Ab Initio Study of Thiol Aqueous Phase Ionization ... - ACS Publications

Aug 2, 1994 - Koopmans ionization energies are 9.68,1.67, and 3.63 eV, ... term, and a discrete hydration shell, leads to a Koopmans value of 5.6 eV...
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J. Phys. Chem. 1994, 98, 10484-10491

10484

Ab Initio Study of Thiol Aqueous Phase Ionization Energies: Methyl Mercaptan and Cysteamine Anny-Odile Colson and Michael D. Sevilla' Chemistry Department, Oakland University, Rochester, Michigan 48309 Received: June 4, 1994; In Final Form: August 2, 1994@

The ionization energies of two thiol model compounds (methyl mercaptan and cysteamine) are calculated at the ROHF/6-31G* level to aid our understanding of the mechanisms involved in DNA radioprotection. Methyl mercaptan, the thiolate anion, and its trihydrated form are fully geometry optimized. The resulting gas-phase Koopmans ionization energies are 9.68, 1.67, and 3.63 eV, respectively. The ionization energy for the solvated methylthiolate anion, CH&(aq), calculated through the use of the SCRF model ( E = 78), the Bom charge term, and a discrete hydration shell, leads to a Koopmans value of 5.6 eV. This result is in good agreement with the corrected vertical solution-phase ionization energy calculated by using the same model (5.4 eV) and with experiment (5.7 f 0.2 eV). The gas-phase ionization energies of cysteamine, its cation, zwitterion, and the pentahydrated form of the latter are reported. We find the Koopmans ionization energy of the anti configuration of the zwitterion to be ca. 6.0 eV. Discrete hydration of the negatively charged sulfur increases the ionization energy by ca. 0.55 eV per water while hydration of the amine group decreases it by ca. 0.1 eV per water. Subjecting the pentahydrated zwitterion to the SCRF model leads to a Koopmans solution-ionization energy of 6.42 eV. These results predict that both the aqueous thiolate anion and the aqueous cysteamine zwitterion will reduce radiation-induced DNA base cation radicals via direct electron transfer as found experimentally. On the basis of energetics of the solvent-solute interactions, we propose that the electron transfer process between the cysteamine zwitterion and DNA cations will be influenced by the intervening solvent and suggest that displacement of the primary solvation layer upon ion pair formation will decrease the zwitterion's ionization energy, thereby facilitating the electron transfer.

Introduction For many years, thiols (RSH) have been known to protect DNA from radiation Owing to a weak SH bond strength relative to most CH bonds,6 thiols are capable of scavenging hydroxyl radicals, thereby reducing the number of OH' radicals susceptible to attack DNA. However, such radioprotection is thought to be negligible under conditions relevant to biological systems.' A second mechanism by which thiols can reduce radical lesions is by what has been termed "chemical repair" in which they react with carbon-centered radicals in DNA by hydrogen addition reaction^:^^ 8, RSH

+ DNA' -DNA + RS'

(1)

The RS' radical, once thought not to be sufficiently reactive to cause damage to DNA, has recently been shown to abstract hydrogens from weak C-H b o n d ~ . ' ~ - lFurther ~ repair by hydrogen atom donation may result in damage fixation if the radical center resulted from an addition reaction, for example by 'OH or A more recently proposed mechanism by which thiols can repair radiation-induced damage is by electron transfer to a DNA cationic radical. Such transfer has been observed between RS- and the DNA base cation radical, G.+,3%l5 as well as between disulfide anion radical, RSSR'-, and DNA'+.16 As part of an ongoing investigation on radioprotective t h i ~ l s , ~ , ' ~inJ ~this J ~ work, we employ ab initio theory to calculate the ionization energies of two thiols: methyl mercaptan and cysteamine in gas phase and in solution. By comparing these results to the previously calculated ionization energies of the four DNA bases,19 we elucidate the possible mechanisms *H.3914

@

Abstract published in Advance ACS Abstracts, September 15, 1994.

0022-365419412098- 10484$04.50/0

by which simple thiols protect DNA by electron-transfer processes and the influence of hydration on these processes. Method of Calculation Ab initio molecular orbital calculations presented in this work were performed on IBM RS 6000, Cray C90, and Cray-YMP computer systems by using the Gaussian 92 program.*O The 6-31G* polarization basis set2' along with the spin-restricted open-shell Hartree-Fock method2* was employed throughout this study to geometry optimize the species of interest. Basis set superposition error (BSSE) was corrected by the counterpoise method.23 To account for possible effects of the solvent on the ionization energies of both model compounds, we used a mixed discrete-continuum model in which the first solvation shell is explicitly included in the solute definition, the remaining shells being modeled by the Onsager reaction field method.24 In this model, the compound is placed in a spherical cavity and immersed in a continuous solvent of desired dielectric constant. To mimic the effect of an aqueous solvent, we chose a dielectric constant of 78. Further considerations of the interactions between the charged species and the solvent are estimated by the use of the Born equation.25 Our solvation model therefore includes solute-solvent dipole-dipole interactions, short-range donor-acceptor-type interactions with the solvent in the first coordinate sphere, and long-range electrostatic interactions of the Born type.26 Cavity formation energies are not included as they are small compared to the uncertainties in the Born term. Moreover, for charged species, they would likely cancel in part with electrostriction effects.

Results and Discussion 1. Methyl Mercaptan. Although this compound presents little interest in terms of its biological potential as a radiopro0 1994 American Chemical Society

J. Phys. Chem., Vol. 98, No. 41, 1994 10485

Thiol Aqueous Phase Ionization Energies

.8 CHQSYHZO),

v

J C

n=O

n= 1

n=2

n=3

n=4

Figure 2. Koopmans energies (bold lines) and corrected vertical energies (dotted lines) of CH3SH, CH&, and CH3S-(H20), (n = 1-4) in gas phase and aqueous solution calculated at the ROHF/6-31G* basis set. The results in aqueous phase are obtained through the use of the SCRF model and the Born charge term. The calculated vertical values were scaled to experiment by using IE(corr) = 0.946IE(calc) 1.387 (see text). The experimental SIE (5.7 f 0.2 eV) was deduced from experiment (see text).

side view

+

d

side view

Figure 1. Optimized geometries at ROHF/6-31G* of (a) CH&(H20); (b) CH3S-(H20)2; (c) CH3S-(H20)3; and (d) CH3S-(H20)4.

tector, its properties as the simplest compound of the mercaptan family can aid in understanding the reactivities of other sulphydryl-containing compounds in radiation biology. In this work, we have geometry optimized CHsSH, its corresponding thiolate anion, and several forms of the hydrated thiolate anion (Figure 1). The calculated C-S bond in CH3SH (1.817 A) is in very good agreement with microwave spectroscopy data (1.819 which also show the molecule to adopt a staggered conformation, as predicted by our calculations. Higher level ab initio calculations2*including electron correlation effect^^^.^^ show no substantial difference with geometries obtained here at ROHF/6-31G*. Upon full geometry optimization of the CH3S- thiolate anion, a slight lengthening of the C-S bond occurs (1.83 A). This geometry is in good accord with MP21 6-3 1G* and MP2/6-3 1+G* optimizations performed on CH3S- 31 as well as experimental geometrical parameter^.^^ Our interest focuses on the ionization energy of the molecule. In obtaining such information, comparison to the previously calculated ionization energies of various DNA fragments is possible. Furthermore, based on this property, predictions about the potential radioprotective role of simple alkyl thiols can be advanced. In this work, we calculate two types of ionization energies: the vertical energy, which is the difference in total energy between the radical and nonradical species in the optimized geometry of the singlet, and the Koopmans ionization energy,34which is the energy of the highest occupied molecular orbital (HOMO). The former tends to underestimate experimental results and therefore usually needs to be corrected, while the latter is generally considered a good estimate of the experimental vertical ionization energy in the gas phase. For instance, in the investigation of the DNA bases ionization energies, we found good agreement (within 0.3 eV) between the Koopmans values and the experimental vertical data.19 At 6-3 1G*, the calculated Koopmans ionization energy of CH3SH is 9.68 eV, as shown in Figure 2. This result is in very good agreement with experimental data (9.44 eV)35and the calculated 19933

Gaussian-2 value (9.55 eV).31 The calculated vertical energy is 8.51 eV, which is 0.93 eV lower than the experimental value. Since a concerted process in which proton transfer from the thiol occurs along with or prior to electron transfer (the energetics is considered below) would allow for radioprotection and because RSH is in equilibrium with RS- in aqueous solution, the thiolate anion has to be examined for its potential radioprotective property. The resulting Koopmans ionization energy of the ion is 1.67 eV, ca. 8 eV lower than that of CH3SH, with degeneracy of the HOMO and HOMO(- 1) energy levels being observed. This calculated ionization energy is 0.2 eV lower than more sophisticated theoretical3 and experimental gas-phase electron affinity of CH3S' (1.861 f 0.004 and 1.871 f0.012 eV).32336337It is important to note that in this instance, because of only slight variations in geometries in going from the anion to the neutral species, the experimentally measured vertical and adiabatic ionization energies are This observation is further verified by the theoretical vertical (0.51 eV) and adiabatic (0.47 eV) ionization energies of CH3S- which we find to lie within 0.04 eV of each other at the 6-31G* level. In this case, the calculated vertical value (0.51 eV) is 1.36 eV lower than the experiment. Efect of Solvation of CH3S- on Ionization Energy. Although the Koopmans IE suggests that all four DNA bases and the deoxyribose would be protected by CH3S-, such a conclusion is not yet reasonable for the effects of solvation have not been considered. In this work, solvation is accounted for by gradually hydrating the thiolate anion through stepwise addition of up to four water molecules. In each step, all the geometrical parameters are optimized, and the resulting geometries of the mono-, di-, tri-, and tetrahydrated thiolate anion are presented in Figure 1. Each of the four water molecules act as hydrogen donors toward the sulfur. We note that in the trihydrated anion, the solvent molecules remain in a trigonal conformation, solely and equally interacting with the anion. We observed an alternate isoenergetic geometry (after accounting for the BSSE) in which all three waters lie on one side of the sulfur anion as a result of hydrogen bonding. Both Koopmans and vertical ionization energies are calculated for each hydrate. Although no experimental ionization energy is available for the hydrates of the thiolate anion, since the calculated vertical values of CH3SH and CH3S- lie below the experimental data, it is reasonable to expect a similar trend for the calculated vertical values

Colson and Sevilla

10486 .I Phys. . Chem., Vol. 98, No. 41, 1994

of the hydrates. As a consequence, we scale the vertical results to experiment by a linear fit of the calculated vertical energies of CH3SH (8.51 eV) and CH3S- (0.51 eV) to the experimental data (9.44 and 1.87 eV, respectively), which yields the relation IE(corr) = 0.946IE(calc)

+ 1.387

(2)

Since the calculated vertical energies of the hydrates of CH3Sall lie between those of CH3SH and CH3S-, eq 2 should yield predictions close to expected experimental values. The corrected gas-phase vertical IEs presented in Figure 2 are in excellent agreement with the calculated Koopmans values, which gave good estimates of experimental vertical gas-phase ionization energies for CH3SH and CH3S-. The gas-phase results show each of the first three water molecules contributes, on average, a 0.6 eV increase to the ionization energy, the trihydrated anion having a gas-phase ionization energy of ca. 3.6 eV, as shown in Figure 2. Additional effects of subsequent hydration layers on the anion ionization energies are investigated by employing the selfconsistent reaction field (SCRF) model described above in which the dielectric constant is chosen to be 78. Furthermore, to account for the ion-dipole i n t e r a ~ t i o na, ~Born ~ charge term25 of the form (3) is added to the energy, where Q is the total charge of the ion, E is the dielectric constant of the solvent (78 in this work), and a0 is the radius of the spherical cavity obtained from the Onsager To our knowledge, there has been no previous application of this model to the determination of Koopmans ionization energies. Comparison of these energies to the corrected vertical values obtained in this model should therefore be informative in validating the use of such a method for the calculation of Koopmans energies in solution. In a first step, we calculate the vertical energies of the thiolate anion and its mono-, di-, and trihydrates with the SCRF method. They amount to 0.59, 1.06, 1.62, and 2.24-eV, respectively. Subsequent correction using eq 2 followed by addition of the Born term yields vertical energies of 4.2, 4.5, 4.8, and 5.4 eV, respectively, as shown in Figure 2. As discussed above, these values are believed to be good estimates of experimental results. The Koopmans energies obtained from the SCRF model and augmented by the Born charge term for the four thiolate anions are presented in Figure 2 and lie by an average of only 0.15 eV above the corrected vertical energies. The vertical and Koopmans energies calculated in aqueous phase (upper part of Figure 2) are truly solution-ionization energies (SIE). We calculate these values as follows SIE = IE,=,,

+ Bom term

(4)

Without consideration of discrete waters of hydration, the calculated SIE for electron abstraction in the process CH3S-(aq)

-.CH3S'(aq) + e-(g)

(5)

is 4.2 (vertical) to 4.4 eV (Koopmans). This result can be compared to the experimental SIE of CH3S- determined from the thermodynamic cycle in Scheme 1, where SIE = -A&(CH3S-) - EA(CH3S') AEh(CH3S.) = 5.7 0.2 eV. fii?h(C&S-) and EA(CH3S') are obtained from the cycle shown in Scheme 2, and h&(CH$') is estimated to be 0.1 eV. All the data presented here are obtained experimentally with the

+

SCHEME 1

*

CH3S- hydration energy (-3.9 f 0.2 eV) deduced from the heat of ionization of e t h a n e t h i ~(which l ~ ~ is assumed to closely resemble that of methanethiol), the solvation enthalpy of CH3SH (-0.26 eV),4l the H+ hydration enthalpy (-11.56 f 0.2 eV) (both corrected by PAV = 0.026 eV to yield the internal energy), and the bond dissociation energy of CH3SH.31 The proton solvation enthalpy is calculated from an average solvation free energy obtained from five separate experimental measurements of the standard hydrogen potential leading to AG's ranging from -254 to -261 kcal/m01~~3~~ and its entropy of hydration -32.7 cal K-' The experimental SIE for reaction 5 (5.7 f 0.2 eV) is 1.31.5 eV higher than the theoretical result obtained above for CH3S-(H20), ( n = 0) (Figure 2). This difference is resolved upon consideration of the mono-, di-, and mhydrated anions ( n = 1, 2, 3 in Figure 2). Indeed, the calculated vertical SIE of the trihydrated ion is 5.4 eV, while its Koopmans SIE more closely agrees with the experimental value. Geometry optimization of a tetrahydrated thiolate anion. in which the fourth proton donor water interacts with the trihydrated anion in a trigonal pyramidal conformation failed due to rupture of the H-bond between the fourth water and the charged sulfur in favor of H-bond formation with a neighboring water. However, a square planar conformation of four waters around the sulfur geometry optimized to the structure shown in Figure Id. It is clear that H-bond formation between neighboring waters significantly weakens the water-sulfur interaction in which the average H-bond length only amounts to ca. 2.62 A. This effect is reflected in the calculated gas-phase Koopmans and corrected vertical ionization energies, which only vary by 0.1 eV from those of the trihydrated anion, while the Koopmans SIE remains unchanged at 5.6 eV ( n = 4 in Figure 2 ) . The results obtained in both configurations suggest that the fourth water molecule will not act as a proton donor as effectively (if at all) as the first three waters and that solvent dynamics at ambient conditions will likely allow, on average, for a net three water molecules acting as proton donors. This result is in very good agreement with the ionic hydration number of HS- (2.8 z t 0.2),26whose hydration thermochemistry has been shown to closely resemble that of CH3S-.44 Since addition of diffuse functions to heavy atoms may impact the ionization energies of the anionic species presented above, the 6-3 1+G(d)//6-3 lG* basis see5 was employed to calculate the gas phase and aqueous phase IEs of CH3SH and its ions. As expected, the use of diffuse functions only slighty affected the ionization energy of CH38H (9.73 eV), while it significantly increased that of CH3S- to 2.21 eV. The 6-31+G(d) Koopmans SIE of the trihydrated anion amounts to 5.9 eV. Since these results appear to more significantly deviate from experiment than the 6-31G* energies, the 6-31G* basis set was employed in the remainder of this work for calculations of ionization energies. DNA Radioprotection by Methyl Mercaptan. Selected energies from Figure 2 are reported in Figure 3 along with ionization energies of various DNA components to which they can be compared. Figure 3 shows the 6-31G*//3-21G Koopmans ionization energies of the four DNA bases19 and the 2'deoxyribose 5'-phosphate (DP) system with a hydrated sodium counterion (1 1.32 eV).33 It also presents the 6-31G*//3-21G

J. Phys. Chem., Vol. 98, No. 41, 1994 10487

Thiol Aqueous Phase Ionization Energies

SCHEME 2 AE= 0.28 ev4'

CH3SH(aq)

t

AE=-0.23ev41

~ E ~ ( c H ~-3.9H.2ev s-)= * CH3S-(g) + H+(gAEh(H+)= C C - 1H1 . 5 i6 k 0- . 2 eb~ ~ q ~'~~ ) + Ht(aq) CH3SH(g) AEo= 15.47 ev31 EA(CH3S*)=-1.86ev32 IP(H*)= 13.60 ev CH3S*(g)+ CH,SH(aq)

+ DNA'+(aq) - DNA(aq) + CH,SH'*(aq) A E = +1 1.7 kcal/mol

(6)

However, although CH3SH appears unable to undergo direct electron transfer, the adiabatic processes shown below are energetically feasible: CH,S-(aq) iDNA'+(aq)

CH3SH(aq)

+ DNA'+(aq)

-

+

CH,S'(aq) DNA(aq) A E = -19.8 k c d m o l (7)

+

+

DNA(aq) CH3S'(aq) H+(aq) AE = -15.5 kcaVmo1 (8)

41

0'

Koopmans IE

SIE

Gas Phase

Figure 3. Koopmans ionization energies of CH&, +NH3(CH&S(Cya) anti and gauche (ROHF/6-31G*), the DNA bases, and the

deoxyribose-phosphatemoiety (ROHF16-3lGW3-21G); adiabatic ionization energies of adenine and guanine in their respective base pairs obtained via the SCRF method with a dielectric constant of 2 to mimic the DNA environment (ROHF/6-31G*//3-21G); 6-31G* solutionionization energies of CH$(H20)3(aq) reported from Figure 2 and (HZO)~+NH~(CH~)~S-(H~O)Z(~~) reported from Table 2 . adiabatic ionization energies of the purines in their respective partially hydrated base pairs, modeled in a DNA environment (G; 6.56 eV; A, 7.27 eV) (the same approach as that used in Colson et al.46was applied at the 6-31G*//3-21G level). As previously reported, the trend in Koopmans ionization energies follows the order G < A < C < T < DP, hence favoring guanine for hole localization. The stabilizing effect of base pairing and partially solvating the systems lead to a decrease in IE, still favoring guanine as the hole acceptor, with an adiabatic IE of 6.56 eV. With a calculated corrected vertical SIE of 5.4 eV (5.7 21 0.2 eV experimental), it appears that the thiolate anion of methyl mercaptan can readily reduce radiationinduced cation radicals of all four DNA bases as well as the sugar moiety. Such repair will involve electron transfer from the thiolate anion to the damaged entities. In this work, we have also considered the monohydrated CH3SH species as a potential electron donor. The binding energy of the first water amounts to 1.95 kcaumol, indicating SH hydrogen-bonds only very weakly as expected. The calculated Koopmans SIE of the monohydrated species obtained in the Onsager model (9.23 eV) and the Born term calculated for the cation radical reveal that electron transfer alone from the thiol to the DNA cation radical (using a calculated adiabatic IE for guanine in a DNA environment of 6.56 eV) is unlikely to occur in the adiabatic process:

Both reactions are predicted to be exothermic. The second reaction is driven by the proton solvation energy and is likely to be slower than the direct electron-transfer process.47 Binding Energies for the Thiolate Anion- Water Complexes. We have calculated the total energy change at T = 0 K for the CH3S-(H20)n-1 H20, where n = reaction CH3S-(H20), 1-4 at the 6-31G* and 6-31+G(d)//6-31G* basis sets, and accounted for the BSSE by the counterpoise method. The differences in zero-point energies were not considered. Each structure involved in the reaction was fully geometry optimized at 6-3lG* as described above and is presented in Figure 1. The resulting 6-31+G(d) BSSE corrected energies for the loss of one water molecule from the mono-, di-, and trihydrated thiolate anions are 12.5, 11.3, and 9.9 kcaumol. (Table 1). The binding energy of the first water is slightly lower than that calculated by Gao et al.,48 who applied some geometry constraints to the system and did not account for BSSE at this level of calculation. Experiments performed on the first three hydrated anionic complexes using pulsed high-pressure mass spectrometry4 reveal stepwise dissociation enthalpies of 15.0, 13.5, and 1 1 . 1 kcal/mol, respectively, in the 238-417 K temperature range, which lie within 2 kcal/mol of our calculated dissociation energies. Earlier in this work, we mentioned a trihydrated and a tetra-hydrated species involving hydrogen bonding among the first hydration layer. In these cases, a larger BSSE is observed, and the BSSE corrected energies amount to 9.8 and 10.8 kcal/ mol, respectively. The latter value differs from experiment by only 1.2 kcal/mo1.44 Although the calculated BSSE corrected binding energies appear to be somewhat underestimated for n = 1-3, we have not accounted for dispersion energy (electron correlation), which will increase the binding energies. 2. Cysteamine. In this work, we geometry optimize cysteamine H*N(CH&SH, its cation H3N+(CH2)2SH, and its zwitterion H3N+(CH&S- as well as various hydrated forms of the latter. The 6-31G* optimized structures are presented in Figures 4 and 5. In treating the neutral free base, we arrive at three possible conformations (Figure 4): one anti and two

-

+

Colson and Sevilla

10488 J. Phys. Chem., Vol. 98, No. 41, 1994

-

TABLE 1: CalculatedaWater Binding Energies for the H20 (kcallmol) Reaction CH~S-(HZO)~ CH~S-(HZO)~-~ ~~~

+

~~~

~~~~

6-31G* AE

6-31+G(d)//6-31G* AE

n

exptlbAH

uncorr

BSSEcorr

uncorr

BSSEcorr

1 2 3 3' 4'

15.0 13.5 11.1

14.3 12.9 11.8 14.5 19.1

12.9 10.4 8.5 (9. (ll.O)d

12.8 12.1 11.2 12.0 15.3

12.5 11.3 9.9 (9.8)d (10.8)d

9.6

Calculated values are total energy difference at T = 0 K and do not account for the small differences in zero-point energies. Sieck, L. W.; Meot-Ner, M. J . Phys. Chem. 1989,93, 1586. Reported as AH in the temperature range 238-417 K. Theoretical results involved H-bonds among waters in the geometry optimized species. Large BSSEs are found in water clusters. Only a few of the calculations involving ghost orbitals were performed and an estimate of the total BSSE was made. a

HzN(CH2)zSH anti

HzN(CH2hSH gauche

@-

*H3N(CHz)zSH

'H3N(CHz)zS anti

HzN(CHz)2SH gauche"

n

n

'H3N(CHz)zS- gauche

Figure 4. Fully geometry optimized structures of cysteamine and its cation and zwitterion at ROHF/6-31G*. The order of stability for the three possible geometries of the free base is gauche > anti > gauche". Although less stable than thegauche configuration in the gas phase, the anti conformer appears to be the preferred form in aqueous solution.

gauche (the hydrogen of the SH group pointed toward the nitrogen in the most stable case, away from it in the other gauche configuration (gauche* in Figure 4)). With regard to the relative stability, the gauche configurations are separated by ca. 2 kcal/ mol, while energetically, the anti conformation lies in between the gauche values. However, in terms of the Koopmans ionization energies, it appears that the anti conformation is the most difficult to ionize, with a calculated Koopmans IE of 9.71 eV, the gauche configurations having nearly equal IUEs (-9.34) (Table 2). The free base is not relevant in biological systems, since, at pH 7, the main forms of cysteamine are the cation H3N+(CH&SH and the zwitterion H3N+(CH&S-. The optimized geometry of the cation for which the calculated Koopmans ionization energy is 13.7 eV is presented in Figure 4. At 6-3 lG* the zwitterion optimizes to two possible configurations (Figure 4), the anti conformation being less stable than the gauche by nearly 20 kcdmol in the gas phase. This is chiefly due to the strong ionic interactions between S- and NH3+ in the gauche conformer, where the heavy atoms are separated by 2.86 8, as opposed to 4.11 8, in the anti configuration. The Koopmans ionization energies for the gauche and anti structures are 7.06 and 6.04 eV, respectively (Table 2). As expected, because of the strong electrostatic interaction, more energy is required to extract an electron from the gauche zwitterion. Vertical and adiabatic ionization energies corrected to equation (2) were also calculated and are presented in Table 2. In the anti conformation, the 6-31G* corrected adiabatic and vertical values are confounded, as observed in methyl mercaptan. On the other hand, the corrected adiabatic ionization energy of the

gauche form is nearly 0.5 eV lower than its vertical counterpart, owing to a considerable geometry reorganization upon radical formation. It is important to note that analogous conformations have been the~retically~~ and e~perimentallf~observed in sulfonium compounds, which in aqueous solution adopt an extended rather than a folded configuration and hence allows for full solvation of charge centers. Therefore, we assume the stabilization energy resulting from solvation of the anti conformation is greater than that resulting from attraction of unlike charges in the gauche conformation and further consider only the anti conformation. To account for possible effects of hydration on the ionization energy of cysteamine, the anti form of the charged aminothiol was hydrated by five water molecules (Figure 5). Since nearly 60% of the total negative charge localizes on the sulfur of the zwitterion as opposed to 80% in the methylthiolate anion, the net number of water molecules acting as hydrogen donors toward the sulfur atom of the zwitterion is likely to be less than three. In a first step, two waters arranged in the optimized geometry of the di-hydrated methyl thiolate anion hydrogen bonded the sulfur of the zwitterion. All parameters were fixed except for the H-bonds, which optimized to 2.58 and 2.50 8, at the 6-31G* level. In a second step, geometry optimization of the parameters involved in positioning three water molecules with respect to the amine group of the fixed ion was performed, leading to hydrogen bond distances of ca. 2.00 8,. The final step consisted of geometry optimizing the H-bond lengths in the pentahydrated structure (Figure 5 ) for which bond angles, torsion angles, and the remaining bond distances were kept fixed in the geometry obtained above. The average H-bond length between the solvent and the charged groups did not change appreciably (Figure 5a,b). Hydration at the sulfur increases the Koopmans ionization energy (by +1.10 eV yielding 7.14 eV) more significantly than hydration at the amine group decreases it (by -0.38 eV yielding 5.66 eV). This can be understood on the basis of charge localization. The electron donation effects of the waters hydrating the amine group on decreasing the ionization energy is small (ca. 0.1 eV per H20), while contribution of the waters directly hydrating the negatively charged sulfur to the increase in ionization energy is more significant (0.55 eV per H20). Simultaneous hydration of both charged ends (Figure 5 ) results in a Koopmans IE of 6.79 eV, an overall increase of 0.75 eV over the non-hydrated species. The corrected vertical gas-phase ionization energy of the pentahydrated species was also calculated and amounts to 6.56 eV. Additional considerations of the effects of solvation of the zwitterion were taken in this work by employing the SCRF method with a dielectric constant of 78, a method which has mainly been used for determining the solvation effects on neutral and ionic species. In the case of methyl mercaptan, we observed that the Koopmans SIE calculated in this model compared well with experiment. Table I1 shows the Koopmans energy of the anti conformation of NH3+(CH&S- is 5.26 eV, while discrete hydration by five waters molecules raises it to 6.42 eV. The corrected calculated vertical values are 5.0 and 5.6 eV, respectively, but should be more representative of adiabatic values with regard to the time scale of events, since in correcting the vertical ionization energies, a full Born charge term for the resultant cation was accounted for. In a truly vertical process, the solvent cannot reorganize and therefore only some fraction of the Born charge term due to the interaction of the solvent with the positively charged amine end of the molecule should be accounted for. This would lead to somewhat higher vertical SIEs and hence to better agreement with the Koopmans values.

J. Phys. Chem., Vol. 98, No. 41, 1994 10489

Thiol Aqueous Phase Ionization Energies

TABLE 2: Calculated 6-316" Ionization Energies of Cysteamine and Its Ions" (eV)

Koopmans vertical adiabatic

9.7 1

9.34

9.33

13.68

7.06 7.15" 6.68"

6.04 5.94" 5.88"

5.26 (5.O)d,e 4.8d

6.79 6.56"

Figures 4 and 5. SCFW method used with dielectric constant E = 78. " Corrected [IE(corr) = 0.946IE(calc) relation in c and the Born charge term. e This value is more representative of an adiabatic result (see text).

g&

u-----180

160

140

120

8 (degrees)

,'

i 1.95A I

a

+ 1.3871.

Corrected by the

:] 60

60

-s:

180

160

140

120

100

80

60

8 (degrees)

Figure 6. Potential energy surface calculated at 6-31G* for the binding of the first water molecule to CH$- and +NH3(CH2)2S- as a function of the H-S-C angle (kcdmol). The values shown are not corrected for BSSE (see Table 1).

2.56A

W

100

6.42 (5.6)d*e

8 b

Figure 5. Geometry of the pentahydrated cysteamine zwitterion calculated at ROHF/6-31G*. The constraints on the geometry and optimization procedures are described in the text. (a) and (b) show the hydration patterns around the sulfur and the amine group viewed along the S-C and N-C axes, respectively. For clarity, the hydrogens of the CH2 groups are not represented.

DNA Radioprotection by Cysteamine. Solely based on the Koopmans ionization energies calculated here and in our previous works in gas-phase model^,^^.^^ both the anti and gauche configurations of the zwitterion appear capable of protecting the sugar moiety of the DNA strand and all four DNA bases, as shown in Figure 3. In aqueous solution, the vertical S E of the pentahydrated zwitterion lies lower than the 6-3 1G*/ /3-21G adiabatic ionization energies of the purines in their respective partially hydrated base pairs, modeled in a DNA environment (G, 6.56 eV; A, 7.27 eV). Hence, reduction of the radiation-induced cation radicals of the DNA bases by electron transfer from the cysteamine zwitterion is predicted to occur. 3. Potential Energy Surface for the Binding of the First Water Molecule to CH3S- and +NH3(CH2)2S-. Since solvation strongly affects the ionization energy of CH3S- and NH3(CH2)2S-, further consideration of the energetics of the solvent-solute interaction is of interest. The binding energy of the first water molecule to the thiolate anion and cysteamine zwitterion was calculated at 6-31G* as a function of the angle formed between the hydrogen of the water and the S-C bond of the ions. The results, which do not account for BSSE (ca. 1.4 kcdmol, see Table 1) are presented in Figure 6. Four angles were used: 75.4", 105.4", 140", and 180". The 75.4" angle results from the geometry-optimized water in the monohydrated cysteamine zwitterion; it was kept fixed in the monohydrated +-

thiolate anion while the other parameters linking the water to the anion were varied. The 105.4"angle results from the fully geometry optimized mono-hydrated thiolate anion. It was kept fixed in the monohydrated cysteamine zwitterion, the other parameters linking the water to the ion were varied. For both thiols, the 180" angle was fixed, while varying the parameters linking the water to the ions. The geometry resulting from this partial optimization was used for a single point calculation performed with a HSC angle of 140". The geometries are reported in the supplementary material. Figure 6 shows a nearly uniform interaction energy of the first hydration water in the vicinity of the methyl thiolate anion, the energy ranging from 12.1 to 14.3 kcal/mol. This is not true, however, of the cysteamine zwitterion for which we observe an increase in binding energy from 6.3 to 13.0 kcal/mol as the water moves around the negatively charged sulfur. The weak binding energies calculated in the 140-180" region are comparable to a single H-bond between two waters.50 It is clear the 140 to 180" region could easily be deprived of solvent in the cysteamine zwitterion, whereas that is not true for the methyl thiolate anion. Conclusions Over the years, mechanisms involved in the radioprotection of DNA by thiols have been the focus of numerous experimental but few theoretical studies.52 Our work using ab initio molecular orbital calculations at the ROHF/6-31G* level was undertaken primarily with the aim of elucidating the radioprotective capabilities of two model compounds toward oxidative DNA ion radicals. Calculations incorporating the effects of solvation show that the ionization energies of thiols in solution are effectively modeled by discrete stepwise hydration by individual water molecules followed by immersion of the hydrated species in a dielectric medium. Our application of the self-consistent reaction field method to the calculation of Koopmans ionization energies in solution

10490 J. Phys. Chem., Vol. 98, No. 41, 1994 is novel. In the case of methyl mercaptan and its thiolate anion, the validity of this model is supported by the very good agreement (within 0.15 eV) between calculated vertical values scaled to experiment and Koopmans-SCRF energies. For methyl mercaptan and cysteamine, calculations predict the solvated thiolate anion and the cysteamine zwitterion can readily reduce radiation-induced cation radicals in DNA via a direct electron-transfer mechanism as expected from experiment.3,15*51,53 The solvating method employed here (which combines the use of discrete water molecules to simulate the first hydration layer and the use of the Born charge term and the Onsager model to mimic further effects of subsequent layers) estimates the net number of proton donor water molecules in the primary hydration layer. In this work, three such waters mimic the experimentally determined solution-ionization energy of CH3S- (5.4-5.6 eV theoretically vs 5.7 f 0.2 eV experimentally). Each water increases the ionization energy of CH3S- by approximately 0.4 eV in a dielectric medium. Note that the solvent polarization effect (Born term) alone accounts for ca. 2 eV. For cysteamine, our calculations show that discrete solvation of the zwitterion by five water molecules and immersion in a dielectric medium ( E = 78) results in a calculated value of 5.6 eV, which is representative of an expected adiabatic result in solution. This value lies below the adiabatic ionization energies of the DNA purines calculated in a DNA environment. From these results, it is clear that the number of waters acting as proton donors in the primary hydration layer around the negatively charged sulfur substantially affects the IEs of both CH3S- and the cysteamine zwitterion (ca. 0.4 eV per H20). It is clear that the dynamics of the first solvation layer will affect the ionization energies of radioprotectors and consequently the driving force toward radioprotection. The fact that cysteamine zwitterion’s waters are less tightly held than for CH$- suggests these dynamics may more greatly affect cysteamine’s ionization energy. In this regard, it is interesting to conjecture on the effects of intervening solvent upon interaction of the DNA cation radical with the cysteamine zwitterion. The solvent between the negative end of the zwitterion and the DNA base pair cation radical will either be maintained in part forming a solvent-shared ion pair or displaced resulting in a contact ion pair.26 The contact ion pair may apply to the interaction of cysteamine with the DNA base pair cations (CG‘+ and TA’+) as the base pair cation’s interactions with water are relatively weak, ca. 4.9 and 2.9 kcallmol per water, respectively!6 Electron transfer from the cysteamine zwitterion to the DNA cation radical would occur more readily in an inner-sphere ion pair than in an outer-sphere ion pair, where transfer likely occurs by tunneling through the intervening waters. Displacement of the zwitterion’s primary solvation layer upon ion pairing also will result in a decrease in the zwitterion’s ionization energy (0.4 eV/molecule) thereby further facilitating electron transfer. Acknowledgment. We thank the National Institutes of Health (Grant ROlCA45424) and the Office of Health and Environmental Research of the Department of Energy (Grant DEFG0286ER60455) for support of this work. We thank the DOE National Energy Research Supercomputer Center at Lawrence Livermore National Laboratory and the DOE supercomputer Center at Florida State University for generous grants of computer time. Supplementary Material Available: Computer outputs of (x,y,z)coordinates for 23 species obtained at the ROHF/6-31G* level (9 pages). Ordering information is given on any current masthead page.

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