may speculate that the methyl group sticks out of the water cluster, while the S−H bond is integrated into the hydrogenbonded network, a different situation from the condensedphase reaction. Looking at the DMDS reactions, the behavior in the gas phase, on the other hand, corresponds very well to the electron attachment to RSSR and cleavage of the disulfide bond as a secondary reaction in pulse radiolysis.24 Quite intriguing is the comparison for CO2•−. In both environments, CO2•− abstracts a hydrogen atom from thiol groups.44 With DMDS, however, gas-phase CO2•− undergoes efficient and selective disulfide bond cleavage, with concomitant formation of a S−C bond. In the condensed phase, charge transfer is observed, essentially the same qualitative behavior as with hydrated electrons.19,20 One possible explanation for this discrepancy is the reduced cage effect in gas-phase clusters. Upon nucleophilic attack of CO2•− at a sulfur atom, a C−S single bond is formed, with the excess electron going into the σ* orbital of the disulfide bond. The thus weakened disulfide bond with bond order 0.5 breaks, and a CH3S radical leaves the cluster. In the condensed phase, the CH3S radical stays in the vicinity, and the disulfide bond may reform. Once charge transfer from CO2•− to the disulfide has taken place, however, back transfer to the linear neutral CO2 will not happen, since it is endothermic. As an alternative scenario, one may speculate that also in the condensed phase, direct charge transfer does not take place, due to the unfavorable Franck−Condon factors associated with the formation of neutral CO2 in the bent geometry of CO2•−, leading to a barrier of unknown height as depicted in Figure 9. In this case, CO2•− will directly cleave the disulfide bond also in the condensed phase, forming RSCOO−. Elimination of CO2 from this unit is calculated to be endothermic by 57 kJ mol−1 at the B3LYP/BS level (66 kJ mol−1 at the M06/BS level) for n =

5. CONCLUSIONS Hydrated electrons and hydrated carbon dioxide radical anions are able to cleave the disulfide bond in DMDS, while the hydrated superoxide radical anion is largely unreactive toward the studied sulfur compounds. Although the superoxide radical anion concentration is used to quantify oxidative stress in biological systems, it is much less reactive than the hydrated electron or carbon dioxide radical anion in the gas phase. The hydrated electron, as well as the carbon dioxide radical anion, reacts with molecular oxygen to form the less reactive superoxide radical anion, a first step to quench radical species and reduce oxidative stress. The reactions of CO2•− with DMDS show that formation of RSCOO− exists as an alternative pathway for disulfide bond cleavage, which may very well be operative also in the condensed phase and in biological environments. In two cases, reaction of MeSH with hydrated electrons and DMDS with hydrated CO2•−, the products formed in gas-phase clusters differ from those in pulse radiolysis studies. These differences offer room for the investigation of the underlying mechanisms, especially dissociative electron attachment. They also indicate that the products formed in electroninduced reaction depend on the size of the solvent environment.

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ASSOCIATED CONTENT

S Supporting Information *

Complete refs 5 and 48; plots of the error function in nanocalorimetry scans of reactions 1, 2, and 3; geometries of CH3S−(H2O)n and (H2O)n•− for n = 1−6 optimized at the M06/BS level; Hess’ law, reactions (9)−(13), including experimental error limits.. This material is available free of charge via the Internet at http://pubs.acs.org. 3833



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-K.S.); [email protected] (M.K.B.). Present Address ¶

Nuclear and Radiation Safety Center, Ministry of Environmental Protection of China, P.O. Box 8088, Beijing, 54 Hong Lian Nan Cun, Haidian, Beijing 100082, P. R. China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from the Deutsche Forschungsgemeinschaft, grant number BE2505/4-2 (M.K.B.), the Alexander von Humboldt-Foundation (Z.S.) and DAAD, PPP Hongkong, project-ID 50750748, is gratefully acknowledged (M.K.B.). The work described in this paper was supported by grants from the Germany/Hong Kong Joint Research Scheme sponsored by the Research Grants Council (R.G.C.) of the Hong Kong and the German Academic Exchange Service of Germany (Reference no.: G_HK006/10) and from General Research Fund from R.G.C. of Hong Kong (Reference No.: CityU 102911) (C.K.S.).

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