Chem. Res. Toxicol. 1992, 5 , 286-292
286
Epoxide Ring Opening and Related Reactivities of Cyclopenta Polycyclic Aromatic Hydrocarbons: Quantum Mechanical Studies James R. Rabinowitz*yt and Stephen B. Little$ Carcinogenesis and Metabolism Branch, Health Effects Research Laboratory, Environmental Protection Agency, Research Triangle Park, North Carolina 27711, and Environmental Health Research and Testing, Znc., Research Triangle Park, North Carolina 27709 Received October 28, 1991 A series of 13 cyclopenta polycyclic aromatic hydrocarbons have been studied using quantum mechanical methods. The three-dimensional molecular structure of each carbocation that might result from the opening of a protonated epoxide ring formed between the carbon atoms completing the cyclopenta ring was computed with AM1. AM1 and ab initio calculations, using a split valence basis set, were then used to predict the direction of ring opening and obtain information about the reactivity of the carbocation. These calculations have shown that for all carbocations studied the cationic charge is well distributed throughout the molecule. The largest CH group charges are approximately 0.3 electron. If the protonated epoxide ring can open so that the nominal charge is on a CH group that is attached to the central ring of an anthracenic core, that carbocation will be greatly favored. For carbocations of this type, the unoccupied a' position (the CH group opposite the position of attachment to the anthracenic core) has as much or more of the cation charge as the nominally charged CH position. The group charges, and other properties related to electrostatic reactivity, clearly favor addition of nucleophiles at the unoccupied a' position over addition a t the nominally charged position. However, when the addition of small nucleophiles at both of these positions is modeled for two such examples, the results favor addition a t the nominally charged position in one case and are equivocal in the other case. The group charges and other reactivities considered characterize the electrostatic part of the interaction. Factors reflecting the electronic configuration of the possible products are also important for determining the interaction. Of the molecules that do not contain an a' carbon atom, only for cyclopenta[cd]pyrene 3,4-epoxide is there a large difference in energy between the two possible carbocations. The favored carbocation for this molecule has the most complete charge delocalization of any carbocation in this study.
Introduction Polycyclic aromatic hydrocarbons (PAHs)' are a pervasive class of anthropogenic chemicals found in the environment. Molecules within this chemical class show considerable variation in toxicity. Some class members are powerful mutagens and animal carcinogens while other molecules show no similar activity after considerable testing ( 1 , 2 ) . The activity of many class members falls between these two extremes. The molecular mechanism of action has been studied in detail for a few class members (3), and that mechanism has been generalized to explain activity within the class. While the details of the molecular mechanism of activity depend on the test system (animal species, organ, etc.) and the molecular structure of the particular PAH being considered, there is always at least one metabolic oxidation step to a more polar and more reactive molecular intermediate. The resultant molecule interacts with a biopolymer to form a covalent bond at a nucleophilic site. For the most carefully investigated specific mechanism of action the relevant metabolic products are epoxides. The PAH-epoxide may bind to the biopolymer by three slightly different paths: (1) it may interact directly with a biopolymer, opening to form the observed adduct, (2) it may be protonated and then interact with the biopolymer forming similar adducts, (3) it may be protonated, followed by ring opening to form a carbocation which binds to electron-rich regions in the * Author to whom correspondence should be addressed. t Environmental
Protection Agency. f Environmental Health Research and Testing, Inc.
biopolymer to form the observed adducts. In each mechanistic sequence the resulting adduct is the same but the manner in which chemical factors in the environment influence the reaction may be different. For instance, it hm recently been suggested that the biopolymer may catalyze the epoxide ring opening. The epoxide is much more likely to be protonated in the vicinity of a DNA polymer because of the substantial increase in proton concentration near the polyanionic DNA polymer (4). The protonated epoxide can open spontaneously to form the reactive hydroxy carbocation. For each slightly different mechanism the molecule that interacts with the biopolymer may be more than a single oxidation step removed from the parent PAH. Cyclopenta polycyclic aromatic hydrocarbons (cPAH) are a subclass of environmentally relevant PAHs. Some members of this subclass have been shown to have carcinogenic (5), genotoxic (6),and tumor-initiating (7) activity in animals and to induce anchorage-independent growth in normal human cells (8). The bond completing the cyclopenta ring (see Figure 1) is shorter and has a greater electron density than most other bonds in the aromatic system. It has similar electronic characteristics to K-region bonds. The bonds connecting these carbon atoms to the PAH backbone are the longest in the molecule but not as long as typical single bonds, and the electron density in those bonds is correspondingly depleted relative to the remainder of the aromatic system. From these electronic considerations, it is reasonble to expect that the Abbreviations: PAH, polycyclic aromatic hydrocarbon; cPAH, cyclopenta polycyclic aromatic hydrocarbon; LUMO,lowest unoccupied molecular orbital; MEP, molecular electrostatic potential.
0893-228x/92/2705-0286$03.00/00 1992 American Chemical Society
Reactivities of cPAHs: Quantum Mechanical Studies
I
8: IV
9
V
7
0
1o
8
VI
vu
X
M S
A
4
XI
to
XI1
xm
Figure 1. The epoxides of the parent cyclopenta polycyclic aromatic hydrocarbons in this study are shown with their numbering. I, aceanthrylene; 11, acenaphthylene; 111, acephenanthrylene; IV, benz[k]aceanthrylene; V, benz[d]aceanthrylene;VI, benzljlaceanthrylene;VII, benz[l]aceanthrylene; VIII, benz[e]aceanthrylene;IX, cyclopenta[cd]pyrene; X, benz[klacephenanthrylene; XI, benz[l]acephenanthrylene; XII, benzLjlacephenanthrylene;XIII, benz[a]acephenanthrylene.
most likely fmt metabolic oxidation step for many cPAHs is epoxidation of the bond completing the cyclopenta ring. However, the electronic structure of the parent PAH is not always the only influence on metabolic oxidation. For example, there is little evidence for the production of the 11,12-epoxide of benzo[a]pyrene even though all straightforward electronic considerations suggest the 11,12 bond is a more likely site for epoxidation than either the 7,8 or 9,lO bond. It may be that these unimolecular indices are not complete descriptors of the electronic considerations in the bimolecular reaction (9) or that the steric considerations of the interaction with the enzyme responsible for oxidation are crucial in determining the sites of oxidation (10). While the modeling of a reaction that includes an electron donor is feasible, there is not enough information available about the structure of the specific enzyme to model the steric considerations of the reaction. In the case of cPAH, however, there is limited experimental evidence to suggest that these unimolecular electronic parameters do predict the most likely epoxidation site (11-13). While it may be difficult to predict the most likely sites of epoxidation from molecular reactivities, the opening of the epoxide ring after protonation or as a result of an interaction with a positively charged region of a biopolymer is spontaneous and is essentially a unimolecular reaction. For epoxide ring opening, simulation of the reaction or models for the reaction based on surrogate
Chem. Res. Tonicol., Vol. 5, No. 2, 1992 287 unimolecular reactivities are likely to be reliable predictors of the reaction products. In a study previously reported (9), we used quantum mechanical methods to compute the electronic reactivities of aceanthrylene 1,2-epoxide (Ia)2J(see Figure l), in order to predict the direction of epoxide ring opening after protonation. We found the 2-hydroxy 1-carbocation (Ib) to be much more stable than the 1-hydroxy-2-carbocation (IC).For both carbocations, we found the positive charge to be distributed throughout the molecule. In the case of the more stable carbocation we found that more positive charge was found associated with the CH6 group than the nominally charged CH1 group. This surprising result and other unimolecular reactivity characteristics [the molecular electrostatic potential (MEP) and the lowest unoccupied molecular orbital (LUMO)] computed for the relevant carbocation suggested addition of a nucleophile was at least as likely at the C6 position as at the nominally charged C1 position and perhaps unusual adducts would be formed. We performed similar calculations for the possible carbocations formed by the epoxide ring opening of acenaphthylene 1,2-oxide (IIa) and acephenanthrylene 4,5oxide (IIIa) and found that, of all five carbocations investigated, only the 2-hydroxy 1-carbocation of aceanthrylene showed unimolecular reactivities that favored addition at any position other than the nominally charged position. However, when we modeled the possible reaction products of the addition of small nucleophiles at the 1and 6 positions of Ib, we found that addition at the 1position (the nominally charged position) was significantly favored and these unimolecular reactivity parameters did not fully characterize the bimolecular addition reaction. The electronic factors introduced by the binding of the small nucleophile were more important than the essentially electrostatic factors modeled by the group charges, MEP, and LUMO. In order to predict the direction of epoxide ring opening and more completely understand the relationship between chemical structure, reactivity properties, and adduct formation, we have now investigated all the similar carbocations that might result from ring opening of the protonated epoxides formed between the carbons that complete the cyclopenta ring of cPAHs with four or fewer six-membered rings and a cyclopenta ring completed by two additional methine carbon atoms (see Figure 1). In this paper, we report the predicted direction of ring opening for the epoxides and the reactivity parameters related to the addition of a nucleophile to the favored carbocation. We have also studied the addition of small nucleophiles to one of these carbocations, the 2-hydroxy 1-carbocation of benz[k]aceanthrylene (IVb),which show a pattern of local reactivities similar to those of the 2hydroxy 1-carbocation of aceanthrylene (Ib). In the case of IVb, unlike Ib, the product of addition of a small nucleophile at C6 is as likely from energetic considerations as the product of addition at the nominally charged C1 position. The starting geometries for all the molecules studied were
obtained from standard bond lengths and bond angles. The five-membered ring in the carbocation is unusual, and a number of geometries were used as starting points. AM1 (14), a semiThe Roman numerals refer to the cPAHs in Figure 1. The a-c designations refer to the following: a, the corresponding epoxide formed between the two carbon atoms that complete the cyclopenta ring; b, the lower energy carbocation that could result from opening of the protonated epoxide (see Table I); and c, the higher energy carbocation. See Figure 1 for the numbering of the molecules.
288 Chem. Res. Toxicol., Vol. 5, No. 2, 1992
Rabinowitz and Little
Horn
""2-;\ IV-r
IV-a
IV-b
Figure 2. Schematic diagram for the protonated epoxide ring opening for molecule IV.
empirical quantum mechanical method, was used to minimize the molecular energy as a function of interatomic position. AM1 has been shown to produce accurate molecular geometries for similar molecules (14-16) and pitive ions (14). The final geometry used is the lowest energy geometry obtained from the various starting geometries. For all reasonable starting points the geometries obtained were not very different. These geometries were then used in single-point ab initio calculations using the Gaussian-86 (17) or Gaussian-88 (18) series of programs with the 3-21g basis set (19). The reported energy differences between the two possible carbocations are from these calculations. AM1 energy differences are also shown. The molecular wave functions obtained from the ab initio calculations were used to compute the other molecular reactivities considered. The atomic charges were computed from the occupied molecular orbitals using the Mulliken method (20) for assigning density to specific atoms. The atomic densities of the LUMO (21) were computed from the unfiiled orbital with the lowest energy, also using the Mulliken method for assigning density to atoms. For each carbocation in this study there is only a single unoccupied orbital with an energy low enough to be considered. The arbitrariness of the Mulliken method for assigning density has been discussed in the literature (22,23),and we have also used other methods for aasigning density to specific atoms. However, in the few examplea we have explored, the results for the type of molecule in this study are not significantly different. The MEP was computed from the molecular wave function using a rapid multipole expansion method that we have shown previously to be accurate at distances of interest (24). The products of the addition of small nucleophiles to the resultant carbocation were studied with the same methods as discussed above. The minimum-energy structure was determined with AM1 and the total energy recomputed with the ab initio method.
Results Table I contains a summary of our resulta. It shows the difference in energy between the two possible carbocation products of the ring opening of the protonated epoxides for the 10 nonsymmetric cyclopenta polycyclic aromatic hydrocarbons in this s t ~ d y .[Three ~ of these molecules have been reported in the previous study (9).]The results of both the single-point ab initio calculations and the AM1 calculations are shown. The predicted direction of ring opening is the same for both methods of calculation and is shown in the second column of Table I (relative to the molecules in Figure 1). Whenever the ring opening could result in the nominally charged CH group being opposite to an unsubstituted CH group that is in the central ring of an anthracenic core [called an a' site ( W ) ] ,like CH6 in I or CH6 in IV,the ring opening to form that particular carbocation will be favored by a large energy increment. A schematic diagram of the ring opening for the cPAH benz[k]aceanthrylene 1,2-epoxide (IVa) is shown in Figure 2. For molecules that do
Figure 3. Schematic diagram for the protonated epoxide ring
opening for molecule X.
not contain an a' carbon the energy differences between the two possible carbocations is smaller. As an example of this class, a schematic diagram of the epoxide ring opening for the cPAH benz[klacephenanthrylene 5,6-epoxide (Xa) is shown in Figure 3. For the lowest energy carbocation for each cPAH considered, including the three symmetric members of the class, the Mulliken charge on the nominally charged CH group, the identification and charge of the most highly charged CH group excluding the nominally charged group, and the LUMO densities at both of these carbon atoms are also shown in Table I. Tables I1 and I11 show the CH group charges and LUMO atomic densities for IVb and Xb, as examples of each class of cPAHs. Table IV contains similar information for IXb, which does not fit into either class. The more stable carbocations of cPAHs IV and V (and to a considerably smaller extent VI-VIII) show a pattern of atomic reactivity characteristics that is qualitatively similar to those of I [reported in a previous study (9)]. For those molecules the atomic charge and LUMO density are much larger at the unoccupied meso position, CH6, than at the nominally charged CH1 position. The difference is more pronounced than for the previously studied Ib. Both of these characteristics are associated with the addition of a nucleophile. This pattern of reactivities is also not seen for any of the higher energy carbocations (that is, Ic-XIIc) (results not shown) where in each case the nominally charged CH group has the largest group charge and largest LUMO density. Both of these Characteristics suggest that these molecules may form unusual adducts with a nucleophile. The molecular electrostatic potentials for the relevant carbocation of IV have been calculated (see Figure 4). They show that there is a more extensive and deeper minimum in the region around C6 than C1 for this carbocation. This result is similar to the results obtained for Ib, but the difference between the two minima is more pronounced in this case. In this example the atomic charges are good descriptors of the electrostatic potentials. The molecular electrostatic potential also suggests the possible formation of unusual adducts. While these molecular characteristics of the carbocation of IV and V suggest the possibility of unusual adducts at C6, the computed energies of the products of the addition of small nucleophiles to the similar carbocation of I showed
Chem. Res. Toxicol., Vol. 5, No. 2, 1992 289
Reactivities of cPAHs: Quantum Mechanical Studies
I
8.50
0.27(1)
0.30(6)
0.20
0.30
3
-
0.35 (1)
0.25(6)
0.34
0.25
I11
0.23
0.27
0.29
16.83
12.66
0.28(6) 0.22(1) 0.30(6)
0.33
IV
0.33(5)h 0.34(4) 0.23 (1)
0.13
0.31
I1
11.83
aO"
V
-
-
0.23 (2)
0.30(7)
0.13
0.30
8.90
8.03
0.28(1)
0.28(6)
0.23
0.26
7.59
7.22
0.27(1)
0.29(6)
0.23
0.26
11.72
9.13
0.28 (5)
0.28(12)
0.23
0.27
12.08
7.20
0.23(3)
0.23
1.79
1.75
0.29
0.12 0.09 0.30
3.26
1.87
0.31(5)h 0.28(4) 0.26 (5)
0.19(9)' 0.18(5) 0.28(6) 0.20(1) 0.25 (6)
0.29
0.24
3.84
2.57
0.26 (5)
0.25 (6)
0.28
0.23
-
-
0.34(6)
0.26 (5)
0.34
0.26
&OH
VI VI1 &OH
VI11
Hoa
IX " O h
X &OH
XI &OH
XI1 XI11 &OH
a Indicates the preferred direction of protonated epoxide ring opening. See Figure 1 for the orientation of the molecular core. *Energy differences between two possible carbocations in kcal/mol. Charge on the nominally charged CH group in units of electrons. The number of the nominally charged carbon is shown in parentheses. (See Figure 1 for numbering.) Charge on the CH group with the largest charge excluding the nominally charged group, in units of electrons. The number of the carbon atom in that group is in parentheses. (See Figure 1 for numbering.) "UMO density on the nominally charged CH group in units of electrons. fLUMO density on the CH group with the largest charge excluding the nominally charged group, in units of electrons. g(-) The two carbocations are identical. "In cases where the energy difference is small, values for both possible carbocations are shown. 'The group with the third highest charge is also shown because the differences are so small. (See Table IV for all group charges.)
atom 1
2 3 4 5 6 7 8 9 10
Table 11. 2-Hydroxy 1-Carbocation Benz[k laceanthrylene (IVb) LUMO group density group charge at atom atom charge 11 0.07 0.13 0.23 0.11 12 0.00 0.04 -0.01 0.09 0.08 13 14 -0.17 0.00 0.05 -0.16 0.06 0.14 15 -0.12 0.31 16 0.30 17 -0.04 0.13 0.21 -0.02 0.03 0.11 18 0.00 19 -0.03 0.05 20 0.05 0.09 0.05
LUMO density at atom 0.00 0.00 0.00 0.00 0.00
0.00 0.06 0.04 0.00 0.10
that from thermodynamic considerations these unusual addition products are not likely to be formed (9). In this study we have computed the energy of molecules that would result from the addition of small nucleophiles to the more stable carbocation of IV (see Figure 5). The results of these calculations are shown in Table V. It can be seen that the addition to IVb is different from the addition to Ib. For IVb the difference between the energy of the products of addition a t C1 and C6 is small. It is not possible to rule out addition at either position for thermodynamic reasons from these results. The lower energy carbocation of M is different from the other carbocations we have studied (see Table IV). The charge on the nominally charged CH group is the smallest charge on any nominally charged CH group in the carbo-
atom
1 2 3 4 5 6 7 8 9 10
atom
1 2 3 4 5 6 7 8 9
Table 111. 4-Hydroxy 5-Carbocation Benz[k]acephenanthrylene (Xb) LUMO atomic density atomic charge at atom atom charge 0.00 11 0.07 0.08 0.07 0.00 12 0.11 0.03 0.00 13 -0.01 0.06 0.00 14 -0.13 0.31 0.29 15 -0.15 0.28 0.30 16 -0.12 0.23 0.15 17 -0.03 0.12 0.03 18 -0.00 0.05 0.00 19 -0.06 0.10 0.06 20 0.00 Table IV. 4-Hydroxy 3-Carbocation Cyclopenta[cd]pyrene (IXb) LUMO atomic density atomic charge at atom atom charge 0.10 0.00 10 0.07 0.16 0.09 11 -0.09 0.23 12 0.01 0.23 0.06 0.00 13 -0.11 0.09 14 -0.12 0.18 0.08 15 -0.01 0.16 0.04 0.00 16 -0.05 0.16 0.08 17 -0.09 0.19 0.12 18 0.00
LUMO density at atom
0.00 0.00 0.00 0.00 0.00 0.00 0.08 0.05 0.00 0.01
LUMO density at atom
0.00 0.00 0.00 0.00 0.00 0.16 0.13 0.00 0.00
cation series studied, and it is the most highly charged CH group in that carbocation. The cationic charge for IXb
290 Chem. Res. Toxicol., Vol. 5, No. 2, 1992
H
Rabinowitz and Little
H
H 0
I
1
-4.0
I
I
-2.0
I
2.0
0.0
I
4.0
I
I
6.0
11
8.0
Angstroms
Figure 4. The molecular electrostatic potential for carbocation Ivb: (black area) >85 kcal/(mol.electron); (medium-shaded areas) 80-85 kcal/ (mol-electron); (light-shaded areas) 75-80 kcal/ (mol-electron). All of the areas inside the solid curves have an electrostatic potential 175 kcal/ (moleelectron).
+
/
H
Y
IV-b
Figure 5. Schematic diagram for the addition of a small nucleophile to carbocation IVb. Table V. Energy Difference between Addition at C1 and C6" benzlklaceanthrvlene (IV) aceanthrvlene (1) -10.8 +0.7 -10.9 -0.5 -13.4 +1.1 (-3.5) -10.7 +2.8 (+0.4) ~
YH
F NH2 OH
~~
Energies E(C1- C6) in kcal/mol. Where two values are given, they refer to alternative geometric isomer pairs.
is more completely shared throughout the molecule than other molecules in this series. Other carboations (Ivband Vb) that have a nominally charged CH group with as small a charge as that of IXb have another CH group with a larger group charge. The LUMO density for IXb is similarly shared throughout the molecule. For IXb there is as much or more delocalization of these reactivities as for IVb and Vb, but in the latter cases the delocalization is primarily the transfer of charge to a single CH group while for IXb this transfer is to CH groups throughout the carbocation.
Discussion The direction of protonated epoxide ring opening for a series of cyclopenta PAHs has been predicted from quantum mechanical calculations of the energy difference
between the two possible hydroxy carbocations. There is qualitative agreement between the ab initio results and the semiempirical AM1 results in this study. In all significant cases the energy difference between the two possible carbocations is greater for the ab initio results than the AM1 results. With the exception of IX the ordering of the molecules is similar for both methods and both methods separate the nonsymmetric molecules in this study into the same two classes (those with a large energy difference between possible carbocations, >7 kcal/mol, and those with a small difference,
