AbInitio SCF MO calculations on the reaction of ...

Ab Znitio SCF MO calculations on the reaction of hydroxyl radical with cytosine BEVERLEY G. EATOCK, WILLIAM L. WALTZ,AND PAULG. MEZEY Department of Chemistry, University of Saskatchewan, Saskatoon, Sask., Canada S7N OW0 Received September 5, 1985

BEVERLEY G. EATOCK, WILLIAML. WALTZ, and PAULG. MEZEY.Can. J. Chem. 64,914 (1986). Ab initio calculations have been carried out on the relative stabilities of various possible products of the reaction between cytosine and the OH radical. These products are of importance in modelling radiation damage to living tissues. The preferred theoretical gas-phase addition site is the C6 ring atom according to these calculations. The analysis of a series of possible contributions to solvent effects strongly suggests the predominance of intermolecular H bonds in stabilizing the experimentally observed C5 adduct.

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BEVERLEY G.EAToCK,WILLIAML. WALTZ,et PAULG. MEZEY.Can. J. Chem. 64,914 (1986). On a effectut de calculs ab initio sur les stabilitks relatives de divers produits possibles pour la rkaction de la cytosine avec le radical OH. Ces produits sont importants dans l'klaboration d'un modkle pour les dommages dfis B la radiation dans les tissus vivants. D'apr2s ces calculs thtoriques, le site prkftrt pour l'addition en phase gazeuse est l'atome de carbone C6. L'analyse d'une skrie de contributions possibles aux effets de solvant sugg2re fortement la prkdominance de liaisons hydrogknes intermoltculaires dans la stabilisation de l'adduit en C5 qui est observk expkrimentalement. [Traduit par la revue]

Introduction Radiation damage to nitrogen heterocycles has attracted interest due to the relevance of such effects for biologically important systems. As an extension of studies on the reactions of OH radicals with N heterocyclic systems (pyridine, imidazole) (1, 2), we have investigated the attack of OH on the pyrimidine base cytosine. Experimental results on the reaction of OH with cytosine agree that ring addition occurs (3,4), but information on site preference for addition is obtained only by indirect methods (esr, pulse radiolysis) leaving results open to some controversy. Ab initio calculations, however, provide a direct aid to identification of favored product isomers, and have been shown to be useful in past studies, supplementing experimental results (1,2,5,6). Radiation damage to cytosine is of interest because of its status as a component of DNA. Consideration of the N bases of DNA as subjects for ab initio studies favors the pyrimidines, both because of their greater susceptibility to OH attack (7) and the simpler problem that the electronic structure of the pyrimidines represents, as compared to the purine bases. Although more information exists regarding reactions of OH with thymine and uracil, the greater experimental controversy for site preference of OH addition to cytosine lent impetus to the choice of cytosine as a subject for these calculations. A recent paper (8) suggested that site selection of OH attack of cytosine is more discriminating than for the analogous reactions with thymine and uracil and that the major product, in opposition to expectations from the results of our previous studies on OH reactions with N heterocycles (1,2), was the species favored on account of electrophilic interactions rather than being the most electronically delocalized product. These factors contributed to the selection of cytosine as the subject for study. In particular, the following two types of processes are of interest:

[I]

OH

+

23 O

[2] OH

+

H abstraction radicals

H

33 O

H

+

OH adducts

+ H20

Hydroxyl radicals are generally thought to add to the ring; however, some evidence also exists for the occurrence of H abstraction (3,4,9) either at the amino group or at N1 (10). Electron spin resonance spectra of the products arising from the reaction of OH with cytosine have yielded ambiguous results as line broadening occurs, obscuring finer splittings. However, all esr studies reported addition of OH at the C5-C6 bond (4,7,11-15). The conclusion by some authors that the C5 adduct is formed is not necessarily reliable as this is based on analogy with the reaction of OH with uracil (7,11,13-15). Both Joshi et al. (11) and Planinic (13) found evidence of the formation of two adducts. Planinic concluded that the preferred C5 adduct was kinetically favored by a ratio of 3:2 (13). Published pulse radiolysis work also supports the theory that two adducts are formed (9,16-18). Hissung and Von Sonntag used pulse radiolysis with both optical and conductimetric detection techniques to conclude that 55% of OH addition occurs at the C5 position (9). This preference for attack at the C5 site is supported by Hazra and Steenken who used pulse radiolysis and the oxidative-reductive characteristics of the nascent radicals (8). These authors however found a ratio of preference for the C.5 adduct to the C6 adduct of 9: 1. In this paper, we present the results of ab initio calculations performed on the reaction of OH with cytosine. Both addition to the cytosine ring and H abstraction were considered as potential reactions. The products resulting from such reactions are depicted in Fig. 1; total energies and energy changes for the reactions [ l ] and [2] have been determined. Calculations were also performed on different tautometers as well as on various internally H-bonded forms of the adducts, and kinetic aspects of the reaction were investiated via the calculation of assumed reaction paths.

Computational methods The total energies of reactants and possible products shown in Fig. 1 were calculated by ab initio SCF MO techniques using a version of the GAUSSIAN 76 program (19,20). Only the STO-3G minimal basis set was employed in the restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF) calculations due to the prohibitively large number of functions required for extended basis set calculations. Due to the limitations on the basis set and the lack of the inclusion of correlation energy terms only large calculated energy differences can serve as basis for chemical conclusions. However, the

915

EATOCK ET AL. H\../H

I

H

H,..

H,-/H

I

H

some degree of arbitrariness in the choice of assumed reaction path. Note, however, that our purpose is to establish upper bounds for activation energies with the given basis set. Any optimization of the reaction path can lead only to an energy lowering, hence these assumed reaction paths provide upper bounds for the theoretical activation energies.


Results and discussion

FIG. 1. Species studied and their designations.

high level of correlation between calculated 3G and 4-3 1G basis results of earlier model studies (2) on various imidazole + OH adducts suggests that an extension of the basis set would not overturn our conclusions. The geometry optimization for the most important species has been canied out to a uniform level of accuracy, higher than that routinely enforced in ab initio studies (vide infra) Initial geometries for both cytosine and species 2-7 were obtained from X-ray diffraction data (21) with the local geometry of the OH addition site assumed to be tetrahedral (1,2,5,6). Initial geometries of species 9 and 10were taken from modified optimized geometries of the analogous keto forms of the adducts. Geometry optimization was performed using the energy gradient method and continued until the average internal coordinate energy derivatives were less than or equal to 0.01 au. Optimization was not canied out as rigorously for species 3 , 4 , 9, and 10 on the basis of the energetic unfavorability for formation of these species. Further refinement is not expected to change the relative energies of the products. The optimized geometries for all species calculated are reported in the appendix in Cartesian coordinate form. The energy of the OH radical was obtained from the study of OH with pyridines (1). Quantitative aspects of the kinetics of the various reactions were obtained by calculating energy along assumed reaction paths. Points for each of the reaction path calculations were obtained by taking the mininum calculated energy of a choice of two different ring geometries: a geometry identical to that of the optimized adduct and a geometry intermediate between that of the optimized adduct and the starting reagent were used for the ring geometry with the C-OH bond length varied. There is

Calculations were performed on potential products arising from the abstraction of H at N 1 and at the amino group and from the addition of OH to the N3-C4 and C5-C6 double bonds. Initially energies were calculated for the keto tautomeric forms of cytosine and the OH adducts. It is not within the scope of this paper to review the many articles published on the tautomerism of cytosine; here we refer only to some theoretical studies on the subject (22-29) including the first ab initio work (22), and a review by Kwiatkowski and Pullman (30) that gives an excellent perspective on this topic. The keto form of cytosine (species 1) has been shown to be the form present in DNA and in solid crystals (30,31). Recent theoretical calculations (MIND0 calculations and other calculations taking some components of solvation into account) (23-25,29) concur with the results from the wide range of experimental techniques (30) supporting the general acceptance of the keto form as the predominant tautomer in aqueous solution despite the considerable controversy in earlier stages of research. Tautomerism of the product of the OH reaction with cytosine (Cy) is an area where no direct experimental information exists. Hazra and Steenken (8) report that substitution at the N1 or N3 positions has little effect on site preference, thus implying a keto tautomeric form as shown in species 4-7 (Fig. 1). Total and relative energies of the keto tautomers of potential products are presented in Table 1. Hydrogen abstraction at the N1 position and at the amino group and addition of OH at N3 are calculated to be unfavorable. The more extensively electron delocalized C6 adduct is calculated to be more stable relative to the C4 adduct and the experimentally found C5 adduct. In earlier theoretical studies (2,22,26-28), an arbitrary value of 40 kJ/mol was set as the limit of significant energy difference; thus, the small energy difference between the C4 and C5 adducts (5 kJ/mol) suggests similar stabilities, and no conclusive ordering in their relative stabilities can be made. Hazra and Steenken's interpretation of their results (8) relies heavily on the assumption that the C4 and C6 adducts are oxidizing radicals while the C5 adduct has reducing properties. As a check on the veracity of these assumptions, ab initio calculations were performed on the optimized geometries of the adducts in question (species 5,6, and 7) with an electron added to each species. Energies (see Table 2) calculated for the addition of an electron to the adducts confirm that the C4 and C6 adducts are better oxidizing agents than the C5 adduct. Because of the systematic disparity between the aqueous phase experimental results and the ab initio calculations involving heterocyclic compounds (2,22,23,29,30) it was decided to investigate the en01 tautomers of the experimentally found C5 adduct and the theoretically preferred C6 adduct. This tautomeric form was chosen on the basis of the favorable relative energy of the en01 tautomer of cytosine as suggested by various theoretical calculations (26-29). The results for these calculations, presented in Table 3, show that the en01 tautomers are less stable than the corresponding keto forms for the adducts and that C6 is still the preferred addition site. Energy was calculated for various geometry modifications to test the effect of internal H bonding. Figure 2 depicts the

CAN. I. CHEM.VOL. 64, 1986

916

TABLE1. Calculated total energies for keto tautomeric forms and relative energiesa speciesb

1 2


3 4 5 6 7

Molecule or radical

E(tot) (au)

AE(kJlmo1)

OH Hz0 Cy (keto form) H abstracted from N1 H abstractedfromNH2 N3-OH-Cy C4-OH-Cy C5-OH-Cy C6-OH-Cy

"Reaction energetics (AE) pertain to process for eq. [l] in text for species 2 and 3 and to process for eq. [2] in text for species 4-7. bSee Fig. 1 for formula of species. T h e geometriesfor species 2 and 4 were not optimized to as high a degree as for other species (see text).

TABLE 2. Calculated total and relative energies for the addition of one electron to hydroxyl adducts Species

Radical

E(tot) (au)

AE (kJ lmol)"

"Relative energy taken with respect to the energy for the appropriate adduct.

TABLE3. Calculated total and relative energies for en01 tautomeric formsa speciesb

8 9 10

Molecule or radical Cy (enol form) C5-OH-Cy C6-OH-Cy

E(tot) (au)

AE (kJ/molY

-387.544Ib -46 1.9097' -46 1.9420'

-1.1 -88.9

BOND LENGTH OF C-OH, prn

FIG.3. Energy variation along approximate reaction paths calculated for addition of OH at the C4, C5, and C6 positions of cytosine. The reactions proceed from right to left. (---) C4-OH-Cy, (---I C5-OH-Cy, -( ) C6-OH-Cy, (a)cytosine + OH at infinite separation. TABLE 4. Calculated upper bonds for activation energies Speciesa

Radical

E, (kJ Imol)

"Reaction energetics are for eq. [2]in text using the energy of species 8 as the starting reactant. %e energy for species 8 is taken from ref. 22. T h e geometries for species 9 and 10 were not optimized to as high a degree as for other species (see text). "See Fig. 1 for formula of species. bSee text.

TABLE5. Calculated spins for radicals Speciesb

FIG. 2. Modified geometries for the C5 adduct to test for internal hydrogen bonding.

modifications in geometry of the keto and en01 forms of the C5 adduct used for the calculations. To increase the proximity between the hydroxyl protons and the lone electron pair of the N atom, both rotation of the amino group (species 12) and displacement of the hydroxyl proton back into the plane of the ring (species 11 and 13) were attempted in order to promote more internal H bonding with the N atoms. Results from these calculations show that these geometries destabilize the adducts

2 4 5 6 7 9 10 Activated complexes of 5 6 7

Radical OH H abstracted from NH2 N3-OH-Cy (keto form) C4-OH-Cy (keto form) C5-OH-Cy (keto form) C6-OH-Cy (keto form) C5-OH-Cy (enol form) (enol form) C6-OH-Cy C4-OH-Cy C5-OH-Cy C6-OH-Cy

T h e expected S-value for doublet is 0.500. bSee Fig. 1 for formula of species.

(keto form) (keto form) (keto form)

Sa

EATOCK ET AL.

TABLE 6. Energies corrected for electrostatic solute-water interaction

Speciesb


2 5 6 7 9 10

Molecule or radical H abs. NHz C4-OH-Cy C5-OH-Cy C6-OH-Cy C5-OH-Cy C6-OH-Cy

Molec. polar.

D

(A3)

Solute-HzO interaction (kJ/mol)

Relative energy" (kJ/mol)

Corrected relative energy (kJ/mol)

3.4 2.4 3.8 3.9 3.6 4.2

10.6 12.2 11.6 11.6 11.5 11.5

-13.1b -7.0 -17.8 -18.4 -15.8 -21.4

183.8 -121.0 -116.0 -200.1 - 1.1 -88.9

170.7 -128.0 -133.8 -218.5 - 16.9 - 110.3

Dip. mom.

"Energy is taken relative to eq. [ I ] for species 2 and eq. [2] for all other species. For species 9 and 10,the en01 fonn of cytosine (species 8) was taken to be the starting reactant. bBecause species 2 is not an isomer of the other products, additional factors such as cavitation and van der Waal's forces are expected to contribute to a different extent to the solvation energy and hence should not be compared to the solvation energies of the other products.

(species 6 and 9) by at least 50 kJ/mol, and hence the energy ordering of the OH adducts is unchanged. To gain some insight into the kinetic aspects of the addition reactions, calculations were performed on assumed reaction paths for addition at the C4, C5, and C6 sites. Such assumed (not optimized) reaction paths, as shown in Fig. 3, are sufficient to give theoretical upper bounds for activation energies. As expected from the near diffusion-controlled rates of reaction (32), the upper bounds for the activation energies for OH addition, given in Table 4, are all small values. From these calculations, it can be seen that attack at the C6 position appears to lead to the kinetically as well as the thermodynamically favored product. No conclusions can be drawn regarding the relative kinetic preference for addition at the C4 vs. C5 positions due to the small difference in calculated approximate activation energies. In Table 5, we have tabulated calculated spin eigenvalues for each of the radical species 2-10. Comparing these values, S , to the expected value of 0.5 for doublets, it is evident that considerable spin contamination has occurred, as is common in UHF calculations. This likely results in lowered calculated energy values for the adducts while calculated energies for cytosine and OH are for the appropriate singlet and doublet states. Thus comparison of energies of the products to the energies of the starting reactants results in exaggerated low theoretical relative energies. The increase of calculated spins along the assumed reaction paths (shown in Table 5) suggests that the greatest degree of mixing of spin states occurs for the assumed reaction path of the C6 adduct, and thus this can possibly explain the apparent lack of an energy barrier for the addition of OH at C6. An important aspect for consideration when comparing theoretical results of studies using isolated molecules with experimental results in solution is solvation. Hydrogen-bonding and electrostatic interactions between solute and solvent molecules are the main solvation features expected to modify the relative stabilities of the isomeric products (33, 34). Whereas in aqueous solution, H bonding is often the dominant effect, a direct, quantitative test of this effect with an ab initio model is prohibitively expensive. On the other hand, it is relatively simple to account for some electrostatic effects. In an attempt to explain the discrepancy between experimental results a n d calculated relative stabilities of tautomers of cytosine, Scanlanand Hillier (23) corrected their calculated energies by including

an estimate of electrostatic interaction. Despite the exclusion of consideration of H-bonding interactions, they were successful in showing the change in relative stabilities of tautomers in vapor phase as opposed to tautomers in solution. Estimates of the water-adduct electrostatic interaction energy have been calculated using the reaction field continuum model (35) and are presented in Table 6. (Calculations are also presented for the product arising from H abstraction at the amino group, despite the nonisomeric nature of this product. Cavitation and van der Wads forces cannot be assumed to have the same effect as for the adduct products.) Molecular polarizabilities have been calculated using the method of Miller and Savchik (36) and the dipole moments have been obtained by ab initio STO-3G calculations. The dielectric constant of water was taken to be 78.54 and the radius of the spherical cavity assumed to be the same as for cytosine, 3.3 A (23). Although the small energy difference involved permits no definitive conclusions as to preference for attack between the C4 and C5 sites, the energies for electrostatic interaction with water show some reordering of relative stabilities of the adducts with the experimentally found C5 adduct now determined to be slightly more preferred than the C4 adduct. However, the small energy difference involved permits no conclusions as to preference of attack between C4 an C5.

Conclusions Ab initio calculations have indicated that the preferred reaction of OH with cytosine is addition of OH to the C6 position, whereas the experimental results in aqueous solution indicate the preference for the formation of the C5 adduct. Interestingly, this experimental-theoretical discrepancy appears to be systematic for a number of N heterocyles (2,22,30). Various factors were considered as the basis for the disparity between experimental and theoretical results including tautomerism of the adducts, kinetic effects, mixing of higher order spin states in calculations of wave functions for doublet adducts, internal H bonding, and solvation aspects. Several plausible explanationshave been excluded and using essentially a process of elimination, the results suggest that intermolecular hydrogen bonding is the major factor leading to the formation of the C5 adduct in solution as the predominant product. Acknowledgments We wish to express our appreciation to the Natural Sciences

918

CAN.I. CHEM.VOL. 64, 1986


and Engineering Research Council of Canada for financial assistance and for a postgraduate scholarship to B. G. E. 1. M. C. ANTHONY, W. L. WALTZ,and P. G. MEZEY.Can. J. Chem. 60, 813 (1982). 2. B. G. EATOCK,W. L. WALTZ,and P. G. MEZEY.J. Comput. Chem. 6, 68 (1985). and J. H. GREEN.^^. J. Radiat. Biol. Relat. 3. M. N. KHATTACK Stud. Phys. Chem. Med. 11, 113, 137, 577 (1966). and B. B. SINGH.Int. J. Radiat. Biol. Relat. 4. M. G. ORMEROD Stud. Phys. Chem. Med. 10,533 (1966). and K. HIRAO.Bull Chem. Soc. Jpn. 52, 287 5. A. IMAMURA (1979). J. Am. Chem. Soc. 97, 6622 6. E. WESTHOEand W. FLOSSMAN. (1975). and H. DERTINGER. Magn. Reson. Biol. Res. Rep. 7. C. NICOLAU Int. Conf. 3rd. 215 (1969). J. Am. Chem. Soc. 105,4380 8. D. K. HAZRAand S. STEENKEN. (1983). Z. Naturforsch. B: Anorg. 9. A. HISSUNGand C. VON SONNTAG. Chem. Org. Chem. 33B, 321 (1978). and L. NIAN-YUN. Radiat. Phys. Chem. 17,293 10. L. REN-ZHONG (1981). 11. A. JOSHI,S. RUSTGI,and P. REISZ.Int. J. Radiat. Biol. Relat. Stud. Phys. Chem. Med. 30, 151 (1976). J. Phys Chem. 94, 3143 (1970). 12. H. TANAGUCHI. Int. J. Radiat. Biol. Relat. Stud. Phys. Chem. Med. 13. J. PLANINIC. 42, 229 (1982). and C. NICHOLAU. Biochim. Biophys. Acta, 199, 14. H. DERTINGER 316 (1970). Biochim. 15. C. NICOLAU,M. MCMILLAN,and R. 0 . C. NORMAN. Biophys. Acta, 174, 413 (1969). 16. L. S. MYERS,M. L. HOLLIS,L. M. THEARD,F. C. PETERSON, and A. WARWICK. J. Am. Chem. Soc. 92,2875 (1970). 17. E. HAYONand M. SIMIC.J. Am. Chem. Soc. 95, 1029 (1973). L. 18. L. S. MYERS,A. WARWICK,M. L. HOLLIS,J. D. ZIMBRICK, M. THEARD, and F. C. PETERSON. J. Am. Chem. Soc. 92,2871 (1970). R. A. WHITESIDE, P. C. HARIHARAN, R. SEEGER, 19. J. S. BINKLEY, J. A. POPLE,W. H. HEHRE,and M. D. NEWTON.GAUSSIAN 76. Quantum Chemistry Program Exchange, 11, 368 (1978). 20. R. POIRIERand M. PETERSON.Modifications of GAUSSIAN 76 (MONSTERGAUSS). DEC Adaptation by R. E. Kari. Laurentian University, Sudbury, Ontario. 21. J. DONOHUE. Arch. Biochem. Biophys. 128, 591 (1968). P. G. MEZEY,and I. G. CSIZMADIA. Theor. 22. J. D. GODDARD, Chirn. Acta, 39, 1 (1975). and I. H. HILLIER.J. Am. Chem. Soc. 10,3737 23. M. J. SCANLAN (1984). B. LESYNG,and A. POHORILLE. Int. J. Quantum 24. R. CZERMINSKI, Chem. 16, 605 (1979). J. R. WHEELER,J. S. KWIATKOWSKI, and B. 25. M. H. PALMER, LESYNG.J. Mol. Struct. 92,283 (1983). 26. P. G. MEZEYand J. J. LADIK.Theor. Chim. Acta, 52, 129 (1979). 27. P. G. MEZEY,J. J. LADIK,and M. BARRY.Theor. Chim. Acta, 54, 251 (1980). 28. E. C. HASS,P. G. MEZEY,J. J. LADIK,and M. BARRY.Theor. Chim. Acta, 60, 283 (1980). J. Mol. Struct. 92, 293 (1981). 29. A. BUDAand A. SYGULA. and B. PULLMAN. Adv. Heterocycl. Chem. 30. J. S. KWIATKOWSKI 18, 199 (1975). 31. D. L. BARKERand R. E. MARSH.Acta. Crystallogr. 17, 1581 (1964). and G. E. ADAMS.Natl. Stand. Ref. Data. Ser. 32. L. M. DORMAN (U.S. Natl. Bur. Stand.), NSRDS NBS 46 (1973). 33. P. BEAKand J. H. WHITE.J. Am. Chem. Soc. 104,7073 (1982). 34. P. BEAK.ACC.Chem. Res. 10, 186 (1977). S. NIR, and T. J. SWISSLER. 35. R. REIN,V. RENUGOPAPKRISHNAN,

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Int. J. Quantum Chem. Quantum Biology Symp. 2, 99 (1975). J. Am. Chem. Soc. 101,7206 36. K. J. MILLERand J. A. SAVCHIK. (1979).

Appendix The information given below is that for the optimized geometry for each species studied in terms of the cartesian coordinates with the units being in angstroms. The numbering of the atoms is that shown for cytosine (Cy) in eq. [I].

TABLEA1 . Species 1 , Cy Atom N1 C2 N3 C4 C5 C6 0 NNH, H1 HNH, H'NH~

H5 H6

X 0.0000 1.2509 1.1653 -0.0239 - 1.2988 -1.2341 2.3099 -0.0892 0.0440 0.7682 -0.9812 -2.2396 -2.1107

Y 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Z

1.3102 0.5891 0.8518 - 1.3957 -0.6537 0.6724 1.1982 -2.7854 2.3289 -3.3270 -3.2676 -1.1784 1.3171

TABLEA2. Species 3, H abstracted from NH2 Atom

X

Y

Z

TABLEA3. Species 4, N3-OH-Cy Atom N1 C2 N3 c4 C5 C6 0 NNH, H1 HNH~ H'NH,

H5 H6 0 H

X

Y

Z

0.0000 1.1865 1.2476 0.1212 - 1.1508 - 1.3695 2.2145 0.1960 0.0648 1.0974 -0.6616 -2.0564 -2.3408 2.595 1 2.8372

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0032 0.0000 0.0000 -0.0033 0.0071 0.0000 0.0000

1.3950 0.6850 -0.7349 - 1.5402 -0.8672 0.4450 1.3854 -2.9583 2.4119 - 3.4222 -3.4974 - 1.4557 0.9240 - 1.3373 -0.3728

919

EATOCK ET AL.

TABLE A7. Species 9, C5-OH-Cy

Table A4. Species 5, C4-OH-Cy


Atom

X

Y

Z

Atom

X

Y

Z

TABLE A6. Species 7, C6-OH-Cy Atom N1 C2 N3 C4 C5 C6 0 NNH~ H1 H5 H6 0 H

Y

TABLE AS. Species 10, C6-OH-Cy

TABLEA5. Species 6, C5-OH-Cy Atom

X

(enol form)

X

Y

Z

0.0000 1.2221 1.2312 -0.0472 - 1.2396 - 1.2236 2.3637 -0.0242 0.1070 0.8544 -0.8785 -2.1816 -2.0973 - 1.2041 - 1.9854

0.0000 0.0000 -0.2636 -0.3807 -0.0275 0.5061 0.1624 -0.8178 0.2890 - 1.0685 -0.8878 -0.0852 0.1284 1.9500 2.2316

1.3400 0.5929 -0.7920 -1.3728 -0.7393 0.6901 1.2196 -2.7116 2.3143 -3.1469 -3.2487 - 1.2652 1.2484 0.7526 0.21 16

Atom

X

Y

Z

(enol form)

Z