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Chem. Res. Toxicol. 1996,8, 499-505
Quantum Mechanical Studies of the Structure and Reactivities of the Diol Epoxides of Benzo[c]phenanthrene Lan Lewis-Bevan,:P$ Stephen B. Little,§ and James R. Rabinowitz*lt Carcinogenesis and Metabolism Branch, Health Effects Research Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 27711, and Integrated Laboratory Systems, Inc., Research Triangle Park, North Carolina 27709 Received November 28, 1994@
Benzo[c]phenanthrene has a crowded bay region that has been called a fjord region. As a result of the interaction between the atoms across the fjord region, it is a nonplanar molecule with a significant barrier between two helical structures. The crowding in the fjord region also affects the three-dimensional structure of the fjord region diol epoxide. Quantum mechanical studies have been performed to determine the structure and reactivities of the fjord region diol epoxides. Eight local minimum energy three-dimensional structures have been found for the trans diol of 1,2,3,4-tetrahydro-3,4-dihydroxybenzo[clphenanthrene 1,2epoxide. They can be characterized by three dichotomies: one between syn and anti, one between quasidiaxial and quasidiequatorial, and the third that depends on nonplanarity of the parent polycyclic aromatic hydrocarbon due to interactions in the crowded bay region, that we have named “in” and “out” based on the position of the epoxide oxygen relative to the distal ring. The structures with the epoxide oxygen on the same side of the saturated ring as the distal ring (in-) are more stable than the structures where the epoxide is on the opposite side (out-). The calculated lowest energy syn and anti structures for the diol epoxide of benzoklphenanthrene are both in-quasidiequatorial, in agreement with experiment. Analysis of the results indicates that the electrostatic interaction across the fjord region could be responsible for the increased stability of the syn-in-quasidiequatorialstructure compared to the syn-inquasidiaxial structure and the stability of the in- structures in general when compared to the out- structures. These calculations suggests that the electrostatic contribution of the distal ring in the fjordhay region may play a part in the interaction with nucleophiles.
Introduction Polycyclic aromatic hydrocarbons (PAHsY are a class of pervasive environmental chemicals. Some members of the class are potent mutagens and animal carcinogens while other class members show little similar activity after considerable testing (1,2). Studies have shown that metabolism is necessary for carcinogenic activity (3, 4 ) , and it is a metabolite of the PAH that binds to DNA (5, 6). For benzo[alpyrene (BaP) and other PAHs with a bay region, the bay region diol epoxide has been shown to be the carcinogenic metabolite (7). “he epoxidation of the 9,lO-bond of the trans 7,8-diol of BaP can form two possible diastereomeric bay region diol epoxides, one where the distal hydroxyl group and the epoxide oxygen are syn and one where they are anti. Each of these diol epoxides has two possible conformations, one where the hydroxyl groups are nearly in the plane of the remainder of the PAH (quasidiequatorial) or one where they are nearly perpendicular to that plane (quasidiaxial). There is little or no barrier to the shift between these two conformations, and only the lowest energy conformation is found for each diol epoxide (8,9). For BaP, experimental studies have shown that the syn
’ U S . Environmental Protection Agency.
i L.L.-B. is a postdoctoral fellow in the curriculum in Toxicology of the University of North Carolina. Integrated Laboratory Systems, Inc. @Abstractpublished in Advance ACS Abstracts, April 1, 1995. Abbreviations: PAHs, polycyclic aromatic hydrocarbons; BaP, benzo[alpyrene; BcPH, benzo[clphenanthrene; 12-MBA, 12-methylbenzlalanthracene.
6
7
Figure 1. The numbering scheme used in this paper for 1,2,3,4tetrahydro-3,4-dihydroxybenzo[clphenanthrene1,2-epoxide.
diastereomer is found to have quasidiaxial hydroxyl groups, while for the anti diastereomer they are quasidiequatorial (9). Additionally, there are two enantiomers for each possible diol epoxide. Large differences in relevant biologial activities have been observed between these various bay region diol epoxide structures of the same PAH (10 , l l ) . Benzo[c]phenanthrene(BcPH) is a PAH with a crowded bay region that has been named a fjord region. BcPH is, at most, a weak carcinogen in animal tests (12). However, some studies have shown that both of its fiord region diol epoxides are among the most active diol epoxides ever tested (13-15). The crowding in the bay region of BcPH has some important effects on its molecular structure. If the molecule was planar, the distance between the two hydrogens in the fjord region (1,12; see Figure 1)would be 0.88 A (16). The steric and electro-
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static interactions between the fjord region hydrogen atoms cause the lowest energy structures to be nonplanar (16, 17). Two helical structures exist with the same energy. The computed barrier for the conversion between these two structures is 6-7 kcallmol (16). Molecular mechanics studies have shown that the steric interaction in the fjord region is still important for determining the structure of the fjord region diol epoxides (18). Previous quantum mechanical studies (19)indicate that for BcPH the syn diastereomer should be quasidiequatorial and the anti diastereomer quasidiaxial. This is the reverse of what is seen experimentally for BaP (8,9). Experimental studies, however, have shown that both the syn and anti forms of the diol epoxide of BcPH have quasidiequatorial hydroxyl groups (9). If the helical structure of BcPH is maintained after the formation of the diol epoxide, there will be two possible structures for each of the structures previously considered for BaP. In one of these structures, the epoxide oxygen is on the same side of the saturated ring as the distal ring of the remaining conjugated system, and in the other, they are on opposite sides. The interaction between the epoxide and the distal ring, particularly, the hydrogen in the fjord region, could provide considerable energy for the stabilization (destabilization) of the various diol epoxide isomers. In the current study we have used computational methods to investigate the structure and reactivities of the conformations of 1,2,3,4-tetrahydro-BcPH 3,4 diol 1,2epoxide to determine the effect crowding in the fjord region has on molecular structure, properties, and the capacity to react with nucleophiles.
Methods The starting geometries for the determination of structure were obtained from previous studies of BcPH (16) using standard bond lengths and angles for the diol epoxide. The epoxide oxygen and the hydroxyl groups were considered in eight classes of initial geometries. The 3,4-hydroxyl groups were always trans relative to one another. The epoxide oxygen was either on the same side (syn) as the 4-hydroxyl group or on the opposing side (anti). Both hydroxyl groups were either near (quasidiequatorial) the plane of the major molecular system or nearly perpendicular (quasidiaxial) to that plane. The distal ring was either on the same side (in) of the saturated part of the system as the epoxide oxygen or on the opposite side (out) (see Figure 2). For each of these eight possible types of starting structures, the dihedral angle of the hydroxyl hydrogens was varied systematically. This gave us many different starting structures for each of the eight conformational types. Additional starting structures were generated from the known planar structure of phenanthrene merged with each of the four relevant tetrahydro trans diol epoxides of cyclohexene. This gave four additional types of starting structures t h a t do not require i d o u t designations. These structures were first minimized with AM1 (20)while the phenanathrene part of the molecule was constrained to remain planar. This provided good alternate starting structures with a planar phenanthrene. These planar structures were then also used as additional starting geometries in the search of the conformation space. AM1 (201,a semiempirical quantum mechanical method, was used to minimize the molecular energy as a function of internal atomic coordinates for each starting geometry. AM1 h a s been shown to produce accurate geometries for similar molecules (16, 20,211 and to produce reasonable results for molecules in this class when molecular energies are compared between similar molecules (22). When more t h a n a single local minimum was found for one of the eight types of structures, the three lowest energy structures were retained for the remainder of the study. These geometries were then used for a series of single point ab initio Hartree-Fock calculations using the Gaussian series
Lewis-Bevan et al. Table 1. Relative Energy (kcal/mol)from Quantum Calculation of Conformers of Diol Epoxides of BcPH la
AM1
0.16 SM2 0.30 hU3-21g 0.0 hf76-31g* 0.56
lb
IC
Id
0.0 4.77 2.36 0.0 3.67 1.88 2.64 10.92 4.39 1.26 8.72 3.33
2a
2b
0.49 0.70 0.83 0.67 0.44 6.71 0.0 4.83
2c
2d
2.72 5.52 2.37 3.92 3.95 15.12 3.95 10.48
of programs (23),and molecular energies and electronic distributions for each structure were obtained. Both the 3-21g and 6-31g* split valence basis sets were used. The 6-31g* basis set, that includes diffise functions on heavy atoms, has the flexibility to provide a more complete description of the charge distribution of these molecules. This increased flexibility is important because of the large partial charge expected on the epoxide oxygen. The same molecular geometries obtained with AM1 were used for single point calculations with the SM2 model in AMSOL (24, 25). These calculations were used to determine the effects of bulk water on the molecular energies and electronic structures. SM2 is a semiempirical program that includes the water surrounding the molecule in a continuum approximation by the addition of reaction field polarization effects, cavitation, dispersion, and hydrophobic effects in a generalized AM1 procedure. For other molecules of this type, molecular geometries obtained by SM2 minimization were found to be nearly identical to geometries obtained with AM1 (22). Therefore, the computer intensive effort of obtaining new molecular geometries was bypassed. Two methods were used for computing atomic charges for comparison and computation of the intramolecular electrostatic interaction. In one method, the Mulliken definition of atomic charge (26) was used, and in the other, atomic charges are obtained from a fit to the computed molecular electrostatic potential of the entire molecule (27). Accessibility of atomic surfaces within the structures was calculated using the algorithm in AMSOL (24).
Results Using A M I , at least one local minimum of each of the eight types of structures of the diol epoxide of BcPH discussed above was found (see Figure 2). Many of the starting geometries minimized to the same final geometries. The starting geometries with planar phenanthrenyl structures all minimized to in- structures with nonplanar phenanthrenyl moieties. Occasionally, a starting geometry with one type of structure would minimize to a geometry of another type. This was particularly true for anti-quasidiaxial starting geometries minimizing to anti-quasidiequatorial minimum energy structures and syn-out-quasidiequatorial minimizing to syn-in-quasidiequatorial. Table 1 shows the relative energies of the lowest energy local minimum for each type of geometry and each quantum mechanical method used in this study. For each quantum mechanical method, the minimum energy structures for each of the four in- structures ( l a , lb, 2a, and 2b) is significantly more stable than the structure of the corresponding out- structure (IC,Id, 2c, and 2d). The energies from both semiempirical methods do not indicate which of the in- structures will be favored, but they do show that Id and 2c will be the favored outconformations. The AMSOL calculations, that include the effects of a bulk water environment, show smaller energy differences between the in- and out- conformations than any of the other methods. The ab initio Hartree-Fock calculations show larger differences between the in- and out- conformers than the semiempirical calculations. They clearly indicate that the anti-quasidiequatorial conformations (2a and 2c) are favored over the anti-quasidiaxial conformations (2b and
Quantum Mechanical Studies: Benzo[clphenanthrene
Chem. Res. Toxicol., Vol. 8, No. 4,1995 501
6 1a syn-in-quasidiequatorial
1b syn-in-quasidiaxial
I
1c syn-out-quasidiequatorial
1d syn-out-quasidiaxial
6 2a anti-in-quasidiequatorial Q
2b anti-in-quasidiaxial
Q
"
6 2c anti-out-quasidiequatorial
2d anti-out-quasidiaxial
Figure 2. The eight local minimum energy structures found with A M 1 for 1,2,3,4-tetrahydro-3,4-dihydroxybenzo[clphenanthrene 1,2-epoxide.la: syn-in-quasidiequatorial;lb: syn-in-quasidiaxial; IC: syn-out-quasidiequatorial;Id: syn-out-quasidiaxial; 2a: antiin-quasidiequatorial;2b: anti-in-quasi-diaxial; 2c: anti-out-quasidiequatorial; 2d: anti-out-quasidiaxial.
2d) for both in- and out- structures and that Id (quasidiaxial) is favored over IC(quasidiequatorial)for the synout- conformation. However, these ab initio calculations show that for the low energy syn-in- structures the quasidiequatorial conformer ( l a ) is favored over the quasidiaxial conformer (lb), in contrast to the results for the higher energy out- structures. These ab initio Hartree-Fock results are in agreement with the experimentally determined configurations of the diol epoxides (9). They show that the lowest energy syn- conformation and the lowest energy anti- conformation both have quasidiequatorial hydroxyl groups ( l a and 2a). For comparison, similar calculations on the bay region diol epoxides of benzo[a]pyrene (BaP) and 12-methylbenz[alanthracene (12-MBA)are reported (see Figures 3 and 4). BaP is planar, and therefore, in- and out-conformers are identical. 12-MBAhas a crowded bay region, similar to that of BcPH. The 12-methyl group is in the bay region. The angular ring and the methyl group are bent in opposite directions to minimize the destabilizing interaction between the methyl hydrogens and the hydrogen on the angular ring that extends into the bay
region. The bay region diol epoxide of 12-MBA has the possibility of eight structures that are similar to the conformers of BcPH. The epoxide oxygen is on the same side of the system as the 12-methyl group for instructures and the opposite side for out- structures. Many different starting geometries were used for each of the eight possible structures, but AM1 found local minima with only six of these structures. No local minima were found with either the syn-out-quasidiequatorial or the anti-out-quasidiaxial,even though a number of starting geometries with those conformations were used. Those starting structures minimized to other types of structures, primarily the corresponding in- structures. The results for BaP are shown in Table 2. The AM1 energy differences between the four local minima obtained for BaP diol epoxide are not significant. However, the ab initio Hartree-Fock calculations clearly show that the syn-quasidiaxial and anti-quasidiequatorial are favored, in agreement with experimental determinations (8)and other quantum mechanical calculations (28,291. The calculations performed on the diol epoxides of 12MBA support the results obtained for BcPH (see Table
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Lewis-Bevan et al.
syn-quasidiequatorial
syn-quasidiaxial
anti-quasidiequatorial
anti-quasidiaxial
Figure 3. The four local minimum energy structures found with A M 1 for 7,8,9,10-tetrahydro-7,8-dihydroxybenzo[ulpyrene 9,lOepoxide.
sy n-in-quasidiequatorial
syn-in-quasidiaxial
syn-out-quasidiaxial
anti-in-quasidiequatorial
anti-in-quasidiaxial
anti-out-quasidiequatorial Figure 4. The six local minimum energy structures found with A M 1 for 1,2,3,4-tetrahydro-3,4-dihydroxy-12-methylbenzo[ulanthracene 1,a-epoxide.
2). The in- conformers are clearly preferred over the outconformers. From these results, it appears as if there are no local minima (or at least, the minima occupy so little of the conformational space that we are unable to find them) for the highest energy out- conformations. The anti-in-quasidiequatorialconformation is favored over the anti-in-quasidiaxial conformation. It appears that the
syn-in-quasidiequatorialconformation is favored over the syn-in-quasidiaxial conformation for the small basis set calculations, but the larger basis set results are equivocal. The electrostatic interaction between the CH1-013CH2 of the epoxide and the CH12 group (the CH group from the distal ring that crowds into the fjord region, see Figure 1)of BcPH was approximated by computing the
Quantum Mechanical Studies: Benzo[c]phenanthrene Table 2. Relative Energies (kcaYmo1) of the Conformations of Diol Epoxides of BaP and of 12-MBA
AM1
hD3-21g
Diol Epoxides of BaP syn-quasidiequatorial 0.84 5.00 syn-quasidiaxial 0.00 2.01 anti-quasidiequatorial 0.56 0.00 anti-quasidiaxial 0.69 8.61 Diol Epoxides of 12-MBA syn-in-quasidiequatorial 0.00 0.00 syn-in-quasidiaxial 3.16 3.37 syn-out-quasidiaxial 5.30 4.24 anti-in-quasidiequatorial 1.87 1.89 anti-in-quasidiaxial 2.17 6.44 anti-out-quasidiequatorial 5.85 4.51
hD6-31a* 3.93 0.90 0.00 5.53
0.00 0.38 1.70 0.64 3.48 3.25
Table 3. Energy (kcaumol) of the Intramolecular Electrostatic Interaction across the Fjord Region between CH12 and the Epoxide group
l a (syn-in-quasidiequatorial) l b (syn-in-quasidiaxial) IC(syn-out-quasidiequatorial) I d (syn-out-quasidiaxial) 2a (anti-in-quasidiequatorial) 2b (anti-in-quasidiaxial) 2c (anti-out-quasidiequatorial) 2d (anti-out-quasidiaxial)
hD6-3lg*, Mulliken charges
hD6-3lg* chelpg, potential derived charges
-6.63 -3.49 2.19 1.20 -3.62 -6.25 1.29 2.00
-3.04 -1.37 1.28 0.03 -1.45 -1.60 0.06 0.96
electrostatic interaction between the appropriate atomic charges. Two different methods were used for obtaining atomic charges from the quantum mechanical results: (1) the Mulliken definition (26)and (2)electrostatic potential derived atomic charges (27) (see Table 3). These results show that the electrostatic interaction across the fjord region significantly stabilizes the in- conformers and destabilizes the out- conformers. For the in- conformations this stabilizing interaction is considerably larger than the interaction between the epoxide and the distal hydroxyl group that has been postulated to stabilize diaxial configurations when calculated in a similar fashion (results not shown). These results also offer a possible explanation for the observation that both the syn- and anti-diol epoxides of BcPH are diequatorial. The electrostatic interaction between the epoxide group and CH12 stabilizes l a significantly more than it stabilizes lb, calculated with either definition of atomic charge, making l a the lowest energy syn conformation. For the anti conformations, in contrast, the calculations using the potential derived charges indicate that the Coulomb interactions across the fjord region are similar for 2b and 2a and it is other differential chemical forces that determine the lowest energy anti conformation. Some computed reactivity properties of the epoxide group of each structure are shown in Table 4. It can be seen that the charges on the epoxide group do not vary significantly as a function of conformation. They are also very similar to the charges obtained for the bay region epoxide of BaP (not shown). The exposed surface area of the individual atoms in the epoxide ring varies considerably with conformation. It can be seen that the epoxide oxygen (013) is less exposed in the in- conformation than the out- conformation. (The exposure in the out- conformation is similar to the exposure of the epoxide oxygen for the diol epoxide of BaP.) Additionally, axial hydroxyl groups significantly shield the epoxide oxygen from the environment. The benzyl carbon in the epoxide ring is shielded for the out- conformations but only slightly less exposed for the in- conformations of BcPH
Chem. Res. Toxicol., Vol. 8, No. 4, 1995 503
diol epoxide than for BaP diol epoxide. For both in- and out- conformations of BcPH and for BaP, the benzyl carbon (Cl) is most exposed and therefore most available for attack in the syn-quasidiequatorial conformation. A measure of the loss of planarity of the phenanthrenyl moiety of the diol epoxides is the dihedral angle formed by the four carbon atoms that comprise the interior edge of the phenanthrene structure (C12 and three contiguous unnumbered carbon atoms in Figure 1). For the instructures, that angle is between 18" and 19" and for outstructures it is between 22" and 25".
Discussion The semiempirical quantum mechanical method AM1 has been used to search the conformational space of the diol epoxide of BcPH. Eight types of conformations have been found that are local minima in the conformation space. They may be characterized by three dichotomies: one between syn and anti, a second between quasidiaxial and quasidiequatorial, and the third that depends on nonplanarity of the parent PAH due to interactions in the crowded bay region, that we have named in- and out- based on the position of the epoxide oxygen relative to the distal ring. The first two of these dichotomies have been discussed extensively in the literature (9). Each of the quantum mechanical methods used shows that the conformations where the epoxide oxygen is on the same side of the saturated ring as the distal ring (in-) are energetically favored over the conformations where the epoxide ring is on the other side of the saturated system (out-). The conformational space is extremely complex, and we have used a number of different starting points for energy minimizations by varying the dihedral angles of the hydroxyl groups. We have used starting conformations where the unsaturated phenanthrenyl moiety is twisted, as it would be in BcPH or planar as it would be in phenanthrene. Even with planar starting conformations considerable twisting is found in the minimum energy structures. Planar starting geometries minimized to twisted structures that are essentially identical to the minimized in- structures. The solution conformation of an adduct of BcPh in a DNA duplex has been determined experimentally (30). In that study the phenanthrenyl ring system was found to be nonplanar, and the dihedral angle formed by the four carbon atoms that comprise the interior edge of the phenanthrene structure was found to be 18.1". Our AM1 results show that dihedral angle to be 19.0" for the same BcPH diol epoxide. We do not know the height or shape of the barriers between these eight types of minimum energy structures obtained. From some anti-quasidiaxial starting points the search has found anti-quasidiequatorial local minima, suggesting that the barrier for this conversion is low (or nonexistent). From some syn-out-quasidiequatorialstarting structures a syn-in-quasidiequatorial local minimum was reached, suggestingthat, in this complex space, there is a path between in- and out- structures that has at most a small barrier. Experimentally, only one s y n and one anti structure have been found for BcPH, and both of those conformations are quasidiequatorial (9). This is in contrast to BaP, where the anti- conformation is diequatorial and the syn- conformation is diaxial (8). The best quantum mechanical results reported in this study, the ab initio Hartree-Fock calculations (see ref 22) agree with the experimental results. For BcPH, diequatorial structures are the most stable local minimum for both syn and anti
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Table 4. Charge and Accessible Surface Area for Diol Epoxides of BcPHa epoxide 0
c1 c2
charge surface area charge surface area charge surface area
la -0.59 39.29 0.22 12.68 0.25 14.67
lb -0.60 34.36 0.21 9.86 0.29 15.96
IC
Id
2a
2b
2c
2d
-0.57 46.84 0.25 8.14 0.27 13.65
-0.60 39.64 0.26 4.63 0.27 17.63
-0.60 37.59 0.21 10.17 0.27 18.34
-0.59 30.72 0.22 11.94 0.26 19.02
-0.58 43.98 0.25 5.04 0.28 17.30
-0.59 39.34 0.27 6.15 0.26 18.92
Charge is in the unit of electron; surface in in the unit of A2.
Table 5. Computed Interatomic Distance (A) across the Fjord Region
la lb IC
Id 2a 2b 2c 2d
H12-0 (epoxide)
H12-Cl
2.18 2.62 3.82 3.83 2.60 2.22 3.83 3.89
2.31 2.37 2.61 2.46 2.37 2.32 2.45 2.60
H12-H1 2.26 2.10 1.81 1.99 2.11 2.24 1.99 1.86
conformations. The difference in energy between the inand out- structures makes it unlikely that the outstructures are experimentally important even if the barrier between in- and out- forms is significant. As a result of the interaction between the two hydrogen atoms in the fjord region, BcPH is a nonplanar molecule. In this nonplanar structure the hydrogens in the (helical) fjord region are 2.04 A apart (17). The distances between nonbonded atoms in the fjord region for the minimum energy structures of BcPH diol epoxide identified in this study are found in Table 5. It can be seen that for all the in- structures the H-H distance across the fjord region is greater than the distance in the parent PAH. The steric repulsion between 013 (the epoxide oxygen) and H12 for the same structures is larger than the steric repulsion between the two fjord region hydrogen atoms (H1 and H12) because of the larger steric radius of the oxygen atom. The results in Table 5 suggest that it is not the difference in the steric interaction that significantly stabilizes the in- conformations relative to the outconformations. The electrostatic interaction across the fjord region has two components, a stabilizing interaction between CH12 and 0 1 3 and a destabilizing interaction between the CH12 and CH1-CH2. For the in- conformations the former is larger than the latter and the electrostatic interaction may be seen as providing the stabilization of the in- conformations relative to the outconformations. Comparing the structures of l a and lb, it can be seen that the stabilizing part of the electrostatic interaction across the fjord region (the interaction between 013 and H12) will be larger for l a and the destabilizing part of the interaction larger for l b (the interaction between H1 and H12). It is the attractive electrostatic interaction across the fjord region that provides the energy for the stabilization of the syn-in-quasidiequatorialconformation ( l a ) relative to the syn-in-quasidiaxial conformation (lb), not the repulsive (destabilizing) steric interaction. For the syn-out conformation where the electrostatic interaction across the fjord region is repulsive, for both structures, the axial conformation is favored, as it is in BaP. It is also interesting to compare the electrostatic interaction across the fjord region for the anti structures. The electrostatic interaction may stabilize 2b relative to 2a, but apparently that interaction is smaller than other interactions that stabilize 2a relative to 2b, as the total molecular energy favors 2a (which is reinforced by
comparing these results to the results in Table 2 for BaP where the energy difference between the two possible anti structures is larger than the energy difference between the two syn structures). From Table 3 it may be seen that the differences between the use of two alternative definitions of atomic charge in the computation of the electrostatic interaction across the fjord region are primarily quantitative. The potential derived charges reduce the size of the interaction compared to the Mulliken charges. The only qualitative difference is that the electrostatic stabilization of 2b relative to 2a is insignificant using the potential derived charges. Considering only the electrostatic interaction computed with potential derived charges, it is easier to see the importance of electrostatics across the fjord region for determining the lowest energy molecular geometry of the diol epoxides of BcPH. It has an important effect for determining the lowest energy syn structure but is not a differential factor for determination of the anti structure. The potential derived charges are a set of atomic charges that best reproduces the molecular electrostatic potential about the complete molecule. Therefore, it includes, in an approximate manner, the effects of other atoms besides those in the fjord region in the computation of the electrostatic interaction across the fjord region. Because of the diffuse basis functions on the oxygen and carbon atoms in the 6-31g* basis set, the Mulliken definition of charge may assign charge segments that are physicaly closer to one atom to another atom. This may account for the qualitative difference between the electrostatic interaction computed with Mulliken charges and potential derived charges. The actual magnitude of the electrostatic interaction across the fjord region will, in any case, be attenuated by any nearby water molecules. The importance of the electrostatic interaction across the fjord region for the stabilization of the syn-inquasidiequatorial conformation of BcPH may provide some insight into the forces that contribute to the binding of nucleophiles to the bay region diol epoxides of PAHs. The contribution to the electrostatic potential of the fjord region proton from the distal ring (and the entire distal ring) of BcPH in and near the fjord region is positive. For all baylfjord region diol epoxides, there is a similar proton that encroaches into the bay region. It contributes to the capacity of the baylfjord region of the molecule to attract negatively charged species or regions of large molecules that have a partial negative charge and orients the molecule relative to charge separation in large molecules. The electrostatic potential in the fjordhay region acts to stabilize zwitterionic intermediates with the correct orientation. This does not necessarily ensure the formation of a chemical bond, which also depends on electronic effects in the final product and postulated intermediates. Additionally, for the low energy inconformations the epoxide surface is shielded by the twisted phenanthrenyl ring system. Similar calculations on the final products and postulated intermediates may provide information on the effects of the unique baylfjord
Quantum Mechanical Studies: Benzo[c]phenanthrene
region structures on the electronic aspects of diol epoxide reactivity.
Conclusions By use of quantum mechanical methods, eight unique, significantly different local minima for the fjord region diol epoxides of BcPH have been identified. Of these, in agreement with experimental results, the most stable structures for both the syn and anti conformations are diequatorial. As in the parent PAH, the diol epoxides minimize the destabilizing interactions in the crowded fjord region by adapting a helical structure. Electrostatic interactions across the fjord region play a major part in stabilizing the unusual syn-quasidiequatorial structure relative to the syn-quasidiaxial structure. Electrostatic interactions across the bay region may be important for determining the reactivities of bay region diol epoxides and the orientation of nucleophiles in the bay region. Further calculations on postulated intermediates and products are needed to understand the relative importance of electrostatic interactions and electronic effects in PAH diol epoxides.
Acknowledgment. We thank Drs. S. Nesnow, C. Waller, and A. Dipple for helpful discussions during the course of this study. The research described in this paper has been reviewed by the Health Effects Research Laboratory of the U S . Environmental Protection Agency and approved for publication. Approval does not signify that the contents necessarily reflect the views and policy of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. The calculations were performed at the National Environmental Supercomputer Center of the U.S. Environmental Protection Agency.
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