J. Phys. Chem. 1992, 96,9181-9194 Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190, 460. (14) Shinohara, H.; Sato, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J. Phys. Chem. 1992,96, 3571. (15) Ross, M. M.; Nelson, H. H.; Callahan, J. H.; McElvany, S.W. J . Phys. Chem. 1992, 96, 5231. (16) Shinohara, H.; Sato, H.; Ohkohchi, M.; Ando, Y.; Kodama, T.; Shida, T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. (17) Yannoni, C. S.; Hoinkis, M.; de Vries, M. S.;Bcthune, D. S.; Salem, J. R.; Crowder, M. S.;Johnson, R. D. Science 1992, 256, 1191. (18) McElvany, S.W. J . Phys. Chem. 1992, 96,4935. (19) Pradeep, T.; Kulkarni, G. U.; Kannan, K. R.; Guru Row, T. N.; Rao, C. N. R. J. Am. Chem. SOC.1992, 114, 2272. (20) Haddon, R. C.; Brus, L. E.; Raghavachari, K. Chem. Phys. Lett. 1986, 125. 459. (21) Gallup, G. A. Chem. Phys. Lett. 1991, 187, 187. (22) Troullier, N.; Martins, J. L. Phys. Rev. B 1992, 46, 1754. (23) McKee, M. L.; Herndon, W. C. J . Mol. Srruct. (THEOCHEM) 1987, 153, 75. (24) R d n , A.; Wbtberg, B. J. Am. Chem. Soc. 1988,110,8701. Rosgn, A,; Wistberg. B. 2.Phys. 1989, 12, 387. (25) Ballester, J. L.; Antoniewicz, P. R.; Smoluchowski,R. Astrophys. J . 1990, 356, 507. (26) Chang, A. H. H.; Ermler, W. C.; Pitzer, R. M. J . Chem. Phys. 1991, 94, 5004. (27) Cioslowski, J.; Fleischmann, E. D. J. Chem. Phys. 1991, 94, 3730. (28) Cioslowski, J. Preprint. (29) Bakowies, D.; Thiel, W. J. Am. Chem. SOC.1991, 113, 3704. (30) Schmidt, P. P.; Dunlap, B. I.; White, C. T. J . Phys. Chem. 1991,95, 10537. (31) Guo, J.; Ellis, D. E.; Lam, D. J. Chem. Phys. Lett. 1991, 184, 418. (32) Dresselhaus, M. S.;Dresselhaus, G. Adu. Phys. 1981, 30, 139. (33) Solin, S.A.; Zabel, H. Adu. Phys. 1988, 37, 87. (34) Johnson, M. T.; Stamberg, H. I.; Hughes, H. P. Sur/. Sci. 1986,178, 290. (35) Belash, I. T.; Bronnikov, A. D.; Zharikov, 0. V.; Palnichenko, A. B. Synth. Mer. 1990, 36, 283. (36) Elser, V.; Haddon, R. C. Phys. Reu. A 1987, 36,4579.
9787
(37) Ruoff, R. S.; Beach, D.; Cuomo, J.; McGuire, T.; Whetten, R. L.; Diederich, F. J. Phys. Chem. 1991, 95, 3457. (38) Cioslowski, J.; Nanayakkara, A. J . Chem. Phys. 1992, 96, 8354. (39) Perdew, J. P.; Zunger, A. Phys. Rev. B 1981, 23, 5948. (40) Ceperley, D. M.; Alder, B. J. Phys. Rev. Lett. 1980, 45, 566. (41) van Duijneveldt, F. B. IBM Research Report RJ945, 1971. (42) Huzinaga, S., Ed. Gaussian Basis Setsfor Molecular Calculations; Elsevier: Amsterdam, 1984. (43) Dunlap, B. I.; Brenner, D. W.; Mintmire, J. W.; Mowrey, R. C.; White, C. T. J. Phys. Chem. 1991, 95, 5763. (44) Brenner, D. W.; Dunlap, B. I.; Harrison, J. A.; Mintmire, J. W.; Mowrey, R. C.; Robertson, D. H.; White, C. T. Phys. Reu. B 1991,44, 3479. (45) Dunlap, B. I. Unpublished results. (46) Dunlap, B. I.; Brenner, D. W.; Mintmire, J. W.; Mowrey, R. C.; White, C. T. J. Phys. Chem. 1991, 95, 8737. (47) Veillard, A. Theor. Chim. Acra (Berlin) 1968, 12, 405. (48) Dunlap, B. I.; Connolly, J. W. D.; Sabin, J. R. J . Chem. Phys. 1979, 71, 3396,4993. (49) Dunlap, B. I.; Mei, W. N. J . Chem. Phys. 1983, 78,4997. (50) Dunlap, B. I.; Cook, M. Inr. J . Quantum Chem. 1986, 29, 767. (51) Jones, R. S.;Mintmire, J. W.; Dunlap, B. I. Int. J . Quantum Chem. Symp. 1988, 22.77. (52) Dunlap, B. I. Phys. Rev. A 1990, 42, 1127. (53) Dunlap, B. I.; Andzelm, J.; Mintmire, J. W. Phys. Rev. A 1990, 42, 6354. (54) Dunlap, B. I.; Andzelm, J. Phys. Reu. A 1992, 45, 81. (55) Dunlap, B. I. Adu. Chem. Phys. 1987,69, 287. (56) Janak, J. F. Phys. Reo. B 1978, 18,7165. (57) Ballester, J. L.; Dunlap, B. I. Phys. Rev. A 1992, 45, 7985. ( 5 8 ) Wolfram Research Muthemutica; Version 2.0; Wolfram Research, Champaign, IL, 1991. (59) Stanton, R. E.; Newton, M. D. J . Phys. Chem. 1988, 92, 2141. (60) Bethune, D. S.;Meijer, G.; Tang, W. C.; Rosen, H. J. Chem. Phys. Lett. 1990, 174, 219. (61) Wang, K.-A.; Wang, Y.;Zhou, P.; Holden, J. M.; Ren, S.;Hager, G . T.; Ni, H. F.; Eklund, P. C.; Dresselhaus, G . ; Dresselhaus, M. S.Phys. Reu. B 1992, 45, 1955.
Ab Initio Molecular Orbital Calculations on DNA Base Pair Radical Ions: Effect of Base Pairing on Proton-Transfer Energies, Electron Affinities, and Ionization Potentials Anny-Wile Colson,+Brent Besler,t and Michael D. Sevilla*-t Department of Chemistry, Oakland University, Rochester, Michigan 48309, and Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: July 6, 1992)
Ab initio molecular orbital calculations have been performed in this study to estimate proton-transfer energies in DNA complementary base pair radical ions and the effect of base pairing on ionization potentials and electron affinities. The calculated (3-21G) adiabatic proton-transfer energy profile for the neutral GC is found to have a single potential minimum, i.e., no stable proton-transfer state, whereas the GC'- shows a second potential minimum which favors proton transfer (AE = -5 kcal). High level calculations (6-31+G(d)) of various uncommon protonation states of the DNA bases and DNA base radical ions were performed to estimate the energy for proton transfer in ' F A , 'C+G, TN-, and CW-. All transfers are energetically unfavorable, but proton transfer in the AT cation radical and anion radical is only slightly endothermic. Base pairing is not found to significantly affect the ionization potential of A or T in the AT base pair. However, base pairing lowers guanine's ionization potential by 0.54 eV while raising cytosine's ionization potential by 0.58 eV. Base pairing revem the order of the ionization potentials and electron affinities of thymine and cytosine which makes cytosine the most electron affinic DNA base and the least likely to be ionized. The order of ionization potentials in base pairs calculated at the 3-21G level is C > T >> A > G. Further investigation was performed on stacked four base (AT/GC) configurations. A 3-21G calculation of the anion radical of the stacked system with the neutral base pair geometries shows the electron localizes on thymine. However, on relaxation of the nuclear framework of the AT/GC system, the electron is found to preferentially localize on cytosine. Calculations on the effect of ion formation on hydrogen bond strengths in base pairs suggest that the hydrogen bonds in the GC anion and cation are greatly strengthened over the neutral GC parent, whereas only the hydrogen bonds in the cation are strengthened in AT over the neutral parent.
Introduction Initial localization of charge in irradiated DNA has been the focus of a number of studies which s& to aid our understanding of primary radiation damage processes in D N A . I - ~ Experiments Oakland University. *Wayne State University.
0022-365419212096-9781$03.00/0
on yirradiated DNA at low temperature have shown that the electron 10CdiZes preferentially on the cytosine base in doublestranded DNA and thymine in single-stranded DNA whereas the hole localizes On guanine in both forms.' The initial localization of the electron and hole on the DNA will depend largely on the electron affinities and ionization potentials, respectively, of the individual DNA bases. It is clear from a number of recent studies Q 1992 American Chemical Society
Colson et al.
C
G
C
H
*G+
Figure 1. Scheme showing electron gain and loss reactions as well as subsequent proton-transfer reactions in DNA base pairs investigated in this work. All base pairs were fully optimized at the 3-21G basis set. The nomenclature indicated below each structure is employed throughout this work.
that proton-transfer reactions between base pair ion radicals or to and from hydrogen-bonded water molecules can also be important determinants of ion radical stabilization and migration in DNA.4*5In our previous work,6 we performed ab initio molecular orbital calculations on DNA bases and their ion radicals in several protonation states in order to elucidate electron affinities, ionization potentials, and proton-transfer reactions. In this work, we perform ab initio molecular orbital calculations with full geometry optimizations to determine proton-transfer energies in DNA complementary base pair radical ions. The effect of base pairing on ionization potentials and electron affinities is also investigated. Base pairing considerably affects the ionization potentials of G and C and the electron affinity of C. Our results suggest a slightly modified model for electron localization in DNA.
Method of Calculation We first performed ab initio SCF full geometry optimizations on the following base pair species: GC, GC'-, ' G V , AT, AT'-, and 'A+T. Subsequently, the product of proton-transfer reactions involving the aforementioned base pairs underwent full optimization with the exception of the neutral AT base pair (Figure 1). Next, we have performed calculations on two classes of DNA components (Figure 2). We completed the series of neutral protonated/deprotonated DNA base radicals (structures I) and nonradical protonated/deprotonated DNA base ions (structures 11) not investigated in our previous study.6 2-matrices, (x,y,z) coordinates, and numbering schemes for all optimized structures are reported in the supplementary material. (See paragraph at the end of the article regarding supplementary material.) Calculations were performed using the Gaussian 90' system of programs on Dec 5000/200, IBM R6000/530, and Cray YMP computers. Spin-restricted open-shell Hartree-Fock (ROHF)* theory was applied throughout this work since UHF calculations
showed large spin contamination? The protonated/deprotonated individual DNA bases were geometry optimized at the 3-21G'O basis set followed by singlepoint calculations at the 6-31G*'' and 6-31+G(d)I2 levels. Partial and full geometry optimizations of the base pairs were carried out at the 3-21G level followed by single-point calculation in the 6-31+G(d) basis set in the case of the fully optimized base pair geometries. Full optimization of the base pairs in which proton transfer had occurred was performed at the 3-21G level. Results and Discussion I. ProhmTramfer Ewrgies in DNA co"entruy Base Pair Radical Ions. A. Proton-Transfer Profdm in GC and cc'-. Because of the widespread occurrence of proton-transfer reactions in chemical and biological processes, numerous experimental and theoretical studies have focused on this phen~menon.'~-'~ More specifically, extensive theoretical investigations on proton-transfer reactions occurring in DNA have been carried In this work, we investigate the effect of transfer of the central proton in the neutral and anioNc radical guanine-cytosine base pair. Full geometry optimizations were performed on GC and GC'- base pairs in the 3-21G basis set. In subsequent single-point calculations, the central proton initially bound to N1 of guanine was moved in 0.1-A increments toward N3 of cytosine without optimization of nuclear coordinates (nonrelaxed case). The results of these calculations for the nonrelaxed proton-transfer energy profies in GC and GC' are shown in Figures 3 and 4, respectively. These figures also present adiabatic proton-transfer profiles calculated for both GC and GC'- (relaxed case). The relaxed or adiabatic profiles were obtained through a set of calculations in which full 3-21G geometry optimizationsof the base pairs were performed at fixed N-H bond distances (shown in Figures 3 and 4).
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9789
DNA Base Pair Radical Ions
H
H
H
-
p '
p '
9
A
*A (+Hi+) H
H
A(+H~+)+ Figure 2. Structures and nomenclatures for DNA protonated and deprotonated bases treated in this work. Structures are shown for the neutral radical bases (I) and the ion nonradical bases (11). non.relaxed
-926.76
-926.76
14
-926.80
.
1
./
adiabatic
d
h
0
non-relaxed adiabatic
2.0
2.2
II
-926.78
1
0
4b
-926.82.
-926.66 4 0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Guanine N1-H Bond Distance (A) Figure 3. Proton-transfer energy profile in GC at the 3-21G level. The nonrelaxed proton transfer, which starts with a fully o timized GC base pair and increments the NI-H proton transfer in 0.1- steps, shows one well-defined minimum at 1.02 A. Due to the presence of a plateau at 1.5-1.8 A, the activation and thermodynamic energies are equivalent and are equal to 3 1.3 kcal. Upon relaxation of the system (full optimization), the activation energy is lowered to 18 kcal and the plateau is shifted to shorter distances. The large black circles are full optimizations. The
1
small light gray circles are simply single-point calculations after shifting the proton by small increments from the optimized positions. The nonrelaxed proton-transfer energy profile in GC shows the expected minimum at a N,-H bond length of 1.02 A but a relatively flat region near the position corresponding to the transfer of the proton to cytosine. Along this nonrelaxed path, 31.3 kcal is required for the proton transfer to occur. This result is in very good agreement with previous theoretical works in which the transfer energy varies between 32 and 42 Upon nuclear relaxation of the system (adiabatic pathway), the activation energy is lowered to 18 kcal and a plateau similar to that in the nonrelaxed path is observed, although it is shifted to shorter distances. The plateau for proton transfer in GC is unexpected as it has been postulated that adiabatic single proton transfer would be characterized by a double potential well.30 Figure 4 presents the calculated proton-transfer energy profile in GC'-. Unlike GC, a double well potential is observed. For the nonrelaxed pathway, a substantial activation energy of 11.4 kcal is required. After proton transfer from guanine to cytosine is complete, the base pair anion is destabilized by 3.3 kcal with respect to the initial structure. If the system is allowed to relax,
-926.824 0.8
I
1.0
1.2
1.4
1.6
1.8
Guanine N1-H Bond Distance (A) Figure 4. Proton-transfer energy profile in G C -at the 3-21G level. In the nonrelaxed pathway, an activation energy of 11.4 kcal is required. As expected, a smaller activation energy is needed along the adiabatic (geometry optimized) pathway. The proton transfer is found to be thermodynamically favored by 4.9 kcal after full optimization. and hence follow the adiabatic pathway, only 4.6 kcal is needed to activate the system which after proton transfer results in a structure 4.9 kcal more stable than the starting base pair anion radical. Since proton transfer can occur much more rapidly than ring relaxation, it is very unlikely that the adiabatic pathway will be followed spontaneously. However, fortuitous inter- and intra-ring vibrational modes could account for a proton-transfer energy profile which resembles that of the adiabatic pathway. Proton tunneling could of course provide an even lower energy pathway. B. Estimntes of Proton-TransTer bergies in ' F A , TA'; 'CC, and CG'-. In order to consider the stabilization of the cationic pyrimidine radicals and the anionic purine radicals by proton transfer, we chose to perform ab initio geometry optimizations on two protonated neutral purine radicals ('A(+Hl+), 'G(+H11+)) which originate from 'A- and *G-, respectively, two deprotonated neutral pyrimidine radicals ('C(-H8+), 'T(-H3+)) which originate from 'C+and 'T', respectively, and the corresponding nonradical ions (+A(+Hl+), +G(+Hl I+), -C(-H8+), T(-H3+)). In addition, we calculated the stabilization energy of the anionic uracil radical by proton transfer from the natural adenine base. The corresponding proton-transfer reactions and calculated energy changes are presented in Table I. As higher
9790 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 TABLE I: Calculated Energy changes for Proton-Trnmfer R-C~~OM witbh DNA B.se Pair Radical Ioas (kcal/mol)
--
proton-transfer reaction AT" +A(+Hl+)'? '(-H3+)' GC" %(+HI l+)'C(-H8+)c A'T 'A(+Hl+)T(-H3+)G'T 'G(+HI 1+)C(-H8+)U'-A 'U(+HS+)A(-HlO+)-
-
-+
3 -.-
-8.35 21.77 12.39 26.79 34.42
Colson et al. TABLE Ik Calculated Energy Changes for Common Proton-Tmmfer Renctiom witbin DNA Base Pair Radical IOM (kcd/mol) exptl'
AE ~
~ ~
4.49 19.05 5.37 16.90 30.29
5.67 19.23 5.38 27.72 3 1.94
-0.2 -2 -2.2
proton-transfer reactions ('G+)C 'G(-Hl+)C(+H3+)+ ('A+)T 'A(-HlO+)T(+H9+)+ ( T ) A 'T(+HS+)A(-HlO+)('C-)G *C(+H3')G(-HI+)-
--
+
-+
AG
3-21G" 3-21Gb pK (kcal) 0.03 1.18 -0.5 -0.7 19.7 -1.94 26.0 28.2 34.66 4.85' 56.85 59.3 -15.98 -4.91 2-3.5 2-4.8
" Full optimization of individual base components.
Full optimization of individual base components. Energy difference algebraically summed. Single-point calculations from optimized 3-21G geometries. 'Thymine and cytosine species optimized at 6-31G'. Determined from equilibrium constants estimated by Steenken.'
Energy difference algebraically summed.6 Full optimization of the base pairs involved. 'Determined from equilibrium constants estimated by Steenken.' dThe H9-thymine bond was kept constant at 1.008 & . due to a tendency of the hydrogen to transfer back to adenine.
basis sets were utilized to investigate the effect of proton transfer from the cationic pyrimidine radicals to the natural purines, discrepancies were observed. Indeed, the "optimal" 3-21G geometries obtained for the cationic cytosine radical (C"), and for the neutral deprotonated thymine radical ('T(-H3+)), varied with the starting geometries. This problem was not observed with other bases. To overcome these difficulties, both pyrimidine cationic radicals and their deprotonated parent radical species ('T(-H3+), 'C(-H8+)) were fully optimized at 6-31G*. No further dependence on starting geometries was observed at this level. Singlepoint calculations were then performed at 6-31+G(d) on all species. Proton-transfer energies were estimated by algebraic summation of calculations on the individual species. Although the stabilization of the cationic thymine radical by proton transfer to the natural adenine base appears to be energetically favorable at the 3-21G level of calculation, all four transfers are energetically unfavorable at the highest basis sets (6-31GS and 6-31+G(d)). Regardless of the level of calculation, it appears that proton transfer in the AT base pairs considered in this set of reactions is the least endothermic processes. Transfer of a proton from the natural cytosine to the anionic guanine radical is the most unfavored process, requiring about 28 kcal to OCCUT at the 6-31+G(d) level, whereas proton transfer from the natural thymine to the anionic adenine radical is the least endothermic process. Steenken4 has estimated the experimental aqueous solution equilibrium constants ( K ) of three of these proton-transfer reactions. Although the trend in experimental K follows the trend in energies at the 6-31+G(d) level, the estimated experimental equilibrium constants for the proton transfer from *T+to A and T to 'A- are and 102.2,respectively, hence predicting energy favorable processes. In studies of electron attachment to oligonucleotides, BemhardzB has reported that adenine successfully competed with the pyrimidines for the excess electron. By considering our proton-transfer results and the substantially greater electron affinity of thymine, the electron-transfer or overall proton- plus electron-transfer process from 'Tto A is predicted to be highly endothermic by 27 or 32 kcal, respectively, at 6-31GS//3-21G and therefore highly unlikely to occur. These results suggest that adenine anion would only form in an unstacked and non-base-paired adenine. Base pairing of adenine anion to thymine should result in rapid transfer of charge to the pyrimidine. A similar reasoning applies to the proton plus electron-transfer process from *C+to G, which would be endothermic by 40 kcal (6-31G5//3-21G). C. Full Optimization of DNA Base Pair hdid IOM: Proton-Trnnsfer Energies. Full geometry optimizations of base pair radical ions involved in the most " m o n proton-transfer reactions were performed with the 3-21G basis set. Table I1 shows these results, including the calculated energy changes obtained in our previous work on individual bases6 and the corresponding experimental px"s determined by Stemken: The starting geometry of the base pairs utilized in the full optimization calculations consisted of the parameters obtained from the previously 3-21G fully optimized individual speck6 Although in our previous work we noticed a high degree of linearity between theoretical Ah"s (obtained from the energy of the individual species involved) and experimental p r s , the Ah"s obtained from ab initio calculations were higher than the M s estimated from the plrs by a factor of 3. While this difference could have been accounted for by the
TABLE IIk Uncorrected Base Pairing Energies (kd/mol)
a
AE (kcal)
-+ + -
G+C-GC
+
G c''G+ + c
GC''G+C
A+T-AT A T'- AT''A* T 'A'T
3-21G -39.85 -58.41 -57.75
6-31+G(d) -23.02 -34.99 -38.05
expt' -21.0
-23.08 -24.02 -30.03
-10.03 -8.79 -17.05
-13.0
"Yanson, I. K.; Teplitsky, A. B.; Sukhodub, L. F. Biopolymers 1979, 18, 1149.
effect of hydrogen bonding to the solvent, we found that it vanished as we fully geometry optimized the base pairs involved in three out of the four reactions considered. Hence, when base pair interaction energies are taken into account in theoretical calculations, theory and experiment predict not only the similar proton-transfer tendency trends but also similar energy changes. II. Base Pairing Energies. A. Uncorrected Base Pairing Energies. Uncorrected base pairing energies calculated at 3-21G and 6-31+G(d) are reported in Table 111. We define the base pairing energy as the difference in energy between the fully op timized base pair and the sum of the energies of the two individually optimized bases. It is therefore an estimate of the total H-bond strength holding the base pairs together. Dispersion interactions are not accounted for in HartretFock calculations. Calculationswere performed for the neutral base pairs and four of the most common radical ions, Le., 'G'C, GC'-, 'A+T, and AT-. The base pairing energy of GC estimated in the 6-31+G(d) basis set (-23 kcal) is in very good agreement with the experimental value of -21 kcal determined from temperaturedependent field ionization mass spectroscopic meas~rements.3~9'~ Del Bene33 reports theoretical HF-SCF calculations which show a 4 kcal higher stabilization energy of GC in the 6-31G(d,p) basis set than ours. Czerminski et al.34and Langlet et report results for GC base pairing energy which are also in good agreement with ours. In the 6-31+G(d) basis set, the base pairing is stronger for 'G+C than for the neutral GC base pair by about 15 kcal. A similar trend is observed for GC'- whose pairing energy is larger than that of GC by 12 kcal. Hence, addition of an electron to cytosine or removal of an electron from the GC base pair strongly stabilizes the dimer. Those results suggest that ionization of the neutral guanine-cytosinebase pair subsequently strengthens the interaction between the two bases and that unpairing will be unlikely to m u r except under harsh conditions. In Table 111, we also report the base pairing energies of the neutral and ionic radical adeninethymine base pair. The AT base pairing energy is in good agreement with previous experimental3' and theoretical ~ t u d i e s At . ~ 6-3 ~ ~l+G(d), ~~ stabilization energies of AT and A T - differ only slightly (2 kcal), whereas ' A T is substantially (7 kcal) more stable than AT. Overall, the results presented in Table I11 clearly show an increase in stabilization of the base pair as an electron is removed from the purine in a base pair, whereas electron addition onto a pyrimidine strengthens the pairing only in the GC pair, likely due to the fact that guanine, unlike adenine, acts as a net proton donor to its complementary base.
DNA Base Pair Radical Ions
B. C ~ r r e c ttoi ~Base ~ Pairing Energies: Dispersion Energy and Basis Set Superposition Error. To fully account for the stabilizationenergy of the neutral base pairs, basis set superposition error (BSSE)and dispersion energy should be accounted for. In this work, BSSE was corrected by the counterpoise method3&in the 6-31+G(d)//3-21G basis set utilizing the expression described by Yang et al.% In this approach, calculations including the full set of ghost orbitals of the complementary base are performed for each monomer, and the resulting energies are subtracted from the energy of the fully optimized base pair. In addition, corrections for monomer distortion energies are accounted for. The values of BSSE for the neutral GC and AT base pairs are 1.89 and 1.38 kcal, respectively, and the resulting corrected values are -21.13 (GC) and -8.65 kcal (AT). Smaller bases sets used in previous calculationswere found to have larger BSSE. For example, Hobza et al.,37using Huzinaga's minimal MINI-I, find a BSSE of 3.82 kcal for GC and 2.62 kcal for AT, and their overall BSSE corrected values slightly vary from ours (-19.57 kcal for GC and -10.25 kcal for AT). Similarly, FBrner's BSSE values at the S T O - 3 G basis set level were 14.11 (GC) and 8.72 kcal (AT), leading to the corrected values of -15.55 kcal for GC and -5.38 kcal for AT.38 To estimate the contribution of the dispersion energy ED,we utilized London's formula3'
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9'791 TABLE Iv: Eond D&trmar (A) between Dowr d Acceptor Atom
Involved in Base Pair Hydrogen Bonds
A=T NI0-09
NI-N3 GEC OII-NB N1-N3
N10-Q
neutral" AT
2.95 2.82 GC 2.91 2.95 2.86
2.98 2.77 GC 2.78 2.91 2.87
anion radical"
cation radical"
AT-
'A+T
2.68 2.88 GC2.93 2.84 2.65
2.68 2.87 'G'C 2.93 2.81 2.66
" Full 3-21G optimization. TABLE V Effect of Base Pairing w DNA
Base Ionizrrtioa
Potentinla (eV)
individual base pair 3-21G
c G
T A
3-21G
individual A
6-31+G(d)
base pair 6-31+G(d)
A
10.00 7.71 9.59 8.42
0.58 -0.54 -0.15 -0.15
6.24 7.08
-0.66 -0.30
Koopmans" 9.00 8.04 9.45 8.48
9.86 7.46 9.34 8.36
0.86 -0.58 -0.11 -0.12
6.90 7.45
6.13 7.14
-0.17 -0.31
9.42 8.25 9.74 8.57
Adiabatic G
A
where N , and Nyrepresent the number of atoms in bases X and Y involved in the base pair; rij is the distance between atom i in subsystem X and a t o m j in subsystem Y. I and a represent the ionization potentials and the optimum atomic static polarizabilities determined by Kang and J h ~ n .In~ the ~ 6-31+G(d)//3-21G dispersion energies for GC and AT are -9.24 and -8.27 kcal, respectively. No calculations involving radicals were performed as the parameters are valid for neutral molecules only. These values must be considered only rough estimates as the London formula is highly approximate.@ Inclusion of the small BSSE and larger dispersion energy in our previously calculated uncorrected base pairing energies accounts for about 30% of the total stabilization energy, leading to an interaction energy of -30.37 kcal for GC and -16.92 kcal for AT, These values are lower than Hobza et al.'s values of -26.41 and -15.98 kca137as well as FBmer et al.'s values of -24.42 and -1 1.64 kca1.38 C. Relaxation Energy on Base Pairing. For the purposes of this work, the base pairing relaxation energy is defined as the difference in energy between the base pair in which the individual bases are allowed to relax upon base pairing (full optimization) and the base pair in which the optimized geometries of the individual bases are held fixed but the bases are allowed to move with respect to each other to form hydrogen bonds (partial optimization). Full and p t h l optimizationsof the neutral and most common ionic radical base pairs were performed in the 3-2 1G basis set. Starting geometries for the partial optimization consisted of the juxtaposition of the fully 3-21G optimized geometry of the individual DNA bases with their complementary base. Base pairing was defined through one hydrogen bond; this parameter constituted the only variable in the optimization process. Hydrogen-bond alignment was respected as much as possible, i.e., close to 180'. A general trend in relaxation energy was obtained: as the base pairing energy increased, the relaxation energy increased. For example, relaxation of adenine and thymine in AT'stabilizes the structure by about 3 kcal, whereas 'A+T and AT base pairs are stabilized by 2 kcal. Relaxation of the bases in GC and 'G+C stabilized the base pair by 5 kcal; GC'- had the largest rearrangement with an 8 kcal stabilization energy. III. Effect of Ion Radical Formation on Hydrogen-Bond Distances in DNA Base Pairs. Hydrogen-bond distances for N H - 0 and NH-N bonds in the neutral and radical ion DNA base pairs optimized in the 3-21G basis set are presented in Table IV. The distancesobtained for the neutral base pairs are in good agreement with X-ray crystal structure analyses;41indeed, the mean deviation
X-ray data'l AT
6.90 7.38
"Typically, the Koopmans ionization potential is taken as the energy of the HOMO. However, for a system of weakly interacting bawa in a base pair, the Koopmans ionization potentials can be estimated by the energies of the HOMO and the HOMO(-1) of the base pair. in hydrogen-bond length from X-ray data in GC base pair is 0.06 A and only 0.04 A in AT. Surprisingly, hydrogen-bond distances are virtually identical for each base pair type regardless of the sign of the base pair ion radical; Le., AT'- has the same distances as 'A+T and GC-has the same distances as 'G'C. These mdts are likely due to a complementary charge distribution for anions and cations suggested in Figure 1. As a general trend, ion radical formation tends to shorten the NH-mO bonds involving the exocyclic nitrogen of the purines while lengthening the NH-0 involving the exocyclic nitrogen of cytosine. We also observe a lengthening of the NH-N bond when the donor nitrogen is on thymine and a shortening of this same bond when the donor is on guanine. lv. E f f e c t o f B a s e p I l i r i n g o a I ~ ~ P ~ m d ~ Affinities. A. Ioaizotioa Potenti&. Experimental and theoretical ionization potentials of DNA bases have been investigated in several studie~.4~-~~ In our previous work6 we reportedthe vertical, adiabatic, and the Koopmans theorem ionization energies of the individual DNA bases to follow the order T > C > A > G. This order was in very good agreement with experimentale v i d e n ~ e a , ~ . ~ ~ suggesting that the hole should localize on guanine. In this work, we have investigated the effect of base pairing on adiabatic and Koopmans ionization potentials at 3-21G and 6-31+G(d). Table V presents the Koopmans ionization potentials of the four DNA bases calculated in the base pairs. The Koopmans ionization potential is only the energy of the HOMO in an individual base. However, for a weakly interacting base pair, the HOMO and HOMO(-1) provide good estimates of the ionization potential of each base within the base pair. These results suggest that base pairing does not significantly affect the ionization potential of A or T in the AT base pair. However, the Koopmans ionization potential of guanine is lowered by 0.54 eV due to base pairing, whereas the ionization potential of cytosine is increased by 0.58 eV. Hence, base pairing affects the ionization potentials of the DNA bases so as to reverse the order of cytosine and thymine and make cytosine the least likely DNA base to be ionized, Le., C > T > > A > G. B. Electron Affinities. Electron affinities of the four DNA bases have been determined theoretically51"4and experimentally by wile^.^^ Berthod et al. first suggested that the pyrimidines were better electron acceptors than the purines. The results of
9792 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 TABLE VI: Effect of Base Pairlag on pyrimidine Adiabatic Electron ACfinitiea (eV) individual base pa? individual base pair 3-21G 3-21G A 6-31+G(d) 6-31+G(d)' A -1.27 -0.75 0.52 -0.98 0.81 C (in CG) -1.79 -1.48 0.04 -0.99 -1.05 -0.06 T (in TA) -1.52 'The base pair electron affinities are properly those of the base pairs AT or GC, not the individual bases T and C. However, the unpaired electron density is contained predominantely on the pyrimidincs (>99%); thus, the purincs a d to perturb (chiefly by H-bond donation) the electron affinitica of the pyrimidines in the basc pair. The electron affinity of T in AT is less affected bccause both T and A act equally as proton donors and acceptors.
Y ~ u n k i nand ~ ~C ~ m p t o nshow ~ ~ trends in electron affinities which compare very well with that of our previous study, Le., T > C > A > G. Surprisingly, Wiley et al.55found the electron affinities of the bases to be in near inverse order to that found here. However, other experimental evidence with mixtures of nucleotides with dinucleotides indicates guanine to be the least electron affinic base, cytosine and thymine having the highest electron affinities.a,56-58 In our previous work,6 we calculated the adiabatic electron affinities of the individual bases at different basis set levels. We now report in Table VI the adiabatic electron affinities of the bases obtained from base pair geometry optimization at 3-21G followed by single-point calculation at 6-3 l+G(d). Electron affinities follow a trend similar to that observed in ionization potentials. Indeed, at the highest basis set, base pairing increases cytosine electron affinity by 0.52 eV whereas thymine electron affinity is decreased by 0.06 eV. Hence, base pairing is responsible for significantly affecting guanine and cytosine electron affinities and ionization potentials; indeed, this phenomenon makes cytosine the most electron affinic DNA base and lowers the already lowest ionization potential of guanine. This effect can be explained by the number of hydrogens supplied to the hydrogen bonds by each base in the pair. In AT base pair, each base donates one hydrogen to the hydrogen bonding whereas in GC, cytosine donates one hydrogen while guanine supplies two. Hence, the net extra hydrogen-bonded proton to cytosine is responsible for the change of electron affinity and ionization potential trends as bases are paired to their conjugate. In proton donor-acceptor systems, the donor and acceptor commonly show the properties found here for guanine and cytosine, respectively, in the GC base pair.59 V. Spin Density Distributions in Stacked Four Base (ATover CC) Anion Radical System. In our previous work, we employed INDO to investigate spin localization in stacked four base systems. In this work, we have further examined spin density distribution in stacked four base confgurations employing Mulliken population analysis.@ ROHF spin densities do not predict sizable "negativespin densities since spin polarization is not accounted for.61 However, the distribution between ring systems should be adequately predicted as this will be dependent on the energy difference between the HOMOS of the individual DNA bases. We performed single-point 3-21G calculations in which the 3-21G geometry optimized neutral AT base pair geometry was stacked 3.4 A above the 3-21G geometry optimized neutral GC pair. On adding an electron to this system, we observed the spin density to localize on the thymine base (96%) with some sharing with cytosine (4%). Upon stacking the anion radical AT geometry above the neutral GC geometry, we found a system stabilized by 0.65 eV, with a lesser mixing of spin (98% on T, 2% on C). Stacking of the neutral AT base pair geometry over the GC anion radical geometry further stabilized the system, moving the spin to cytosine (99% on C, 1% on T). Finally, a system consisting of the neutral AT geometry stacked over -G(-Hl)C(+H3)' geometry was investigated. As shown in Table VII, this latter system was the most stable. As a consequence, the electron is predicted to fully localize on cytosine. A similar set of calculations involving the AT base pair stacked over the CG pair was carried out; Le., the purine base was stacked over the pyrimidine instead of the purine over purine. Although the trend in relative energy or the site of electron localization in each system did not change, the sharing of spin density between bases was affected by modifying
Colson et al.
W: Effect of Nuclear Framework Relaxation on Anion Spin Deasity in AT/(% .ndAT/CC"
TABLE
C
T
neutral geometries AT/GC
G
0.04' 0.96 0.00 0.W 0.99 0.01 A T - geometry/neutral GC 0.02b 0.98 0.00 0.W 1.00 0.00 c'-G geometry/neutraI AT 0.99' 0.01 0.00 1.W 0.00 0.00 'C(+H3)G(-H,)- geometry/ 1 . w 0.00 0.00 neutral AT 1.OW 0.00 0.00
A
re1 energy (eV)
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.21 -0.65 -0.54 -1.06 -1.23 -1.20 -1.55
"All base pair geometries used in the stacked system were optimized, b A T above GC (purine stacked over purine). 'AT above CG (purine
stacked over pyrimidine).
t 0.0
--2
-0.65
tu
-1.06
-1.20
I
\ / AT G C*-
/
AT 'G(-H) CH.
Reaction Coordinate F i i 5. Schematic diagram showing the ab initio MO (3-21G)energy
changes involved in electron transfer from thymine to cytosine followed by adiabatic proton transfer from guanine to cytosine in a stacked four base system. The transition state [ATGC]'- is the energy of the anion in the neutral ATGC geometry before nuclear relaxation whereas all other states are lowest energy adiabatic ground and transition states. the configuration of the stacked system. Indeed, the calculation involving the neutral geometries predicts the spin density to be shared between thymine (99%) and guanine (1%), while previously shared between thymine and cytosine. As anion geometries (AT-, C'G, *C(+H3)G(-Hl)-) are utilized, no further spin mixing is observed. Figure 5 presents the electron localization and migration upon proton transfer and base pair geometry changes. Based on these calculations, a new model for electron localization in DNA emerges: when adding an electron to the stacked system composed of the neutral base pairs geometries, the anion is predicted to initially localize on thymine. Then, as the system relaxes, the site of the unpaired electron shifts to the cytosine base where it is further stabilized by transfer of a proton from guanine to cytosine as shown in Figure 6.6
Conclusions Ab initio molecular orbital calculations have been performed in this study to determine proton-transfer energies in DNA complementary base pair radical ions and the effect of base pairing on ionization potentials and electron affiities. These calculations support the following tentative conclusions. 1. Base pairing lowers@e's Koopmans ionization potential by 0.5 eV and increases that of cytosine by the same amount, without substantiallyaffecting those of adenine and thymine. Base pairing also increases cytosine's electron affinity by 0.5 eV and has no significant effect on thymine. These calculations suggest cytosine to be the most electron affinic base in double-stranded DNA and thymine to be the most electron affinic in singlestranded DNA in accord with recent Calculations performed on relaxed stacked configurations of AT over GC a h predict the site of the unpaired electron to localize on cytosine.
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9793
DNA Base Pair Radical Ions
-1.06 ev
4.14ev I
I
Nudear Framework Relaxation
Proton T r a d e r
I
Figure 6. Overall electron-transfer energy diagram from thymine to cytosine in a stacked four base system upon nuclear framework relaxation followed by proton transfer from guanine to cytosine. Calculations were performed a t the 3-21G level.
Thus,the order of ionization potentials for base paired DNA bases
is C > T >> A > G. 2. Stabilization of pyrimidine cation radicals and purine anion radicals by proton transfer to their complementary bases is predicted to be energetically unfavorable. Estimates from experim e n d results4suggest similar trends, although one proton transfer reaction was suggested to be favorable. 3. Full geometry optimizations of the complementary base pair radical ions involved in proton-transfer reactions leads to proton-transfer energies comparable to free energies calculated from aqueous solution equilibrium constants: suggesting small AS values for proton transfer.6 4. Transfer of the central proton in the GC base pair anion radical is energetically (adiabatic) favorable with a small activation energy, whereas the GC neutral nonradical parent shows no stable proton-transfer state. 5. GC base pair cation and anion radicals are found to have substantially (SO-70%) greater pairing energies than the neutral GC parent. AT base pair anion radical pairing energy is comparable to that of the neutral AT, whereas AT cation radical has a pairing energy 70% higher than the pairing energy of AT. Moreover, as the base pairing energy increases, the nuclear framework relaxation energy on base pairing increases. Since the pairing energies are mainly a result of H-bond formation, our results suggest these bonds to be strengthened by ion radical formation. 6. In base pairs and stacked base pairs, the unpaired electron density is largely localized on the pyrimidines. When utilizing the neutral geometries in the stacked anion system, some spin sharing is observed between cytosine (4%) and thymine (96%). As the nuclear framework is allowed to relax, the unpaired electron localizes on cytosine (99%) and little mixing occurs (1% o n thymine). Spin delocalization is found to extend only to the base immediately above or below the anionic site, not to the complementary base. 7. Calculations of the overall electron-transfer energy in a four base system predicts cytosine anion to be ca.0.4 eV lower in energy than T.' This would represent the predicted minimum activation energy toward electron migration in DNA. Proton transfer from guanine to the cytosine anion increases the stabilization energy by ca. 0.2 eV and therefore increases the barrier to electron migration slightly. Experimental measurements of the activation
energy of protonation of thymine anion in DNA have been made and are in the range 0.4-0.65 eV.62 Since the first step in the protonation reaction is likely the migration of the trapped electron from cytosine to thymine, the results are in good accord with those found here. It is clear that in DNA the H-bonding of the DNA bases to the first layer of water as well as the DNA backbone will affect the properties of the ion radical species formed in DNAs5 As a consequence, in future efforts, the effects of waters of hydration and the ribose phosphate backbone on the electron affinities and ionization potentials of DNA bases will be investigated. Acknowledgment. We thank the Office of Health and Environmental Research of the Department of Energy (Grant DEFG028ER60455) and the National Cancer Institute of the National Institutes of Health (Grant ROlCA45424) for support of this work. The Chrysler Corp. is gratefully acknowledged for their donation of Cray YMP time for this work. We are grateful to Dr. M. Szczeiniak-Bryant for many helpful discussions. Supplementary Material Available: Tables of optimized parameters, including the numbering systems for 10 DNA base pair species at 3-21G, 13 DNA base species at 3-21G, and 4 DNA species at 6-31Gr, and the total energies of the DNA bases considered in this work and a figure of the structures of 12 protonated/deprotonated neutral radical and ionic bases considered in our previous work6 (21 pages). Ordering information is given on any current masthead page. References a d Notes (1) (a) Sevilla, M. D.; Becker, D.; Yan, M.; Summerfield, S . R. J. Phys. Chem. 1991,95,3410. (b) Yan, M.; Becker, D.; Summerfield. S.;Renke, P.; Sevilla, M. D. J. Phys. Chem. 1992,96, 1983. (2)(a) Bernhard, W. A. J . Phys. Chem. 1989,93,2187. (b) Bernhard, W.A. NATO ASI Ser. H 1991,54,141. (3) Hllttermann, J.; RBhrig, M.; KBhnlein, W. Int. J. Radiat. Biol. 1992, 61,299. (4)(a) Steenken, S.Chem. Reu. 1989,89,503.(b) Steenken, S.Abstracts, 38th Meeting of the Radiation Research Society, 1990; p 55. (c) Steenken, S.Free Radical Res. Commun., in press. (d) Steenken, S.;Telo, J. P.; Novais, H. M.;Candeias, L. P. J. Am. Chem. Soc. 1992,114,4701. ( 5 ) S ~ I O MM., C. R. In The Early Effects of Radiation on DNA; Fielden, E. M., O'Neill, P., Eds.; Springer-Verlag: Berlin, 1991;pp 111-124. (6)Colson, A. 0.; Baler, B.; Close, D. M.; Sevilla, M. D. J. Phys. Chem. 1992,96,661. (7)Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.;Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. GAUSSIAN 90,Revision H; Gaussian, Inc.: Pittsburgh, PA, 1990. (8) (a) Roothaan, C. C. J. Reu. Mod. Phys. 1960.32, 179. (b) Binkley, J. S.;Pople, J. A.; Dobash, P. A. Mol. Phys. 1974,28, 1423. (9) Szabo, A.; Ostlund, N. S.In Modern Quantum Chemistry; McGrawHill Publishing: New York, 1989;p 107. (10)Binkley, J. S.;Pople, J. A.; Hehre, W. J. J . Am. Chem. Soc. 1980, 120, 939. (11) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 66,217. (12)Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J . Compur. Chem. 1983,4, 294. (13) Proton-Transfer Reactions; Caldin, E., Gold, V.,Eds.;Chapman and Hall: London, 1975. (14)Bohme, D. In Ionic Processes in the Gas Phase; Almester Ferreing, M. A.. Ed.:Reidel: Dordrecht. Holland. 1984: D 1 1 1. (15) Scheiner, S.Ace. Chem. Res. 1985, 18, -174. (16) Roszack, S.;Kaldor, U.; Chapman, D. A.; Kaufman, J. J. J. Phys. Chem. 1992,96,2123. (17)LBwdin, P. 0.Adu. Quantum Chem. 1965, 2, 213. (18) Scheiner, S.;Harding, L. B. J. Phys. Chem. 1983, 87, 1145. (19)Scheiner, S.Acc. Chem. Res. 1985,18, 174. (20)Nanata. C.: Ada. M. J. Mol. S t r c t . ITHEOCHEW 1988.179.451. (21)Aiza, hi. i.Coiput. Chem. 1988,9,362. (22)Clementi, E.;Mehl, J.; von Niessen, W. J. Chem. Phys. 197L54.508. (23)Clementi, E.;Corongiu, G.; Detrich, J.; Chin, S.; Domingo, L.Int. J . Quantum Chem. 1984,18,601. (24) Scheiner, S.;Kern, C. W. Chem. Phys. Letr. 1978,57,331. (25)Scheiner, S.; Kern, C. W. J. Am. Chem. SOC.1979,101, 4081. (26)Rein, R.; Ladik, J. J . Chem. Phys. 1964,40, 2466. (27)Rein, R.; Harris, F. E. J. Chem. Phys. 1965, 42, 2177. (28)Rein, R.; Harris, F. E. J . Chem. Phys. 196$,43, 4415. (29)Lunell, S.;Sperber, G. J. Chem. Phys. 1967,46, 2119. (30)Lijwdin, P. 0.Reu. Mod. Phys. 1963,35. (31)Yanson, I. K.; Teplitsky, A. B.; Sukhodub, L. F. Biopolymers 1979, 18, 1149. (32)Sukhodub, L. F.Chem. Reu. 1987,87,589. (33)Del Bene, J. E. J. Mol. Struct. (THEOCHEM) 1985, 124, 201.
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(34) Czerminslri, R.; Kwiatkowslri, J. S.;Person, W. B.; Szczepaniak, K. J . Mol. Strucr. 1989, 198, 297. (35) Langlet, J.; Claveric, P.; Caron, F.; Bceuve, J. C. Int. J. Quantum Chem. 1981; 19, 299. (36) (a) Boys, S.F.; Bemardi, F. Mol. Phys. 1970, 19, 553. (b) Yang, J.; Kestner, N. R. J. Phys. Chem. 1991, 95, 9214. (37) Hobza, P.; Sandorfy, C. J. Am. Chem. Soc. 1987, 109, 1302. (38) F6rner. W.; Otto, P.; Ladik, J. Chem. Phys. 1984, 86,49. (39) Kang, Y. K.;Jhon, M.S. Theor. Chim. Acta 1982, 61, 41. (40) Szczginiak,M.M.;Scheiner, S.;Hobza, P. J. Mol. Struct. (THEQ CHEMI 1988.179. 177. (4 1) ‘Sasaengk, W. Princplcs of Nucleic Acid Structure; Springer-Verlag: New York, 1984; p 123. (42) Doughty, D.;Younathan, E. S.;Voll, R.; Ahdulnur, S.;McGlynn, S . P. J. Electron Socctrosc. Relat. P h e o m . 1978, 13. 379. (43) Lin, J.; Yu;C.; Peng, S.;Akiyama, I.; Li, K.; Li Kao Lee;LeBreton, P. R. J. Am. Chem. Soc. 1980, 102,4627. (44)Dougherty, D.;McGlynn, S.P. J. Chem. Phys. 1977,67, 1289. (45) LeBreton, P. R.; Yang, X.;Urano, S.;Fetzer, S.;Yu, M.;Leonard, N. J.; Kumar, S . J. Am. Chem. Soc. 1990, 112, 2138. (46) Bodor, N.; Dewar, M.J. S.;Harget, A. J. J. Am. Chem. Soc. 1970, 92, 2929. (47) Lifschitz, C.; Bergmann, E. D.; Pullman, B. Tetrahedron Lett. 1967, 46, 4583. (48) Orlov, V. M.;Smirnov. A. N.; Varshavsky, Y. M. Tetrahedron Lerr. 1976, 48,4377.
(49) Hush, N. S.;Cheung, A. S.Chem. Phys. Lett. 1975,34, 11. (50) Aktekin, N.; Pamuk, H. 6. Chim. Acta Turc. 1982, I . (51) Berthold, H.; Giessner-Prettre, G.: Pullman, A. Theor. Chim. Acta 1966,5, 53. (52) Pullman, B.; Pullman, A. Quantum Biochemistry; Wdey-Interscience: London, 1963. (53) Younkin, J. M.;Smith, L. J.; Compton, R. N. Theor. Chim. Acta 1976, 41, 157. (54) Compton, R. N.; Yoshioka, Y.; Jordan, K. D. Theor. Chim. Acta 1980, 54, 259. (55) Wiley, J. R.; Robinson, J. M.;Ehdaie, S.;Chen, E. C. M.;Chen. E. S.D.; Wentworth, W. E. Biochem. Biophys. Res. Commun. 1991,180,841. (56) Gregoli, S.;Olast, M.;Bertinchamp, A. Radiat. Res. 1982,65,202. (57) Sevilla, M.D.; Clark, C.; Holroyd, R. A.; Pettei, M. J. Phys. Chem. 1976, 80, 353. (58) Sevilla, M. D. In Excited States in Organic Chemistry and Biochemistry; Pullman, B.; Goldblum, N., Eds.;Reidel: Baston, 1977; pp 15-26. (59) Mulliken, R. S.;Person, W. B. Molecular Complexes; Wiley-Interscience: New York, 1969. (60) Szabo, A.; Ostlund, N. S. In Modern Quantum Chemistry; McGraw-Hill: New York, 1989; p 151. (61) Hinchliffe, A. Ab Initio Determination o j Molecular Properties; Adam Hilger: Bristol, 1987; p 152. (62) Grislund, A.; Ehrenherg, A.; Rupprecht, B.; Tjilldin, B.; Strijm, G. Radiat. Res. 1975, 61, 488.
Theoretical Study of the Blnding of Ethylene to Second-Row Transition-Metal Atoms Margareta R. A. Blomberg,* Per E. M. Siegbahn, and Mats Svensson Institute of Theoretical Physics, University of Stockholm, Vanadisviigen 9, S - 1 1 346 Stockholm, Sweden (Received: May 14, 1992)
The binding between ethylene and all second-row transition-metal atoms from yttrium to palladium has been studied by using size-consistent correlation methods and large basis sets. The binding energy curve as a function of atomic number of the metal shows the usual characteristic minimum in the middle of the row. This minimum is mainly due to the logs of exchange when the bonds are formed. The strongest bonds are formed by the atoms to the right, for which covalent and donation-back-donation bonding is optimally mixed. The atoms to the left form metallacyclopropanes in which the C-C bond is entirely broken. Comparisons are made both to the componding reaction with methane and to the bonding between metal cations and ethylene.
r.
~atrodu~ti~n The coordination of alkenes to transition-metal centers constitutes the starting point in several important catalytic reactions, e.g., the saturation of unsaturated hydrocarbons and the polymerization of ethylene and other olefins. The polymerization of ethylene, catalyzed by transition-metal compounds in the Ziegler-Natta process, has been thoroughly studied the past decades,’ but the reaction mechanisms are still not well understood. For example, the mechanism for the carbon-carbon bond-forming polymer chain propagation step is still controversial. It is expected that the strength of the metal-ethylene bond influences the C< bond-forming step, which is described as insertion of the ethylene moiety into the metal-alkyl bond. A starting point in an investigation of the mechanisms for ethylene polymerization is therefore to determine the variation in the metal-ethylene binding energies over different metals. In this paper we summarize the results from such a study for the second-row transition-metal atoms. The interaction between a transition-metal complex and ethylene is often described by the Chatt-Dewar-Duncanson mechanism in which ethylene donates parts of the r-electrons to an empty a-orbital of the metal. This weakens the *-bond of ethylene, which makes the r*-orbital lower in energy so that electrons will be accepted from a backdonating d-orbital of the metal atom. This is a very general binding mechanism, which can broadly describe practically all metal-ligand bonds. If the metal-olefin bonding is studied more in detail, it is possible to characterize at least three rather different bonding types that are preferred, as will be demonstrated below, by different transi-
tion-metal atoms. In the first of these bonding types, the backdonating d-orbital on the metal is initially doubly occupied. In the second bonding type this orbital is only singly occupied. In the third bonding type, finally, the binding has a normal covalent character and a metallacyclopropaneis formed. In the latter case the reaction between ethylene and the metal atom can be described as an oxidative addition reaction in which the r-bond of ethylene is broken and two metal-carbon a-bonds are formed. This reaction has many similaritieswith other oxidative addition reactions that have been studied previously, such as the reaction between methane and second-row transition-metal atoms.* It should be added that the distinction between the three types of bonding is not entirely strict. It is, for example, clear that the purely covalent bonding in the metallacyclopropane is just an extreme of the donation bonding with doubly occupied donating orbitals. In the analysis below we have, nevertheless, still found it useful to discuss the binding mechanisms in terms of these three different bonding types. The binding between ethylene and transition metals has been systematically studied previously both theoretically3and experimentally: for the case of positively charged metal ions. By charging the metals it has been possible to study the reactions by mass spectrometric methods, which has given important information about both reaction pathways and enthalpies. Detailed comparisons have then provided valuable information about the accuracy of both the calculations and the experiments. The positively charged transition metals are of considerable interest by themselves, but one of the major goals of these studies has also
0022-36S4/92/2096-9794S03.00/00 1992 American Chemical Society
