196
J . Am. Chem. Soc. 1991, 113, 196-201
mixture was extracted with CH2CI2. The organic layer was dried over Na2S04and the solvent was removed at reduced pressure. The residue was purified by flash chromatography (1:3 CH2CI2-petroleum ether) to afford 743.3 mg (83%) of 52 as a light yellow solid: mp >244 OC; ‘ H N M R 6 8.48 (d, JI,3 = JII,13 = 1.5, 2 H, H - I , H-13), 7.48-7.41 (m, 5 H, H-3, H - l I , H-3’, H-4’, H-5’), 7.16-7.12 (m, 4 H, H-4, H-10, H-2’, H-6’), 2.62 (t, Js.6 = 6.6, 4 H, H-6, H-8), 2.43 (t, Js,6 = 6.6, 4 H, H-5, H-9); ”C N M R 6 143.9, 138.7, 138.3, 138.1, 137.3, 134.1, 131.6, 130.7, 130.0, 128.7, 127.9, 120.2, 120.1, 104.0, 28.7, 27.5; M S (El, 70eV), m/z (relative intensity) 539 (4), 541 (7), 543 (3); M S (FAB, Xe) exact mass m / z 541.998 75, found 541.992 80. calcd for C29H20Br2N
the solvent was removed at reduced pressure. The residue was purified by flash chromatography (1:4 EtOAc-CH2C12)to afford 112 mg (65%) of 50 as a light green powder: ‘H N M R 6 8.25 (d, JI,3 = 1.1, 2 H, H-I, H-13), 8.06 (d, J3r,4,= 9.4, 4 H, H-4’, H - 5 9 , 7.58-7.51 (m, 3 H, H-3”, H-4”, H - 5 9 , 7.48 (d, J3,4= 7.5, 2 H, H-3, H - l I ) , 7.36-7.27 (m, 8 H, H-4, H-10, H-3‘, H-6’, H-2”, H-6”), 6.78 (d, J1,,3, = 2.7, 4 H, H-l’, H-8’), 2.86 (t, J5.6 = 6.5, 4 H, H-6, H-8), 2.61 (t, J5.6 = 6.5, 4 H, H-5, H-9); M S (FAB, Xe), m / z 858 ( M + H ) , exact mass calcd for C59H43N@., m / z 858.333 16, found 858.331 50.
Acknowledgment. W e thank Lori Bostrom and Kurt Saionz
2,12-Bis(2,7-dimethoxy-9-acridinyl)-7-phenyl-5,6,8,9-tetrahydrobenz- for synthetic contributions and Scott Wilson for assistance with
[ajlanthracene 14-Cyanide (50). To a solution of 105 mg (0.19 mmol) of 52 in 4 0 m L of THF at -90 OC was added 0.42 mL (0.394 mmol) of n-butyllithium in hexane. The resulting solution was stirred for 0.5 h and was transferred via canula to a solution of 199 mg (0.58 mmol) of 52 in I O mL of THF at -78 OC. The mixture was stirred for 3 h and was quenched with a saturated aqueous solution of ammonium chloride at -78 OC. The mixture was extracted with CHlC12and dried over Na2S04,and
the X-ray analysis. Funding from the National Institutes of Health (GM39782) and the National Science Foundation (CHE58202) is gratefully acknowledged. S.C.Z. acknowledges a Dreyfus Teacher-Scholar Award, an Eli Lilly Granteeship, and an N S F Presidential Young Investigator Award. Z.Z. thanks the University of Illinois for a Departmental Fellowship.
Complexation of Nucleotide Bases by Molecular Tweezers with Active Site Carboxylic Acids: Effects of Microenvironment Steven C. Zimmerman,* Weiming Wu, and Zijian Zeng Contribution from the Department of Chemistry, University of Illinois, Urbana, Illinois 61801, Received May 31, 1990
Abstract: In chloroform-d molecular tweezer 1 forms a 1:l complex (Job plot) with 9-propyladenine (4). Changes in the UV-visible absorption spectrum of 1 upon addition of 4 and the changes 1 and 4 induce in each other’s IH NMR spectrum
are consistent with those of a complex comprised of hydrogen bonds and a-stacking interactions. The microenvironment around the carboxylic acid group in 1 markedly alters its complexation behavior relative to a simple carboxylic acid such as butyric acid (Lancelot, G. J . Am. Chem. SOC.1977, 99,7037-7042). The association constants for the 1-4 and butyric acid-5 complexes are 25000 M-’ ( 2 9 8 K ) and 160 M-’ ( 3 0 3 K), respectively. Butyric acid prefers a type 1 hydrogen bonding pattern while 1 adopts a type 7 pattern. The nucleotide base selectivities follow the order G > C > A > U for butyric acid and A > G >> C > U for 1. The presence of protic solvents markedly decreases the strength of the complex between 1 and 4. Two analogues of 1 have also been studied, molecular tweezer 2 and 3. Both lack the dimethylamino substituent found in 1, while 3 has a spacer unit that is fully oxidized. The association constants for the 2-4 and 3-4 complexes are 14000 and 120000 M-I, respectively.
Noncovalent interactions are of fundamental importance to all biological processes. This has inspired the study of host-guest chemistry whose goals include the development of artificial enzymes and the understanding of complexation phen0mena.l While the small, usually nonpeptidic, organic hosts bear little resemblance to natural “receptors” such as enzymes and antibodies, they have several distinct advantages. They provide a more manageable degree of structural complexity. Furthermore, hosts with different functional group orientations, varying degrees of flexibility, and modified electronic properties can be synthesized and compared. Additionally, synthetic receptors are often soluble in several solvents, and because these molecules are constructed of covalent bonds they maintain their structural integrity in a wide range of media. Well-chosen changes in structure and solvent provide invaluable insights into molecular recognition phenomena. Our interest in this area has been with receptors, called “molecular tweezers”,* which complex aromatic guests through a-sandwiching3 and, more recently, through wsandwiching com( I ) (a) Cram, D. J. Angew. Chem., In!. Ed. Engl. 1988,27, 1009-1 112. (b) Host Guest Complex Chemistry, Macrocycles; VBgtle, F., Weber, E., Eds.; Springer-Verlag: New York, 1985. (c) Rebek, J . , Jr. Science (Washington, D.C.))1987,235, 1478-1484. (d) Lehn, J . M. Angew. Chem., Int. Ed. Engl. 1988. 27,89-1 12. (e) Diederich, F. Ibid. 1988,27,362-386. (f) Breslow, R. Arc. Chem. Res. 1980,13, 170-177. (2) Chen, C.-W.; Whitlock, H. W., Jr. J . Am. Chem. SOC.1978, 100, 4921-4922.
0002-7863/91/1513-196$02.50/0
bined with hydrogen b ~ n d i n g . ~These ,~ latter receptors were inspired by two quite different areas of research. The first involves the study of bichromophoric molecules that can bind to DNA by bis-intercalationn6 The second area is the study of protein-DNA re~ognition.~It was proposed several years ago that amino acid side chains might recognize D N A bases and base-pairs through a-stacking and hydrogen-bonding interactions. The most selective (3) (a) Zimmerman, S. C.; VanZyl, C. M. J . Am. Chem. Soc. 1987,109, 7894-7896. (b) Zimmerman, S. C.; VanZyl, C. M.; Hamilton, G. S.J . Am. Chem. SOC.1989,1 1 1 , 1373-1381. (c) Zimmerman, S. C.; Mrksich, M.; Baloga, M. J . Am. Chem. SOC.1989,I l l , 8528-8530. (4) Zimmerman, S. C.; Wu, W. J . Am. Chem. SOC.1989,111,8054-8055. (5) Reports dealing with hydrogen-bonded (host-guest) complexes have appeared with increasing frequency. Representative examples: Rebek, J., Jr.; Nemeth, D. J . Am. Chem. SOC.1985,107,6738-6739. Aarts, V. M. L. J.; van Staveren, C. J.; Grootenhuis, P. D. J.; van Eerden, J.; Kruise, L.; Harkema, S . ; Reinhoudt, D. N. J . Am. Chem. SOC.1986,108, 5035-5036. Pirkle, W. H.; Pochapsky, T. C. Ibid. 1986,108, 5627-5628. Kelley, T. R.; Maguire, M. P. J . Am. Chem. S o t . 1987,109, 6549-6551. Kilburn, J. D.; Mackenzie, A. R.; Still, W. C. Ibid. 1988,110, 1307-1308. Chang, S.-K.; Hamilton, A. D. Ibid. 1988,110, 1318-1319. Bell, T. W.; Liu, J. J . Am. Chem. Soc. 1988,1 IO,3673-3674. Sheridan, R. E.; Whitlock, H. W., Jr. Ibid. 1988,110,4071-4073. Ducharme, Y.; Wuest, J. D. J . Org. Chem. 1988,53, 5789-5791. Ebmeyer, F.;Vogtle, F.Angew. Chem., Int. Ed. Engl. 1989,28, 79-81. Tanaka, Y.; Kato, Y.; Aoyama, Y. J . Am. Chem. SOC.1990,112, 2807-2808. See also ref 13. (6) Wakelin, L. P. G.Med. Res. Reo. 1986,6 , 275-340. (7) See: Saenger, W. Principles of Nucleic Acid Structure; SpringerVerlag: New York, 1984; Chapter 18 and references therein.
0 1991 American Chemical Society
Complexation of Nucleotide Bases by Molecular Tweezers Table 1. IH NMR Complexation Shifts (A6) in 9-Propyladenine and Related Compounds Induced by Molecular Tweezer 1 in Chloroform-d at 298 Ka H-2 NH-6 NMe-6 H-8(7)‘ CH2N-9 0.17 -0.40 0.42 0.07 0.07 -0.31 -0.14 0.34 0.08 0.46 0.22 -0.02 ‘Shifts at ca. 40% saturation of guest. bPositive values represent upfield shifts. eH-8 in 4 and 8, H-7 in 9.
J . Am. Chem. SOC.,Vol, 113, No. 1. 1991 o.8
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1
guest 4 8 9
binding contacts between proteins and nucleic acids were realized to involve at least two hydrogen bonds, requiring t h e carboxylic acid group of aspartic or glutamic acid, t h e amide group of asparagine or glutamine, or the guanidinium group of arginine.* Numerous model studies have confirmed the feasibility of these interactions both in solutiong and in the solid state.I0 While this “direct readout” mechanism has been observed recently in t h e X-ray structures of repressor-DNA operator complexes,’’ recognition mechanisms that do not require direct hydrogen bonds appear possible. l 2 Using concepts found in these two areas of research, we sought to develop small molecules that could selectively bind mono- or polynucleotides. Other investigators have been similarly motivated and synthetic receptors for each of t h e four common nucleotide bases have been reported in the past several years.I3 These receptors were designed to hold multiple hydrogen bond donor and acceptor groups, and sometimes a single a-stacking surface, in a spatial arrangement that is complementary t o only one nucleotide. The results have been impressive. Most show selectivity in binding nucleotide bases and several form exceptionally stable complexes. By contrast, our approach was an empirical one. We chose to use a single carboxylic acid group, already shown by Lancelot14 to bind only moderately well to nucleotide bases and with only slight selectivity in chloroform (vide infra). The question to be answered was whether these unassuming properties might be improved by removing the carboxylic acid group from bulk solvent and placing it deep within t h e cleft of our molecular t w e e ~ e r s .The ~ work described herein has shown t h e answer t o be in the affirmative. Just as the properties of amino acid side chains held in clefts and depressions on t h e surface of proteins are altered by the m i c r ~ e n v i r o n m e n t , ’so ~ are the carboxylic acid groups in molecular tweezers 1-316 strongly influenced by the surrounding aromatic cleft in their interaction with nucleotide bases. (8) Bruskov, V. 1. Mol. Biol. (Moscow) 1975, 9, 245-249. Pabo, C. 0.; Jordan, S. R.; Frankel, A. D. J . Biomol. Strucf. Dyn. 1983, I , 1039-1049. (9) See: Lancelot, G.; Mayer, R.; HCltne, C. Biochim. Biophys. Acta 1979, 564, 181-190 and references therein. (IO) Sasada and co-workers have been very active in this area: Fujita, S.; Takenaka, A.; Sasda, Y. Bull. Chem. SOC.Jpn. 1984, 57, 1707-1712 and references therein. ( I I ) Jordan, S. R.; Pabo, C. 0. Science, (Washington, D.C.) 1988, 242, 893-899. Matthews, B. W. Nature (London) 1988, 335,294-295. (12) Otwinowski, 2.;Schevitz, R. W.; Zhang, R.-G.; Lawson, C. L.; Joachimiak, A.; Marmorstein, R. Q.;Luisi, B. F.; Sigler, P. B. Nature (London) 1988, 335,321-329. (13) Rebek, J., Jr.; Askew, B.; Ballester, P.; Buhr, C.; Jones, S.;Nemeth, D.; Williams, K. J . A m . Chem. SOC.1987, 109, 5033-5035. Hamilton, A. D.; Van Engen, D. Ibid. 1987, 109, 5035-5036. Rebek, J., Jr.; Williams, K.; Parris, K.; Ballester, P.;Jeong, K. S. Angew. Chem., In?. Ed. Engl. 1987, 26, 1244-1245. Feibush, B.; Saha, M.; Onan, K.; Karger, B.; Geise, R. J . Am. Chem. SOC.1987,109,7531-7533. Hamilton, A. D.;Pant, N. J . Chem. Soc., Chem. Commun. 1988,765-766. Jeong, K. S.; Rebek, J., Jr. J . A m . Chem. Soc. 1988, 110,3327-3328. Askew, B.; Ballester, P.; Buhr, C.; Jeong, K. S.; Jones, S.; Parris, K.; Williams, K.; Rebek, J., Jr. Ibid. 1989, 111, 1082-1090. Williams, K.; Askew, B.; Ballester, P.; Buhr, C.;Jeong, K. S.; Jones, S.; Rebek, J., Jr. Ibid. 1989, 1 1 1 , 1090-1094. Goswami, S.;Hamilton, A. D.; Van Engen, D. Ibid. 1989, 111, 3425-3426. Adrian, J. C., Jr.; Wilcox, C. S. Ibid. 1989, 111, 8055-8057. (14) Lancelot, G. J . Am. Chem. SOC.1977. 99. 7037-7042. These studies were performed at 303 K. (15) Cf.: Fersht, A. R. Enzyme Structure and Mechanism, 2nd ed.; Freeman: New York, 1985; pp 165-166. (16) Zimmerman, S. C.; Zeng, Z.; Wu, W.; Reichert, D. E. J . Am. Chem. SOC.previous paper in this issue.
0.2
If
0.0 0.000 0.001 0.002 0.003 0.004 0.005 [9 P ropy I ad e n i ne] ( M)
-
Figure 1. Plot of the upfield chemical shift (‘H NMR) of anthracene H-10 resonance in 1 as a function of added 9-propyladenine (4).
Results and Discussion Complexation of 9-Propyladenine. T h e self-association of 1 was studied over a wide concentration range. From to M chloroform, the absorption spectra of 1 obeyed Beer’s law. From lo4 to IO-* M chloroform-d, the IH NMR chemical shifts of the anthracene rings in 1 were found to change by
