Chem. Res. Tonicol. 1990,3, 292-295
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Articles Aralkylation of 2’-Deoxyguanosine: Medium Effects on Sites of Reaction with 7-(Bromomethyl)benz[ a ]anthracene Guo K. Pei and Robert C . Moschel* BRI-Basic Research Program, NCI-Frederick Cancer Research Facility, Frederick, Maryland 21 701 Received January 19, 1990
Product distributions were determined for reactions between 2’-deoxyguanosine, or its anion, and 7-(bromomethyl)benz[a]anthracene in either acetone/H20 (1:l) or 2,2,2-trifluoroethanol at 50 or 70 “C. The exocyclic amino-substituted product, iV-(benz[a]anthracen-7-ylmethyl)2’-deoxyguanosine, was always the major nucleoside product formed in these reactions, although ita yield was higher in reactions involving 2’-deoxyguanosine anion than the neutral nucleoside. Reaction with the anion also led to formation of the l-substituted 2’-deoxyguanosine and a guanidinoimidazole nucleoside resulting from reaction of 2’-deoxyguanosine at carbon 5. Reactions of 2’-deoxyguanosine anion in 2,2,24rifluoroethanol are shown to produce significant amounts of the N2-substituted product, which is difficult to prepare by other routes.
Introduction In aqueous media, solvolytically reactive derivatives of the polycyclic aromatic hydrocarbon carcinogens react with DNA at the exocyclic amino groups of the bases (1, 2). Reaction at the N2-positionof 2’-deoxyguanosine residues is generally most extensive, but the total yield for such products is usually very low. Consequently, if millimolar amounts of these products are required for study or for derivatization and incorporation into synthetic DNA, their preparation through isolation from DNA reactions is impractical. Unfortunately, there is no convenient method for total synthesis of N2-substituted 2’-deoxyguanosines as an alternative preparative route. As a result, we are examining direct substitution reactions with 2‘-deoxyguanosine under various conditions with the aim of producing larger quantities of the N2-aralkyl-substituted derivatives than are produced in aqueous reactions. To this end, we have examined the reactions between the directacting carcinogen 7-(bromomethyl)benz[a]anthracene (7BrMeB [ a ]A) and 2’-deoxyguanosine (dGuo) (Figure 1) under different solvent conditions. We selected this reactive hydrocarbon for study because it is easily prepared, it was the fmt carcinogen shown to react with the exocyclic amino groups of the DNA bases under aqueous conditions (3),and the salient features of its reactivity would be expected to be representative of those exhibited by a variety of more complex aralkylating agents (1,2). We previously studied the reaction of guanosine with simple benzylating agents under different solvent conditions (4-7). These agents, like 7-BrMeB[a]A (3), react extensively at the 7-pition of guanine nucleosides in polar aprotic solvents (e.g., dimethylacetamide or dimethylformamide). Under neutral aqueous conditions, benzylating agents and 7-BrMe[a]A react with the exocyclic amino group of guanine nucleosides although the latter 0893-228x/90/2703-0292$02.50/0
exhibits much higher site selectivity (3,8). Under alkaline aqueous conditions (6,r) we observed that guanosine anion trapped a higher proportion of benzylating agent than was trapped by the neutral nucleoside. While this was not unexpected, it was surprising to observe that those benzylating agents that could readily ionize in H 2 0 and produce a significant proportion of N2-substituted product with the neutral nucleoside produced still higher yields of the same product in reactions with guanosine anion, even though the exocyclic amino group of the anion is not formally charged. Since 7-BrMeB[a]A (Figure 1)would be expected to ionize fairly readily in mixed aqueous solvent (i.e., acetone/H20, 1:l) (9),or in solvents of higher ionizing power but lower nucleophilicity such as 2,2,2trifluoroethanol (TFE) (9),we anticipated, by analogy with our benzylation studies, that reactions between dGuo or 2’-deoxyguanosine anion (dGuo-) and 7-BrMeB[a]A in these solvents would produce larger amounts of the desired N2-substituted product [Le., P-(benz[a]anthracen-7-y1methyl)-2’-deoxyguanosine)(N2)] than are produced in totally aqueous nucleoside or DNA reactions. To test this, we measured the extent of formation of this product (N2) as well as l-(benz[a]anthracen-7-ylmethyl)-2’-deoxy-
guanosine (N-1) and 4-(benz[a]anthracen-7-ylmethyl)-5guanidino-l-(~-~-2’-deoxyribofuranosy1)imidazole ((2-5) together with the solvolysis products 7-(hydroxymethyl)benz[a]anthracene (7-HOMeB[a]A) and 7-[(2,2,2-trifluoroethoxy)methyl]benz[a]anthracene (7-TFEMeB[a]A) produced in these reactions at either 50 or 70 OC (Figure 1). The results of these investigations are summarized below. Materials and Methods 2’-Deoxyguanceine was purchased from Pharmacia, PisCataway, NJ. 7-(Bromomethyl)benz[a]anthracene was prepared as de@ 1990 American Chemical
Society
Chem. Res. Toxicol., Vol. 3, No. 4, 1990 293
Aralkylation of 2”-Deoxyguanosine 0
H
O w H 0‘
0
H0
dGuo
-
d?),
9
dGuo-
H 0‘
(CH3)ECO/HE0
RBr
RBr
these pooled fractions to dryness afforded 0.22 g (28%) of chromatographically homogeneous product ‘H NMR b 2.36 (m, 1,H-2’a),2.74 (m, 1,H-2’8), 3.63 (m, 2, H-5’),3.92 (m, 1,H-4’), 4.48 (m, 1,H-3’), 4.97 (t,l,OH-5’, exchanges with DZO), 5.39 (d, 1,OH-3’, exchanges with D20), 5.50 (d, 2, ArCH2, JCH = 4.2 Hz,changes to a singlet on addition of D20),6.42 (t, 1, 6.92 (t, 1,N2H,JwCH = 4.2 Hz,exchanges with D20),7.63-9.57 (m, 12, Ar + H-8),10.14 (8, 1, 1-NH, exchanges with D20); +ve FAB MS m / z 508 ([M + HI+), 392 ([B + 2Hl+), 241 ([C&sI+). 4(Benz[a]anthracen-7-ylmethyl)-5-guanidino-l-@-~-2’-deoxyrib furanosy1)imidazole(C-5) eluted in fractions 79-95 ‘H NMR 6 2.03 (m, 1, H-2’a), 2.29 (m, 1,H-2’@),3.41(d, 2, H-5’),3.69 (m, 1,H-4’),4.22 (m, 1,H-3’),4.50 (s,2, ArCH,), 5.12 (br s, 1,OH-5’, exchanges with D20),5.45 (br s, 1,OH-3’, exchanges with DzO), 5.58 [br, s,4, 2(NH2),exchange with D20], 5.79 (t, 1,H-1’1, 7.27 (s, 1,H-2), 7.53-9.32 (m, 11,Ar); +ve FAB MS m/z 482 ([M HI+), 366 ([B 2H]+), 241 ([CIJ-Il,]+). l-(Benz[a]anthracen-7ylmethyl)-2’-deoxyguanosine(N-1) eluted in fractions 96-132 ‘H NMR 6 2.15 (m, 1,H-2’a), 2.45 (m, 1,H-2’@),3.48 (m, 2, H-59, 3.76 (m, 1, H-49, 4.29 (m, 1,H-3’),4.86 (t, 1, OH-5’, exchanges with D20),5.20 (d, 1,OH-3’, exchangea with D20),6.05 (t, 1,H-1’1, 6.28 (s,2, ArCHJ, 6.43 (s,2, “W2,exchange with D20), 7.61-9.55 (m, 12, Ar + H-8); +ve FAB MS m/z 508 ([M HI+), 392 ([B + 2H]+), 241 ([Cl&s]+). 7- [(2,2,2-Trifluoroethoxy)methyl]benz[a]anthracene (7-TFEMeB[a]A)eluted in fractions 135-151: ‘H NMR 6 4.37 (q,2, FaCCH2, J Q ”=,9.4 ~Hz), 5.67 ( ~ $ 2ArCHJ, , 7.60-9.58 (m, 11,Ar); E1 MS m/z 340 ([MI+), 241 ([Cl,H18]+). Analysis of the Aralkylation of 2’-Deoxyguanosine by 7-(Bromomethyl)benz[a]anthracene as a Function of Solvent and Temperature. 2’-Deoxyguanosine hydrate (3.5 X l0-l mol) was dissolved with constant stirring in 10 mL of either acetone/H20 (1:l)or TFE in the presence or absence of triethylamine (TEA) (7.2 X lo-‘ mol) at either 50 or 70 OC as indicated (Figure 2). 7-(Bromomethyl)benz[a]anthracene (1.6 X lo-‘ mol) was added, and stirring was continued for 4 h. At the end of this period the reaction mixtures were diluted with 650 mL of MeOH/H20 (400.250) and warmed as necessary to dissolve any suspended solids. Acetonitrile (350 mL) was then added to ensure that all components remained in solution. An aliquot (50 pL) of this solution was then withdrawn and was mixed with 200 pL of CH&N/H20 (25:75), and this was loaded onto a 4.6 X 250 mm Beckman Ultrasphere ODS column eluted at 1mL/min with a linear gradient of 0.5 mM TEA in CHSCN/H20 (2575) to 0.5 mM TEA in CH&N over 30 min. UV absorption was continuously monitored at 290 nm to quantify products containing the benz[a]anthracenyl chromophore. These data were supplied directly to a Hewlett-Packard 3350 Laboratory Automation System (LAS) for electronic integration of peak areas. The percentage of the total area contained in each individual reaction component peak was calculated. Retention times for the individual reaction products were as follows: N2, 9.2 min; C-5, 12.8 min; N-1, 14.3 min; 7-HOMeB[a]A, 20.5 min; 7-TFEMeB[a]A, 28.1 min. Data reported (Figwe 2) are the average of at least three determinations.
DI.
CF3CH20H
+
+
0
+
+
N2
+
RROCH,CF, OH a n d / o r
N-1
R=
(py I
CHI-
Figure 1. Products and conditions for the reactions between 2’-deoxyguanosine or its anion and 7-(bromomethyl)benz[a]anthracene.
scribed previously (3,101. 7-(Hydroxymethyl)benz[a]anthra~ne (11) was prepared by hydrolyzing 7-BrMeB[a]A in acetone/H20 (7:3) ovemight. Sephadex LH-20 was purchased from Sigma Chemical Co., St. Louis, MO. All other chemicals were from Aldrich Chemical Co., Milwaukee, WI. ‘H NMR spectra were recorded on a Varian XL 200 instrument interfaced to an Advanced data system. Samples were dissolved in dimethyl-d6 sulfoxide with tetramethylsilane as internal standard. Positive ion (+ve) and negative ion (-vel fast atom bombardment (FAB) mass spectra (MS) were obtained with a reversed-geometryVG Micromass ZAB-2F spectrometer interfaced to a VG 2035 data system. A mixture of dithiothreitol and dithioerythritol(1:l) was used as FAB matrix. High-pressure liquid chromatography was carried out at room temperature on Beckman Ultrasphere ODS columns (5” particle size) by using two Waters 6OOOA pumps, a Model 660 solvent programmer, a Model 450 variablewavelength UV detector, and a Model U6K sample injector. Preparation of N2-(Benz[a]anthracen-7-ylmethy1)-2’deoxyguanosine (W),1-(Benz[ a ]ant hracen-7-y lmet hyl)-2‘Results and Discussion l), 4-(Benz[a ]anthracen-7-ylmethy1)-5deoxyguanosine (Nguanidine1 - ( 8 - D ~ d e o x y ~ b o f ~ ~ (C-51, y l ) and ~ ~ ~ l e The nucleoside and solvolysis products produced in 7-[(2,2,2-Trifluoraethoxy)methyl]benz[a ]anthracene (7these reactions are illustrated in Figure 1. From reactions TFEMeB[a]A). To a stirred suspension of 1.0 g of 2‘-deoxybetween dGuo- and 7-BrMeB[a]A carried out in TFE in guanosine hydrate in 80 mL of gently refluxing TFE was added the presence of TEA, three nucleoside products, N2, N-1, 1 mL of triethylamine. When nearly all the 2‘-deoxyguanosine and C-5, together with the solvolysis product 7-TFEMhad dissolved, 0.5 g of 7-BrMeB[a]A was added and the resulting eB[a]A were isolated. These were separated by column Suspension was stirred for 4-5 h. The reaction was then allowed chromatography on Sephadex LH-20 (see Materials and to cool to room temperature and was stirred for an additional 20 Methods). The UV absorption spectra for all these h. The solvent was removed under reduced pressure, and the residue was treated with 100 mL of H20 containing 1 mL of products were quite similar and were dominated by the triethylamine with stirring for 1 h to remove unreacted 2’highly absorptive 7-substituted benz[a]anthracenyl chrodeoxyguanosine. The suspension was suction filtered, and the mophore present in each product. Mass spectral data for recovered solid was suspended twice with stirring in 50 mL of each product were consistent with the assigned structure. MeOH/H20/concentrated NH40H (7525:3) for 10 min. The Additional confirmation of their identity was provided by suspension was filtered both times, and the clear filtrates were ‘H NMR spectra in DMSO-& For the N2 product the two pooled and loaded on a 2.8 X 70 cm Sephadex LH-20 column methylene protons adjacent t o the N2-position appeared eluted with MeOH/H20/NH40H (75:253) at 1 mL/min. UV as a doublet at 6 5.50 (J = 4.2 Hz). These protons were absorption was continuously monitored at 280 nm and fractions coupled to a single exchangeable N2-amino proton which (10 mL) were collected. ZP-(Benz[a]anthracen-7-ylmethyl)-2’deoxyguanosine (P) eluted in fractions 50-78. Evaporation of appeared as a triplet at 6 6.92 (J = 4.2 Hz). On addition
Pei and Moschel
294 Chem. Res. Toxicol., Vol. 3, No. 4, 1990
AcetonelWater, 70' TFE, 70' of DzO,the resonance of the amino proton disappeared and 100 7 the two-proton doublet at 6 5.50 changed to a singlet. The resonance for the exchangeable proton at the 1-position 80 of this product appeared as a broad singlet at 6 10.14 prior to the addition of D20. A similar d o d i e l d resonance for 60 an exchangeable 1-proton was absent in the spectrum for i-the N-1 product, which is consistent with the site of subd 40 stitution of the hydrocarbon in this adduct. In this case, the methylene protons were observed as a singlet at 6 6.28. 20 The resonance for the two N2-amino protons in this adduct 0 also appeared as a singlet at 6 6.43 and disappeared on addition of D20. Also produced in these reactions was AcetoneITEAIWater, 70e TFEITEA, 70' 4-(benz[a]anthracen-7-ylmethyl)-5-guanidino-l-(P-~-2'100 7 deoxyribofuranosy1)imidazole (C-5). This is a polycyclic 1100 - 1 aromatic hydrocarbon analogue of a type of product previously identified in our studies of guanosine benzylation (6-8,12-14). Several aspects of its 'H NMR spectrum (see Materials and Methods) are consistent with this structure. First, the resonance for the two methylene protons of the hydrocarbon moiety appears at 6 4.50, which is at higher field than is observed for attachment of this group to a heteroatom (see above) and indicates attachment of the methylene to a carbon in this particular adduct. The 0 resonance for the four exchangeable amino protons in this structure appears as a broad singlet at 6 5.58. The resoTFE, 50' AcetonelWater, 50° nance for the H-2 proton of the imidazole ring in this 100 7 100 I 1 molecule appears at 6 7.27, which is indicative of the guanidinoimidazolestructure since the corresponding H-8 proton resonance for an intact substituted 2'-deoxyguanosine appears at much lower field within the complex pattern of resonances for the aromatic protons of the hydrocarbon residue. The 'H NMR spectrum for 7-TFEMeB[a]A is included under Materials and Methods. In Figure 2 we present, in histogram format, the percentage yield for the various reaction products under the solvent and temperature conditions indicated. Products were separated and quantified on the basis of peak areas TFEITEA, 50' AcetoneITEAIWater, 50° by using a high-pressure liquid chromatographicseparation and an automated electronic area integration system. l 80 o o r ] Yields are reported as the percentage of BrMeB[a]A converted to product, and they are the average of at least three determinations. As indicated in panels A and B of Figure 2, neutral dGuo fails to compete with solvent for d 40 reaction with 7-BrMeB[a]Aat 70 "C. Only 7-HOMeB[a]A and 7-TFEMeB[a]A are detected in these reactions even 20 though nucleoside products are stable under these reaction conditions. The formation of a small amount of 7-HOM0 eB[a]A in TFE is probably a result of reaction with H20 present in the commercial solvent. When TEA is included in these reactions to cause reaction between dGuo- and Ez c-5 N-1 7-BrMeB[a]A, the formation of nucleoside products in! Y Unknown N2 7-HOMeB(a)A 07-TFEMeB(a)A creases dramatically. As indicated in panel C of Figure 2,30% of 7-BrMeB[a]A is converted to the N2, C-5, and Figure 2. Histogram of the percentage yield of 7-(bromoN-1 products in an approximate ratio of 5:2:1 in acetone/ methyl)benz[a]anthracene-derived 2'-deoxyguanosine and solTEA/H20 at 70 "C. In TFE/TEA at 70 "C (panel D), volysis products as a function of solvent and temperature. 52% of the carcinogen reacts with nucleoside to produce the same products in a ratio of 8:1:2. In addition to these presence of TEA, the yields for products W, N-1, and C-5 products, an unexpected product (Le., "unknown" in Figure again increase substantially (panels G and H), although 2) is formed in 9% yield. Preliminary 'H nuclear magnetic they differ little from the yields observed in the respective resonance and mass spectroscopic data indicate that it is solvents at 70 "C. Overall, it appears that the reaction structurally related to the C-5 product, although it appears between neutral dGuo and 7-BrMeB[a]A is much more to have the elements of TFE covalently attached. A final sensitive to changes in reaction temperature than is the structural assignment for this new product will require same reaction involving dGuo-. For reactions involving additional spectroscopic investigations. the latter, higher yields of nucleoside products are produced in TFE rather than in acetone/HzO as solvent. A t 50 "C, neutral dGuo reacts with 'I-BrMeB[a]A to Indeed, the greatest amount of IP-(benz[a]anthracen-7produce the N2 product as the only nucleoside product. ylmethyl)-2'-deoxyguanosine is produced in TFE/TEA Its yield is 4 times greater in TFE (Figure 2, panel F) than solvent at 50 "C. in acetone/H20 at this temperature (panel E). In the
z
n
2o
Aralkylation of 2'-Deoxyguanosine
Figure 2 also illustrates that the N-1 and C-5 products are produced only in the presence of TEA in these solvents. While this was expected for formation of the N-1 product, it was not expected for the C-5 product since our previous studies with related benzylating agents (6-8, 12-14) indicated that these types of products were produced in reactions with neutral guanine nucleosides as well as with the anionic form. Until the mechanistic details of their formation are understood, it would serve little purpose to speculate on why the C-5 product is not produced in 7BrMeB[a]A reactions with neutral dGuo. The data s u g gest, however, that this type of product would probably not be expected in aqueous DNA reactions under neutral conditions. Certainly, many more studies are required to establish the range of electrophiles and reaction conditions that are capable of producing these products. Lyle et al. (15) previously examined the reaction between 6-([13C]chloromethyl)benzo[a]pyreneand 2'-deoxyguanosine as a function of pH and determined that the sites of reaction with dGuo in neutral dioxane/H20 (1:l) were the 7- and N2-positions, while at pH 10.5, the sites were the 1-and Os-positions. These observations clearly differ from ours, and it remains to be seen if this is a result of a medium effect or if the reactivities of 6-([13C]chloromethyl)benzo[a]pyrene and 7-BrMeB[a]A toward dGuo are radically different. It may be that an Os-substituted dGuo is also produced with 7-BrMeB[a]A, but it might decompose under our reaction conditions to produce solvolysis and/or nucleoside products as we observed previously with other Os-substituted guanosines (6, 7). Additional experiments will be required to test these possibilities. Nevertheless, the solvent effect data we have presented here indicate that the reaction of the anion of guanine nucleosides in TFE with selected solvolytically reactive electrophiles can provide significantly higher yields of the types of products typically formed only in aqueous reactions, although in much smaller amounts. This reaction system is likely to be very useful for the preparation of a wider variety of N2-substituted guanine nucleosides in the absence of an alternative totally synthetic route.
Acknowledgment. We are indebted to B. Hilton, G. Chmurney, and J. Klose for determining the 'H NMR spectra and to C. Metra1 and J. Roman for determining the mass spectra. This work was supported by the National Cancer Institute, DHHS, under Contract NO1-CO74101 with BRI. The contents of this publication do not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the US.Government. Registry No. dGuo, 961-07-9; dGuo-, 127209-29-4; N2, 127209-30-7;N-l,127209-31-8; C-5,127209-32-9;7-TFEMeB[a]A, 127209-33-0; 7-HOMeB[a]A, 16110-13-7; RBr, 24961-39-5; (C-
Chem. Res. Toxicol., Vol. 3, No. 4, 1990 295 HS)&O, 67-64-1; HzO, 7732-18-5; CF,CH20H, 75-89-8.
References (1) Dipple, A., Moschel, R. C., and Bigger, C. A. H. (1984) Polynu-
clear Aromatic Carcinogens. In Chemical Carcinogens(Searle, C. E., Ed.) ACS Monograph 182, 2nd ed., pp 41-163, American Chemical Society, Washington, DC. (2) Dipple, A., Moschel, R. C., and Hudgins, W.R. (1982) Selectivity of alkylation and aralkylation of nucleic acid componenQ. Drug Metab. Rev. 13, 249-268. (3) Dipple, A., Brookes, P., Mackintosh, D. S., and Rayman, M. P. (1971) Reaction of 7-bromomethylbenz[a]anthracenewith nucleic acids, polynucleotides and nucleosides. Biochemistry 10, 4324-4330. (4) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1979) Selectivity in nucleoside alkylation and aralkylation in relation to chemical carcinogenesis. J. Org. Chem. 44, 3324-3328. (5) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1980) Aralkylation of guanosine by the carcinogen N-nitroso-N-benzylurea. J. Org. Chem. 45,533-535. (6) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1981) Dissociation of 06-(p-methoxybenzy1)guanosinein aqueous solution to yield guanosine, p-methoxybenzylguanosines and 4-(p-methoxybenzyl)-5-guanidino-1-&~-ribofuranosylimidazole. J. Am. Chem. SOC. 103, 5489-5494. (7) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1984) Substituent-induced effects on the stability of benzylated guanosines: Model systems for the factors influencing the stability of carcinogen-modified nucleic acids. J. Org. Chem. 49, 363-372. (8) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1986) Reactivity effects on site selectivity in nucleoside aralkylation: A model for the factors influencing the sites of carcinogen-nucleic acid interactions. J. Org. Chem. 51, 4180-4185. (9) Bentley, T. W., and Schleyer, P. v. R. (1977) Medium effects on the rates and mechanisms of solvolyticreactions. Adu. Phys. Org. Chem. 14, 1-67. (10) Dipple, A., and Slade, T. A. (1970) Structure and activity in chemical carcinogenesis: Reactivity and carcinogenicity of 7bromomethylbenz [a]anthracene and 7-bromomethyl-12-methyl benz[a]anthracene. Eur. J. Cancer 6,417-423. (11) Badger, G. M., and Cook, J. W. (1939) The synthesis of growth-inhibitory polycyclic compounds. Part I. J. Chem. SOC., 802-806. (12) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1981) A novel product from the reaction of p-methylbenzyl chloride with guanosine in neutral aqueous solution. Tetrahedron Lett. 22, 2427-2430. (13) Carrell, H. L., Zacharias, D. E., Glusker, J. P., Moschel, R. C., Hudgins, W. R., and Dipple, A. (1982) X-ray crystallographic proof of electrophilic attack at the pyrimidine/imidazole ring junction in guanosine. Carcinogenesis 3, 641-645. (14) Moschel, R. C., Hudgins, W. R., and Dipple, A. (1986) Electrophilic attack at carbon-5of guanine nucleosides: Structure and properties of the resulting guanidinoimidazoleproducts. In The Role of Cyclic Nucleic Acid Adducts in Carcinogenesis and Mutagenesis (Singer, B., and Bartsch, H., Eds.) IARC Scientific Publications 70, pp 235-240, Oxford University Press, London, U.K. (15) Lyle, T. A., Royer, R. E., Daub, G. H., and Vander Jagt, D. L. (1980) Reactivity-selectivity properties of reactions of carcinogenic electrophiles and nucleosides: Influence of pH on site selectivity. Chem.-Biol. Interact. 29, 197-207.
