1366
J . Phys. Chem. 1990, 94, 1366-1372
D.O.E. of the views expressed in this work. We acknowledge use of a graphics program for plotting vibrational eigenvectors that was developed in the laboratories of Professor R. Mathies at the University of California-Berkeley. We also thank Dr. Janusz Golus for help with the preparation of several compounds used
in this study and Mark Bartelt for his expert assistance in constructing several electronic components. Registry No. ISN, 14390-96-6; D2. 7782-39-0; Ru(bpy)32+, 151 5862-0.
An Investigation of Tautomerism in Adenine and Guanine Michael Sabio,? Sid Topiol,*qt and William C. Lumma, Jr. Department of Medicinal Chemistry, Berlex Laboratories, Inc., I IO East Hanover Avenue, Cedar Knolls, New Jersey 07927 (Received: August 21, 1989)
The geometries of an extensive set of adenine and guanine tautomers have been optimized by using the semiempirical AM1 method and ab initio Hartree-Fock method with the 3-21G basis set. Tautomeric energies were evaluated at the AM1 and the Hartree-Fock STO-3G and 3-21G levels. For selected (low-energy) tautomers of both adenine and guanine, correlation effects were evaluated at the MP2 (Maller-Plesset perturbation theory to second order) level with the 6-31G and 6-31G*(5D) basis sets. For the same selected tautomers of guanine, geometry optimizations were performed at the RHF/6-3 1G*(5D) level. The results suggest that the most stable form of these purines at the AMI, STO-3G, and 3-21G levels is 1A and ZA, Le., the one usually assumed. At the RMP2/6-31G*(sD)//RHF/6-31G*(5D) level (with the zero-point energy correction estimated at the RHF/3-21G//RHF/3-21G level) there is an enol tautomer of guanine which is less than 2 kcal/mol higher in energy than the most stable (keto) tautomer. All other tautomers are unlikely to be experimentally observed in the gas phase. In these systems, the effects due to polarization functions and correlation do not seem to be additive.
Introduction The purines adenine and guanine serve as the base portions of the cyclic nucleotide second messengers adenosine 3'5'-cyclic monophosphate (CAMP) and guanosine 3'5'-cyclic monophosphate (cGMP), respectively, in addition to their presence in the ribonucleosides adenosine and guanosine (and the corresponding deoxyribonucleosides). Knowledge of the physical chemical properties of these purines is therefore clearly essential in understanding many important biochemical processes. The nature of the purine base is likely to play a direct role in the selectivity of recognition of cAMP and cGMP at their sites of action as second messengers, as well as at the active sites of enzymes at which they are hydrolyzed (e.g., their corresponding phosphodiesterases).] Moreover, their properties could also be indirectly involved in the recognition of their corresponding cyclic nucleotides. For instance, it has been suggested2 that the relative conformation about the sugar-to-base bonds of cAMP and cGMP, which varies with the base portion, is a primary component in the recognition of ligands at cGMP phosphodiesterase 11. One of the more fundamental properties to consider in understanding these purines is the relative tautomeric states. Tautomerism can affect both direct and indirect features, such as those mentioned above, of the parent systems. For example, models in which ligand tautomerism is involved in activation of histamine-H2 receptor^^-^ and serotonin receptors6,' have been proposed and may have implications for other receptor systems.* Similarly, active site tautomerism plays a role in the mechanism of action of enzymes such as the serine protease^^^'^ or DNAse." Indeed, it has already been suggested that guanine tautomerism plays a role in biochemical processes.12.'3 Computational chemical studies permit a direct analysis of such properties. In studies appearing in the literature, the structures of the most likely forms of adenine and guanine, as well as other selected tautomers, have been determined.14-24 These studies include examinations of the most likely tautomers by semiempirical approaches as well as ab initio calculations with the STO-3G and 3-21G basis sets. In this work, we study an extensive series of tautomers of adenine and guanine by using the semiempirical AMI method as well as ab 'Present address: Sandoz Research Institute, East Hanover, New Jersey 07936.
0022-3654/90/2094-1366$02.50/0
initio Hartree-Fock and Maller-Plesset perturbation theory calculations to second order with the STO-3G, 3-21G, 6-31G, and 6-31G*(5D) basis sets. The present results should facilitate the understanding of the role of tautomers of these moieties in biological systems and processes.
(1) Charbonneau, H.; Beier, N.; Walsh, K.A,; Beavo, J. A. Proc. Natl. Acad. Sci. USA 1986,83, 9308. (2) Wells, J. N.; Garst, J. E.; Kramer, G. L. J . Med. Chem. 1981, 24, 954. (3) Topiol, S.; Weinstein, H.; Osman, R. J . Med. Chem. 1984, 27, 1531. (4) Weinstein, H.; Mazurek, A. P.; Osman, R.; Topiol, S . Mol. Pharmacol. 1986, 29, 28. (5) Weinstein, H.; Chou, D.; Johnson, C. L.; Kang, S.; Green, J. P. Mol. Pharmacol. 1976, 12, 738. (6) Osman, R.; Weinstein, H.; Topiol, S.; Rubenstein, L. Clin. Physiol. Biochem. 1985, 3, 80. (7) Oman, R.; Topiol, S.; Rubenstein, L.; Weinstein. H. Mol. Pharmacol. 1987, 32, 699. (8) Topiol, S . Trends Biochem. Sci. 1987, 12, 419, (9) Blow, D. M.; Steitz, T. A. Annu. Reu. Biochem. 1970, 39, 63. (IO) Hartley, B. S.; Shotton, D. M. In The Enzymes; Boyer, P. D., Ed.; Academic Press: New York, 1971; Vol. 3, pp 323-373. (11) Suck, D.; Oefner, C. Nature 1986, 321, 620. (12) Topal, M. D.; Fresco, J. R. Nature 1976, 263, 285. (13) Rhoads, R. E.; Hellman, G. M.; Remy, P.; Ebel, J. P. Biochemistry 1983, 22, 6048. (14) Davis, A.; Warrington, B. H.; Vinter, J. G. J. Comput.-Aided Mol. Design 1987, 1 , 97. (15) Norinder, U . J. Mol. Srruct. (THEOCHEM) 1987, 151, 259. (16) Walker, G. A.; Bhatia, S. C.; Hall, J. H., Jr. J . Am. Chem. Soc. 1987, 109, 7629. (17) Del Bene, J. E. J. Cfiem. Phys. 1983,87, 367. (18) Les, A,; Kukawska-Tarnawska, B. J. Mol. Struct. (THEOCHEM) 1986. 148. 45. (19) Bartzsch, C.; Weiss, C.; Hofmann, H.-J. J. Prakr. Chem. 1984,326, 407. (20) Sygula, A.; Buda, A. J. Mol. Struct. (THEOCHEM) 1983, 92,267. (21) Sygula, A.; Buda, A. J. Mol. Struct. (THEOCHEM) 1985,121, 133. (22) Zielinski, T. J.; Breen, D. L.; Haydok, K.; Rein, R.; MacElroy, R. D. Int. J . Ouant. Chem. 1978, 3 5 5 . (23)Latajka, Z.; Person, W. B.; Morokuma, K. J. Mol. Srruct. (THEOC H E W 1986, 135, 253. (24) Thewalt, U.; Bugg, C. E.; Marsh, R. E. Acta Crystallogr. 1971,827, 2358.
0 1990 American Chemical Society
Tautomerism in Adenine and Guanine
Methods The geometries of tautomers of adenine and guanine were optimized by the AMI method2s as implemented in AMPAC (version 1 .00)26and at the a b initio Hartree-Fock level with the 3-2 1G basis set27as implemented in the GAUSSIAN 82 system of programs.28 Except for the hydrogen atoms of -NH2 and = N H in adenine and -OH, -NH2, and = N H in guanine, all atoms were constrained to be coplanar in the optimizations. In addition, molecular energies of the structures derived from the AM1 geometry optimizations were calculated ab initio with the STO-3G29 and 3-21G basis sets. In this paper, the notation X//Y denotes that the energy of a structure derived by method Y is calculated with the use of method X . Selected tautomers of guanine were fully optimized (without any constraints of planarity) at the RHF/6-31G*(5D)M level. For selected tautomers of adenine and guanine, energies were evaluated by using the 6-31G3' and 63 1G*(5D)3' basis sets with and without inclusion of correlation effects at the MP232(Merller-Plesset perturbation theory to second order) level. The SAS software33was used to create the graphs.
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1367 TABLE I: Relative Molecular Energiesa and Dipole Moments of Tautomers of Adenine in Geometries Optimizedb by the AM1 Method and ab Initio with the 3-21G Basis Set re1 energy, dipole methodC kcal/mol moment, D ~~
1 2 3 4
2.18 [2.18] 2.32 2.64 2.38
H
Results and Discussion Extensive Series of Tautomers. In Tables I and 11, we present the relative energies and dipole moments of adenine and guanine calculated at the A M I / / A M I , STO-3G//AMI, 3-21G//AM1, and 3-21G//3-21G levels with the constraints described in the Methods section. For comparison, the values in brackets are taken from Norinder's AMI studyI5 in which he allowed complete geometry optimization. We are primarily interested in the properties of adenine and guanine as fragments of cAMP and cGMP, for which only those tautomers of adenine and guanine with a hydrogen atom at position 9 (Le., the lower "imidazole" nitrogen atom in Tables I and 11) are models; this hydrogen atom is used as a substitute for the sugar portion. For cAMP (dipole moment 3.0 D34),only adenine tautomers (see Table I) A (the global minimum), E, F, K, and L fulfill this requirement. It should be noted that the 9-methyl derivatives of adenine and adenosine exist predominantly as the 6-amino tautomer in aqueous and nonaqueous From an examination of the energies, it is clear that A is the only stable form (more stable than the next feasible tautomer of this subset by 15.6 kcal/mol at the 3-21G//3-21G level) and, therefore, A is the only one which should be used to model CAMP. This is consistent with the results of previous studies based on NDDO, CNDO/2, and STO-3G calculation^.'^^^^^^^ Similar conclusions, based on a more limited analysis of MNDO and STO-3G results, were reached by Sygula et aLzoand Davis et aI.,I4 respectively. These results agree with the recently reported analysis of microwave spectroscopy of ad-
0.0 [O.O]d 0.0 0.0 0.0
1 2 3 4
7.0 [6.7] 6.4 13.3 11.0
6.01 [5.90] 5.95 7.04 7.54
1 2 3 4
11.6 [11.6] 18.3 14.1 14.2
4.54 [4.54] 4.39 4.74 4.81
1 2 3 4
19.6 19.61 24.6 26.0 26.5
8.82 [8.51] 8.76 9.77 9.85
13.6 13.61 16.3 15.6
3.39 [3.38] 3.35 3.89 3.66
1 2 3 4
16.0 24.0 24.4 24.3
4.04 4.17 4.88 4.95
1 2 3 4
18.7 [18.7] 25.1 23.3 22.8
2.99 3.17 3.43 3.72
1 2 3 4
15.2 [15.2] 24.4 22.4 22.4
3.78 [3.78] 3.82 4.40 4.3 1
1
22.1 32.4 30.0 30.4
3.66 3.81 4.74 4.76
2 3 4
14.5 [14.5] 26.4 21.2 21.4
1.67 I1.671 1.59 2.19 2.25
1 2 3 4
24.1 [24.0] 34.6 33.4 34.0
1 2 3 4
22.5 [22.5] 34.9 33.6 33.7
1 2 3 4
2 3 4
18.5
I
H
I
(25) Dewar, M. J. s.;Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J .
Am. Chem. SOC.1985, 107, 3902.
(26) Stewart, J . J. P. AMPAC, version 1.00 Quantum Chemislry Program Exchange, program no. 506, 1986. (27) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. (28) Binkley, J. S.; Frisch, M. J.; Raghavachari, K.; Whiteside, R. A,; Schlegel, H. B.; DeFrees, D. J.; Seeger, R.; Pople, J. A,; Fluder, C. GAUSSIAN 82 Revision H Version, 1984; Department of Chemistry, Carnegie-Mellon University, Pittsburgh, PA 15213. (29) Hehre, W. J.; Stewart, R. F.; Pople, J . A. J . Chem. Phys. 1969,51, 2657. (30) Hariharan, P. C.; Pople, J . A. Chem. Phys. Lett. 1972, 66, 217 (modified for this study so that the basis set contains five, not six, second-order Gaussian functions). (31) Hehre, W. J.; Ditchfield, R.; Pople, J . A. J . Chem. Phys. 1972, 56,
8.43 8.42 10.09 9.79
7.91 [7.60] 7.89 9.34 9.31
1 9 L - l
LLJI.
(32) Mdler, C.; Plesset, M. S. Phys. Reu. 1934, 46, 618. (33) VMS SAS Production Release 5.16 (1986), SAS Institute Inc., Cary, N C 27512. (34) De Voe, H.; Tinoco, J., Jr. J . Mol. Biol. 1962, 4 , 500. (35) Lord, R. C.;Thomas, G. J., Jr. Spectrochim. Acta 1967,23A, 2551. (36) Kyogoku, Y.; Lord, R. C.; Rich, A. J. Am. Chem. SOC.1967,89,496. (37) Bergmann, E. D.; Feilchenfeld, H. W.; Neiman, 2. J . Chem. SOC.B 1970, 1334. (38) Pullman, B.; Berthod, H.; Dreyfus, M. Theor. Chim. Acta 1%9, IS, 265. (39) Mezey, P. G.;Ladik, J. J . Theor. Chim. Acta 1979, 52, 129.
"The total molecular energy (in au) of adenine is -64.027365 ( A M I / / A M I ) , -458.608 198 ( S T O - 3 G / / A M I ) , -461.881 138 (32 I G / / A M l ) , and -461.901 518 (3-21G//3-21G). the optimizations of adenine tautomers in this work, all atoms except the -NH2 and =N-H hydrogen atoms are constrained to be copolanar. cMethods of calculation: ( I ) AMI A M I ; (2) STO-3G//AMI; (3) 3-21G//AMI; (4) 3-21G//3-21G. The values in brackets are taken from Ulf Norinder's study in which he allowed complete geometry relaxation. See J . Mol. Struct. ( T H E O C H E M ) 1987, 151, 259-269.
.P
1368 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Sabio et ai.
TABLE 11: Relative Molecular Energiesa and Dipole Moments of Tautomers of Guanine in Geometries Optimizedb by the AM1 Method and ab Initio with the 3-21G Basis Set re1 energy, dipole re1 energy, dipole methodC kcal/mol moment, D method' kcal/mol moment, D I 2 3 4
0
O
H
0.0 [O.O]d 9.6 0.0 0.0 { O . O ) ~
5.91 [5.91] 5.43 7.09 7.18
1 2 3 4
22.6 33.9 34.5 37.2
6.40 6.47 7.77 7.61
I 2 4
20.1 [20.4] 29.4 28.4 30.3
6.15 6.07 7.65 7.32
1 2 3 4
24.9 36.6 38.7 40.8
I 2 3 4
16.4 [16.7] 27.2 22.6 25.0
5.01 [5.01] 4.97 5.79 5.64
1 2 3 4
19.9 31.9 29.6 32.1
4.72 4.71 5.63 5.39
1 2 3 4
23.8 35.5 35.4 37.2
6.55 6.62 8.22 8.11
1 2 3 4
28.0 41.1 43.9 45.5
6.90 7.08 8.92 8.65
1 2 3 4
21.2 [21.3] 24.3 33.4 31.8
5.43 [5.82] 5.65 5.93 7.95
1
27.9 34.5 47.5
7.26 7.59 8.73
1 2 3 4
1.5 [1.5] 13.9 2.7 2.5 (2.61
1.85 [1.66] 2.41 2.06 2.44
1 2 3 4
14.6 [14.2] 26.3 20.3 20.4
9.56 (9.481 8.67 11.41 12.24
1 2 3 4
7.5 [7.3] 20.3 8.0 8.4
4.07 [3.83] 3.1 I 5.00 5.21
1
2 3 4
11.0 [11.1] 23.9 10.4 12.8
5.68 [5.53] 4.87 6.55 6.3 I
1 2 3 4
11.6 [11.7] 25.9 12.8 15.3
8.09 7.40 9.70 9.56
1 2 3 4
6.9 [7.2] 22.0 4.4 7.0
2.22 [2.24] 2.53 2.47 2.60
1 2 3 4
6.9 [7.2] 22.3 4.9 7.1
3.34 2.94 4.15 4.17
I 2 3 4
3.2 [3.3] 0.0 3.3 4.9 (5.01
2.97 [2.91] 2.80 3.44 3.49
1 2 3 4
6.4 [6.5] 5.4 8.4 9.8
3.70 [3.74] 3.80 4.21 4.03
1 2
4.5 [4.6] 1.2 3.8 5.7
3.37 3.31 4.27 3.92
3
ri
H
H
k0
8.33 8.52 10.57 10.23
H
I
2 3 4
a local minimum was not found
I
H
3
4
2 3 4
13.3 12.7 18.4 19.7
4.00 4.22 5.02 4.96
1 2 3 4
18.4 [18.7] 27.6 25.7 28.0
4.02 [4.02] 3.86 4.58 4.42
1
H
1 2 3 4
17.3 [17.3] 18.7 22.3 22.3
5.34 [5.20] 5.30 5.72 5.93
1
2 3 4
17.5 [17.5] 20.3 24.5 24.5
7.40 7.63 8.63 8.97
1 2 3 4
23.1 35.2 34.2 37.5
3.50 3.97 4.68 4.75
H H- 0
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1369
Tautomerism in Adenine and Guanine TABLE I1 (continued) method' 1
ORH
2 3 4
re1 energy, kcal/mol
dipole moment, D
methode
re1 energy, kcal/mol
dipole moment, D
19.3 [19.6] 29.1 25.8 28.3
3.42 [3.43] 3.58 4.69 4.44
2 3 4
29.8 [29.6] 46.8 44.4 47.5
6.38 6.50 7.74 7.74
28.7 42.6 45.1 48.1
2.70 3.14 3.48 3.60
2 3 4
25.8 [26.1] 31.3 30.9 33.7
4.13 4.40 4.29 4.64
24.2 35.8 35.2 37.4
1.17 1.19 1.33 1.20
1 2 3 4
25.6 [25.9] 30.2 29.7 32.0
2.14 [2.14] 2.20 1.67 1.86
34.8 52.2 53.1 56.6
8.54 8.91 10.67 10.85
1
32.6 40.7 44.6 47.1
6.00 6.41 7.15 7.18
31.7 38.7 41.8 43.7
4.31 4.67 5.12 5.14
1
A 1
H ' 0
2 3 4 H
H ,
0
1
2 3 4
1
2 3 4 1
2 3 4
30.6 [28.8] 46.0 44.1 46.9
7.24 [5.83] 7.40 9.01 9.15
I
ORH
A OHH
1
2 3 4
I
H
1 2 ,3 4
H
1
2 3 4
34.6 53.9 55.0 58.7
8.19 8.56 10.19 9.81
"The total molecular energy (in au) of guanine is -75.799363 (AMI//AMI), -532.459814 (STO-3G//AMI), -536.338 131 (3-21G//AMI), and -536.365 252 (3-21G//3-21G). the optimizations of guanine tautomers in this work, all atoms except the -OH, -NH,, and =N-H hydrogen atoms are constrained to be coplanar. 'Methods of calculation: ( 1 ) AMI//AMI; (2) STO-3G//AMI; (3) 3-21G//AMl; (4) 3-21G//3-21G. dThe values in square brackets are taken from Ulf Norinder's study in which he allowed complete geometry relaxation. See J . Mol. Sfrucr. (THEOCHEM),1987, 151, 259-269. 'The values in braces are taken from 2.Latajka et al.'s study in which the guanine molecule is completely planar.
enine together with 3-21G calculations''' of structures A, E, and B and infrared results.41 The largest differences in relative energies and dipole moments of our AM1 geometry optimizations from the corresponding values of Norinder's study are 0.3 kcal/mol and 0.3 1 D, respectively: the good agreement supports the notion that our constraints in the adenine optimizations are reasonable. Although complete relaxation of the geometry would lower the molecular energy of each tautomer, it would not be expected to significantly change the relative stabilities of the tautomers.1s*21 One observes that the 3-21G//AM1 and 3-21G//3-21G relative energies are quite similar (Le., the differences are less than 3 kcal/mol) which suggests that the AM1 and 3-21G structures are in good agreement. It should also be noted that relative to the energy of the global minimum, the A M l / / A M l tautomeric energy is lower than the corresponding 3-21G//3-21G energy for each of the other tautomers (see Figure 1); the rank order of the tautomers at the AMI//AMl and 3-21G//3-21G levels is almost the same (the relative position of F to G is the only difference). Thus, the 3-21G//AMl and A M l / / A M l methods provide good quantitative and qualitative approximations, respectively, to the 3-21G//3-21G tautomerization energies. This is analogous to similar findings reported43 for histamine tautomerism using the (40) Brown, R. D.; Gcdfrey, P. D.; McNaughton, D.; Pierlot, A. P. Chem. Phys. Lerr. 1989, 156, 61. (41) Nowak, M. J.; Lapinski, L.; Kwiatkowski. J. S. Chem. Phys. Leu. 1989, 157, 14. (42) (a) Radchenko, Ye. D.; Plokhotnichenko, A. M.; Blagoi, Yu. P. Biofirika 1984, 29, 553. (b) Stephanian, S. G.; Sheina, G. G.; Radchenko, E. D.; Blagoi, Yu. P. J . Mol. Sfrucf.1985, 131, 333. (c) Sheina, G. G.; Radchenko, E. D.; Stephanian, S. G.; Blagoi, Yu. P. Srudia Biophys. 1986, 114, 123. (43) Topiol, S. J. Compuf. Chem. 1987, 8 , 142.
older semiempirical MNDO and M I N D 0 / 3 methods in place of the AM1 method. In Table 11, the tautomers of guanine which may serve as a model for cGMP are A, C, E, F, I, K, M, N, 0, P, Y, Z, AA, and BB. Of this set, at the A M l / / A M l , 3-21G//AM1, and 3-21G//3-21G levels, only A (the global minimum at these levels), I, and K are sufficiently low in energy (Le., with relative energies less than 6 kcal/mol) to be considered. The relative energies of the keto and enol tautomers at the RHF/3-21G//RHF/3-21G level are in agreement with those recently reported.44 At the STO-3G//AM1 level, this subset is reduced to only I and K, I being 9.6 kcal/mol more stable than A. Zielinski et al. predictedz2 that I is more stable than A by 18.7 kcal/mol (or 10.0 kcal/mol after a correction for solvent effects) when geometries are optimized with the MINDO/2 method. Tautomer I is also expected to be more stable than A by 7.220 and 8.223.45kcal/mol when geometries are optimized with the MNDO method and a b initio with the STO-3G basis set, respectively. (See ref 20 for a discussion of the reliability of the MNDO method in the prediction of the relative stabilities of guanine tautomers.) The RHF/321G//RHF/3-21G level calculations thus predict that guanine occurs predominantly as A in the gas phase. In certain experimental conditions, the enol tautomers I and K may exist, according to solution studies of inosineM and the interpretationz3 of the infrared spectrum of guanine. This is in good agreement with earlier r e ~ u l t s . ' ~ Szczepaniak J~ et aL4' concluded that the 2(44) Person, W. B.; Szczepaniak, K.; Szczesniak, M.; Kwiatkowski, J. S.; Hernandaz, L.; Czerminski, R. J . Mol. Sfrucr. 1989, 194, 239. (45) Nishimura, Y.; Tsuboi, M.; Kato, S.;Morokuma, K. Bull. Chem. Soc. Jpn. 1985, 58, 638. (46) Chenon, M. T.; Pugmire, R. J.; Grant, D. M.; Panzica, R. P.; Townsend, L. B. J . Am. Chem. Soc. 1975, 97, 4636.
1370 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Sabio et al.
Re l a t i v e Energy 30
(kcal/mol)
2s
, AMl//AMI
__--,,
c
, 20
15
1 0
5
0 1
2
3
T o u t o m % r
5
4
R a n k
6
( b y
7
8
3--21G//3--21G
9
1 0
1 1
12
e n e r g ~ e s )
Figure 1. The AMI//AMI, STO-3G//AMI, 3-21G//AMI, and 3-21G//3-21G relative energies (in kcal/mol) of adenine tautomers ranked according to the 3-21G//3-21G energies. The letter designations above the horizontal axis are those used to label the tautomers in Table I. 50
Re 1 a ti ve Energy 5 0
(kcalhol)
40
30
20
1 0
0
T o u t c m e r
R a n k
( b y
3--21G//3--21C
e n e r g i e s )
Figure 2. The AMI//AMI, STO-3G//AMI, 3-21G//AMI, and 3-21G//3-21G relative energies (in kcal/mol) of guanine tautomers ranked according to the 3-21G//3-21G energies. The letter designations are those used to label the tautomers in Table 11. No local minimum was found for tautomeric form V when a geometry optimization was performed ab initio with the 3-21G basis set. The rank of V was determined by a comparison of its 3-21G//AMI energy with those of all other tautomers.
amino-6-keto and 2-amino-6-enol forms are present in approximately equal amounts in studies of 9-methylguanine in an argon matrix at I5 K. In contrast, Thewalt et al.24concluded that A is the only tautomeric form in crystals of guanine monohydrate. More elaborate studies (see below) were therefore performed to more reliably address this question. A comparison of the AMI results to those of Norinder (who allowed complete geometry relaxation) reveals that the high-energy DD structure provides the only significant deviations (Le., 1.8 (47) Szczepaniak, K.; Szczesniak, M.; Person, W. B. Chem. Phys. Lett. 1988, 153, 39.
kcal/mol and 1.41 D). This indicates that greater freedom in the optimization would change the relative stability of this structure without significantly changing the relative energies of the other tautomers. The 3-21G//AM1 and 3-21G//3-21G relative energies are comparable (see Figure 2), but the agreement is not as good as in the adenine calculations. As with adenine, relative to the energy of the global minimum, the AMl//AMl tautomeric energy is lower than the corresponding 3-21G//3-21G energy for each of the other tautomers. All in all, it appears that the 321G//AMl and A M l / / A M l (for relative tautomeric energies less than ca. 20 kcal/mol) results again provide quantitative and qualitative agreement, respectively, with the 3-21G//3-21G results.
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1371
Tautomerism in Adenine and Guanine
TABLE III: Total Molecular Energies (in nu) and Relative Energies (in kcal/mol) of Selected Adenine and Guanine Tautomen" RHF/ 3-21G// RHF/3-21G
RHF/ 6-3 1G / / RHF/3-21G
RHF/ 6-3 lG*( 5D)// RHF/3-21G
-461.901 5 18
-464.295552
-464.516211
(0.0)
(0.0)
H
'?
RMP2/ 6-31G// RHF/3-21Gb
RMP2/ 6-31G*(5D)// RHF/3-21G
zero-point energf corrected MP2
-465.251947
-465.921999
estimated
(0.0)
(0.0)
-461.87667 1 (1 5.592)
-464.270249 (15.878)
-464.493237 (14.416)
-536.365252
-539.137688
-539.389390
(0.0)
(0.0)
-539.392620 (0.0)
(0.0)
(0.0)
-465.907935 (8.825)
estimated (8.331)
-465.231608 (12.763)
-465.901 236 (1 3.029)
estimated (1 3.296)
-540.21 1426
-540.965735 (0.0)
estimated
-464.500018 (10.161)
-461.884061 ( 10.95 4)
(0.0)
RHF/ 6-3 1G*( 5D)// RHF/6-31G1(5D)
(0.0)
(0.0)
-536.357378 (4.941)
-539.130440 (4.548)
-539.386465 (1.836)
-539.389997 (1.646)
-540.20 1900 (5.978)
-540.9621 19 (2.269)
estimated (1.874)
-536.356179 (5.693)
-539.127631 (6.31 1)
-539.38461 5 (2.996)
-539.388189 (2.780)
-540.199963 (7.193)
-540.961 170 (2.865)
estimated (2.496)
#In the 3-21G geometry optimizations, all atoms, except the amino and imino hydrogen atoms (in adenine), and the amino and hydroxyl hydrogen atoms (in guanine), were constrained to be coplanar. In the 6-31G*(5D) geometry optimizations, complete relaxation was permitted. Relative energies are given in parentheses. bThe frozen-core approximation was used in the RMP2 calculations. In this column are relative energies derived from the values of the previous column after (3-21G) zero-point vibrational energy differences are considered. The zero-point vibrational energies calculated at the 3-21G level are (from top to bottom row) 68.978,69.245, 71.765, 71.370, and 71.396 kcal/mol after scaling by 0.9 (see e.g., Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986, and references therein). dThe adenine tautomer B does not serve as a model for CAMP however, it has been included in this table because of recent interest (see the Results and Discussion section).
It is also worth noting that, if the relative stabilities of all guanine tautomers (not only those substituted at the 9-position) were considered, then the list of possible tautomers would also have to include B and, depending on the experimental conditions, D, G, H, and J (see Table 11). Discussions of low-energy tautomers not substituted at the 9-position, based on theoretica114,15*20~z1~z3 studies, may be found in the literature. Basis Set and Correlation Effects for Selected Tautomers. We have selected various low-energy tautomers of adenine and guanine for further investigation by more extensive ab initio methods. These results are presented in Table 111. For the guanine tautomers A, I, and K, we note first that at the highest level of computation presented here (RMP2/631G*(5D)//RHF/3-21G including a zero-point energy correction obtained at the 3-21G level) the energy differences are significantly reduced. At this level of theory, tautomer I is only 1.87 kcal/mol higher in energy than A and would thus be expected to exist in significant amounts in the gas phase. This would support the recent interpretationz3 of the infrared spectrum. At the SCF level, comparison of the first two columns of Table 111 shows that the 6-31G tautomeric energies are very similar to those obtained with the 3-21G basis set if the same (3-21G) geometries are used for both. On the other hand, while still using the 3-21G geometries, we see from columns 2 and 3 that adding polarization functions to the basis set (Le., 6-31G*(5D) versus 6-31G) lowers the relative energy of the guanine tautomer I by 2.7 12 kcal/mol so that it is only 1.836 kcal/mol less stable than tautomer A. If the tautomers are evaluated with the same basis set (6-31G*(5D)), but with the different structures obtained from
3-21G and 6-31G*(5D) geometry optimizations, the results are very similar (columns 3 and 4). In general, the most extensive basis set used in this study seems to lower the relative energy of I and K in a manner which is not very dependent on structure. Inclusion of correlation effects at the MP2 level (without polarization functions) raises the relative energy of the guanine tautomers I and K in the 6-3 lG//RHF/3-21G calculations (columns 2 and 5). In the 6-31G*(5D)//RHF/3-21G calculations (columns 3 and 6) the relative energy of I is again raised; however, that of K is lowered by a negligible amount. We also note that the effects of correlation and polarization functions do not seem to be additive here. For instance, the relative preference of tautomer I shifts from 4.548 to 1.836 kcal/mol (columns 2 and 3) with a change from the 6-31G to the 6-31G*(5D) basis set, Le., a decrease of 2.712 kcal/mol. Starting with the same results (column 2), inclusion of correlation at the MP2 level yields an increase in the relative tautomeric preference of 1.430 kcal/mol. The sum of these two changes in tautomeric preference, -1.282 kcal/mol, is not equal to the change which occurs when polarization and correlation effects are both included, Le., a lowering of 2.279 kcal/mol (columns 2 versus 6). All in all, polarization functions seem to lower the energy differences between these (I and K) tautomers (columns 2 versus 3 or 5 versus 6) and MP2 corrections most often increase these differences (columns 2 versus 5 or 3 versus 6); however, the effects do not seem to be additive. For adenine, tautomers A, B, and E were reevaluated. Because the results for guanine tautomeric preferences at the higher levels were not very sensitive to the geometry used (3-21G versus 631G*(5D)), we used only the 3-21G optimized structures for
J . Phys. Chem. 1990, 94, 1372-1376
1372
adenine. It is clear that the E tautomer is still significantly higher in energy at all levels of calculation presented here. As with guanine (see above), the 6-31G results agree very well with the 3-21G results at the Hartree-Fock level. Here also, the effects of including polarization and correlation are not additive with respect to the predicted tautomeric preference. In light of recent disagreements based on interpretations of the infrared spectra of adenine41v42 as to whether tautomer B is present in an inert argon matrix, we have also evaluated the RMP2/631G*(5D)//RHF/3-21G energy (with an estimate of the zeropoint energy) of tautomer B (Table 111). We find that B is 8.3 kcal/mol higher in energy than A at this level of calculation and is therefore predicted not to be present in significant quantities.
Conclusions At the semiempirical AMI and Hartree-Fock (using STO-3G and 3-21G basis sets) levels performed here, extensive tautomerism
studies suggest that both adenine and guanine moieties substituted at the 9-position will exist predominantly as their usually depicted tautomeric forms in the gas phase. However, more extensive studies on selected low-energy tautomers, with extended basis sets at the MP2 level and including zero-point energy corrections, predict that the guanine moiety substituted at the 9-position may also exist, in significant amounts, as the enol tautomer. Thus, predictions of the tautomeric preferences of these systems at the ab initio level are sensitive to the basis set used and degree of correlation included. The present results suggest that these (correlation and polarization) effects should not be evaluated separately as they do not appear to be additive in these systems.
Acknowledgment. We are very grateful to Guy Talbot for very valuable technical assistance. Registry No. Adenine, 73-24-5; guanine, 73-40-5
ESR Measurement of the pK, of Carboxyl Radical and ab Initio Calculation of the Carbon-I3 Hyperfine Constant A. S. Jeevarajan, Ian Carmichael, and Richard W. Fessenden* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 26, 1989) The pKa value for the equilibrium C02H t H++ C02- has been determined by ESR methods to be -0.2 f 0.1. Formate that had been enriched in I3C was reacted with SO4'- in steady-state photolysis experiments to produce the radical. The positions of the hyperfine lines did not vary over the pH range 1.6-10 but were found to change continuously as the acid concentration was increased over the range pH 1.2 to about pH 0, where the intensity became too small for study. The change in average hyperfine constant followed the behavior expected for a simple equilibrium under the assumption that exchange of the acid proton is rapid. The I3C hyperfine constants in aqueous solution were found to be 146.4 f 0.1 G for COT and 166 f 5 G for C02H. Ab initio molecular orbital calculations generally agree with the measured hyperfine constants and predict a higher value for the neutral form of the radical. ESR lines from a reduced oxalate radical containing 13C were also detected.
Introduction Several different values have been reported for the pKa of the equilibrium C 0 2 H z H+
+ C02-
(1)
The most widely based on different types of accepted value of 1.4 is based on the change in absorption at 250 nm in pulse-irradiated solutions of formate or formic acid.4 A discussion of the earlier measurements is given in that paper. This radical appears in a number of chemical processes and is commonly used as a reducing agent in studies of the redox reactions of a wide variety of compounds at various pH values. A recent papers has reported a detailed study on the reduction potential of the couple C 0 2 / C 0 2 -(and also for the alcohol radicals) and makes use of the pKa value to determine that the radical is completely in the dissociated form under the experimental conditions used. The pK, is not used directly in calculating the potential of this couple but does affect the value calculated for C 0 2 / C 0 2 H . A study of the couple CO,/HCO, has also been made? and here, the value of the pK, of C 0 2 H directly affects the standard potential
and free energy change. Other discussions of the thermodynamics involving C02- also depend on the pK,.' From these and other examples, it is clear that knowledge of this pK, value is important. Where ESR methods can be applied, they are often superior to optical methods of determining pKa values in that identification of the spectra of the two forms of the radical is more definitive and no assumptions are needed regarding how the chemical yields vary as the pH is changed. The most favorable situation for ESR occurs when the acidic proton is rapidly exchanged with water or H+ and the ESR parameters of the two forms of the radical are significantly different. In such a case, the observed ESR parameter, hyperfine coupling (hfc) or g factor, represents the concentration-weighted average for the two forms. The fact that the ESR spectrum can be followed continuously as pH is changed gives great confidence that the identification of the corresponding radical is correct. Previous work has shown that COz- can be prepared photolytically by the reactions
s2082-E ! +. SO4'- + HC02-
( I ) Buxton, G. V.; Wilmarth, W. K. J . Phys. Chem. 1963, 67, 2835. (2) Gutlbauer, F.; Getoff, N. Z . Phys. Chem. 1966, 51, 255. (3) Fojtik, A.; Czapski, G.; Henglein, A. J . Phys. Chem. 1970, 74, 3204. (4) Buxton, G. V.: Sellers, R. M . J . Chem. Soc., Faraday Trans. I 1973,
69, 555. ( 5 ) Schwarz,
H. A.; Dodson, R. W. J . Phys. Chem. 1989, 93, 409. (6) Surdhar, P. S.: Mezyk, S. P.:Armstrong, D. A. J . Phys. Chem. 1989, 93, 3360.
0022-3654/90/2094- 1372$02.50/0
SO4'-
+ HC02H
-
2~0;-
+ H+ + S042C02'- + 2H' + Sod2C02'-
(2) (3a)
(3b)
and that the g factor of C 0 2 - does not vary over the pH range 4.6-0.8.8 More recent work on the radical HP03-9has shown ( 7 ) Koppenol, W.
H.; Rush, J. D. J . Phys. Chem. 1987, 91, 4429.
0 1990 American Chemical Society
