J. Org. Chem. 2001, 66, 7313-7319
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Thermodynamic Aspects of Dicarboxylate Recognition by Simple Artificial Receptors Brian R. Linton,‡,§ M. Scott Goodman,‡,| Erkang Fan,‡,⊥ Scott A. van Arman,‡,# and Andrew D. Hamilton*,† Departments of Chemistry and Molecular Biophysics and Biochemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, and Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
[email protected] Received April 23, 2001
Recognition of dicarboxylates by bis-functional hydrogen-bonding receptors displays divergent thermodynamics in different solvent systems. NMR titration and isothermal titration calorimetry indicated that neutral bis-urea and bis-thiourea receptors form exothermic complexes with dicarboxylates in DMSO, with a near zero entropic contribution to binding. The increased binding strength of bis-guanidinium receptors precluded quantitative measurement of binding constants in DMSO, but titration calorimetry offered a qualitative picture of the association. Formation of these 1:1 complexes was also exothermic, but additional endothermic events occurred at both lower and higher host-guest ratios. These events indicated multiple binding equilibria but did not always occur at a discrete 2:1 or 1:2 host-guest molar ratio, suggesting higher aggregates. With increasing amounts of methanol as solvent, bis-guanidinium receptors form more endothermic complexes with dicarboxylates, with a favorable entropy of association. This switch from association driven by enthalpy to one driven by entropy may reflect a change from complexation involving the formation of hydrogen bonds to that promoted by solvent liberation from binding sites. Introduction One approach to understanding the controlled bimolecular association that is fundamental to most aspects of biological function relies on the design and synthesis of small-molecule receptors that are capable of selectively recognizing a substrate under physiological conditions. A traditional approach would position hydrogen-bonding, electrostatic, or hydrophobic functionality on a receptor complementary to the desired guest.1 Often not considered, but of seminal importance, is the role that the solvent plays in this association. Solvent participation was explicitly neglected in the earliest small-molecule receptors binding in nonpolar organic solvents, but increases in importance as host-guest design progresses into more competitive solvents, such as water.2 An indication of solvent participation can be gleaned from the enthalpic and entropic contributions to association. The decrease in rotational and translational degrees * To whom correspondence should be addressed. E-mail:
[email protected]. † Yale University. ‡ University of Pittsburgh. § Current address: Department of Chemistry, Bowdoin College, 6600 College Station, Brunswick, ME 04011-8466. | Current address: Department of Chemistry, Buffalo State College, 1300 Elmwood Ave., Buffalo, NY 14222. ⊥ Current address: Department of Biological Structure, University of Washington, Box 357742, Seattle, WA 98195. # Current address: Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17604-3003. (1) Several others have designed receptors capable of substrate recognition in water using electrostatic and hydrogen-bonding forces. Torneiro, M.; Still, W. C. Tetrahedron 1997, 53, 8739-8750. Hossain, M. A.; Schneider, H.-J. J. Am. Chem. Soc. 1998, 120, 11208-11209. Rotello, V. M.; Viani, E. A.; Deslongchamps, G.; Murray, B. A.; Rebek, J., Jr. J. Am. Chem. Soc. 1993, 115, 797-798. (2) Adrian, J. C., Jr.; Wilcox, C. S. J. Am. Chem. Soc. 1991, 113, 678-680.
of freedom that occurs with bimolecular assembly is an entropically unfavorable event, present in all receptorsubstrate interactions. The favorable entropy often observed with hydrophobic binding is largely associated with the liberation of solvent from lipophilic surfaces, compensating for the immobilization of the substrates, although enthalpic effects can still be the driving force.3 Enthalpic effects are balanced between solvation of the substrates, interactions within bulk solvent, and the attraction between host and guest. These effects have been documented in lipophilic cyclophane hosts,3 but remain to be established with receptors that rely on hydrogen bonding. An analysis of solvent effects on hydrogen-bonded complexes is complicated by the dissipation in binding affinity that occurs in highly polar solvents, such as alcohols or water. Competition with solvent for binding sites makes association more difficult. Any investigation must be able to quantify both the high binding affinity observed in less polar solvents and the low binding affinity observed in highly polar media. One approach is to create a series of synthetic receptors that provide the same presentation of hydrogen-bonding sites but differ vastly in their binding strength. In our continuing exploration of the recognition of biologically relevant substrates, we have designed simple artificial receptors that complex dicarboxylate derivatives.4 This paper focuses on association in increasingly competitive solvents, from dimethyl sulfoxide (DMSO) to water, and attempts to illustrate the thermodynamic consequences (3) Smithrud, D. B.; Wyman, T. B.; Diederich, F. J. Am. Chem. Soc. 1991, 113, 5420-5426. (4) Fan, E.; VanArman, S. A.; Kincaid, S.; Hamilton, A. D. J. Am. Chem. Soc. 1993, 115, 369.
10.1021/jo010413y CCC: $20.00 © 2001 American Chemical Society Published on Web 10/11/2001
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Linton et al. Chart 1
of binding in these various solvent systems. All receptors bind exothermically in DMSO, but become more endothermic with added methanol. In addition, calorimetric analysis demonstrated multiple binding equilibria which were not detectable using NMR titration. Results and Discussion Successful evaluation of the thermodynamic effects of solvent on dicarboxylate recognition requires a series of receptors that can form similar complexes but with varying binding strengths. This arises from the difficulty in the quantitative measurement of association for a single receptor in a variety of solvents, because the complexation is either too strong in less polar solvents or too weak in more polar solvents. Previous results from our group4-6 and others7-9 have shown that bidentate hydrogen-bonding groups are complementary to a variety of oxyanions.10 This paper focuses on three binding moieties, ureas (1, X ) O), thioureas (1, X ) S), and guanidiniums (1, X ) NH2+), that all show binding to
carboxylate, with the association strength increasing with hydrogen bond donor acidity.4 Attachment of two of these bidentate binding groups onto a semirigid spacer unit (5) Linton, B. R.; Hamilton, A. D. Tetrahedron 1999, 55, 6027-6038. (6) Salvatella, X.; Peczuh, M. W.; Gairi, M.; Jain, R. K.; SanchezQuesada, J.; deMendoza, J.; Hamilton, A. D.; Giralt, E. Chem. Commun. 2000, 1399-1400. (7) Sebo, L.; Schweizer, B.; Diederich, F. Helv. Chim. Acta 2000, 83, 80-92. (8) Berger, M.; Schmidtchen, F. P. J. Am. Chem. Soc. 1999, 121, 9986-9993. Berger, M.; Schmitchen, F. P. Angew. Chem., Int. Ed. 1998, 37, 2694-2695. Czekalla, M.; Stephan, H.; Habermann, B.; Trepte, J.; Gloe, K.; Schmidtchen, F. P. Thermochim. Acta 1998, 313, 137144. (9) Schneider, S. E.; O’Neil, S. N.; Anslyn, E. V. J. Am. Chem. Soc. 2000, 122, 542-543. Anzenbacher, P., Jr.; Try, A. C.; Miyaji, H.; Jursikova, K.; Lynch, V. M.; Marquez, M.; Sessler, J. L. J. Am. Chem. Soc. 2000, 122, 10268-10272. Sasaki, S.; Mizuno, M.; Naemura, K.; Tobe, Y. J. Org. Chem. 2000, 65, 275-283. Snellink-Ruel, B. H. M.; Antonisse, M. M. G.; Engbersen, J. F. J.; Timmerman, P.; Reinhoudt, D. N. Eur. J. Org. Chem. 2000, 1, 165-170. Xie, H.; Yi, S.; Yang, X.; Wu, S. New J. Chem. 1999, 23, 1105-1110. Davis, A. P.; Lawless, L. J. Chem. Commun. 1999, 9-10. Metzger, A.; Anslyn, E. V. Angew. Chem., Int. Ed. 1998, 37, 649-651. Kelly, T. R.; Kim, M. H. J. Am. Chem. Soc. 1994, 116, 7072. Gross, R.; Durner, G.; Gobel, M. G. Liebigs Ann. Chem. 1994, 49-54. Hamann, B. C.; Branda, N. R.; Rebek, J., Jr. Tetrahedron Lett. 1993, 34, 6837-6840. Smith, P. J.; Reddington, M. V.; Wilcox, C. S. Tetrahedron Lett. 1992, 33, 6085-6088.
creates synthetic receptors that can complex dicarboxylates via four hydrogen bonds. In this way, we have linked the hydrogen-bonding groups to a xylylene spacer to form bis-urea 2, bis-thiourea 3, and bis-guanidiniums 4 and 5 (see Chart 1). This series of artificial receptors permits an exploration of dicarboxylate recognition in competitive solvents such as DMSO, methanol, and water. Although these receptors successfully complex a variety of dicarboxylates, two representative substrates were chosen for this study: glutarate 6 and a more rigid analogue adamantane-1,3-dicarboxylate 7. Both dicarboxylates were employed as their bis-tetrabutylammonium (TBA) salts. The fidelities of these ditopic interactions were supported by several techniques. Job’s analysis verified the 1:1 stoichiometry for all three complexes.11 Positioning within the host-guest complex was indicated by detection of intermolecular NOEs. For example, irradiation of the central aromatic protons of bis-guanidinium 4 in complex with both glutarate and 5-nitroisophthalate produced enhancement of the guest proton signals, as shown in Figure 1. Figure 1c shows the aromatic region of the NMR spectrum of the complex (Figure 1b) along with the difference spectrum produced from irradiation of the receptor aromatic signals (at right). Molecular modeling12 indicated energetic minima with the anticipated positioning of each carboxylate interacting with one binding group.13 Attempts to grow crystals suitable for X-ray analysis using these receptors produced only polymeric complexes with bidentate hydrogen bonding between binding group and carboxylate, similar to previous urea-carboxylate structures.14 No crystals comprised of a distinct 1:1 complex were obtained. Further evidence for the presence of hydrogen bonding upon complexation was the large (>1 ppm) changes in the chemical shifts of both urea and thiourea NH protons with added guest; guanidinium NH protons were broadened under these conditions. NMR titration (10) For comprehensive reviews of anion recognition, see the following: Bianchi, A.; Bowman-James, K.; Garcia-Espan˜a, E. Supramolecular Chemistry of Anions; Wiley-VCH: New York, 1997. Schmitchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609-1646. Seel, C.; Gala´n, A.; deMendoza, J. Top. Curr. Chem. 1995, 175, 101-132. Snowden, T. S.; Anslyn, E. V. Curr. Opin. Chem. Biol. 1999, 3, 740-746. Beer, P. D.; Schmitt, P. Curr. Opin. Chem. Biol. 1997, 1, 475-482. (11) Connors, K. A. Binding Constants; John Wiley and Sons: New York, 1987. (12) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440-467. Molecular modeling was carried out using the Amber force field. (13) Mitchell, J. B. O.; Thornton, J. M.; Singh, J. J. Mol. Biol. 1992, 226, 251-262. This provides a survey of guanidinum-carboxylate complexes through calculation and a survey of protein crystal structures. (14) Zafar, A.; Geib, S. J.; Hamuro, Y.; Hamilton, A. D. New J. Chem. 1998, 22, 137-141.
Thermodynamic Aspects of Dicarboxylate Recognition
J. Org. Chem., Vol. 66, No. 22, 2001 7315 Table 1. Association Constants for Dicarboxylates in D2O/MeOD Mixturesa host guest 10% D2O 20% D2O 25% D2O 50% D2O 75% D2O 4 5
6 7 6 7
2800 3300 3100 5000
850 1300 1100 1500
720 780 840 1160
230 300 290 440
130 320
Measured by NMR titration at 20 °C. Ka are listed in M-1. Errors in Ka 50 000 M-1), but addition of more competitive solvents reduced the association strength to a measurable level due to favorable solvation of both host and guest. In an effort to gauge the effect of solvent composition on association, binding titrations were performed in a variety of aqueous methanol mixtures, and the association constants for both bis-guanidiniums binding to both dicarboxylates are shown in Table 1. Suprisingly, little selectivity is observed between either receptor or either dicarboxylate, with both bisguanidiniums being flexible enough to match the spacing of the dicarboxylates. The slightly tighter binding of the more rigid adamantane-dicarboxylate is probably a reflection of its increased preorganization The decline in association strength with added water reinforces the presumption that a more strongly hydrogen bond donating solvent will reduce binding affinity through increased solvation of both host and guest, as compared to methanol. Association was still observed, however, in high percentages of water suggesting that recognition in aqueous media can be achieved with the accumulation of multiple charged hydrogen-bonding groups. Thermodynamics of Dicarboxylate Association. The thermodynamic contributions to dicarboxylate binding for all four receptors (2-5) were investigated by both van’t Hoff analysis of NMR titrations as well as ITC. NMR titration data from 10 to 50 °C are shown in Table 2, in two solvent systems. The data for bis-urea 8 and bis-thiourea 9 were determined using d6-DMSO as the solvent, but due to the increased strength of the bisguanidinium-dicarboxylate association, data for derivatives 4 and 5 were determined in 25% D2O/MeOD. Van’t Hoff analysis of the variable temperature data permitted determination of the enthalpic and entropic contributions (16) Breslauer, K. J.; Freire, E.; Straume, M. Methods Enzymol. 1992, 211, 533-567. Freire, E.; Mayorga, O. L.; Straume, M. Anal. Chem. 1990, 62, 950A-959A. Kamiya, M.; Torigoe, H.; Shindo, H.; Sarai, A. J. Am. Chem. Soc. 1996, 118, 4532-4538. Berger, C.; Jelesarov, I.; Bosshard, H. R. Biochemistry 1996, 35, 14984-14991. Livingstone, J. Nature 1996, 384, 491-492. Ladbury, J. E. Structure 1995, 3, 635-639. Ladbury, J. E.; Wright, J. G.; Sturtevant, J. M.; Sigler, P. B. J. Mol. Biol. 1994, 238, 669-681. Cooper, A.; McAlpine, A.; Stockley, P. G. FEBS Lett. 1994, 348, 41-45. Ream, J. E.; Yuen, H. K.; Frazier, R. B.; Sikorski, J. A. Biochemistry 1992, 31, 55285534. (17) Arena, G.; Casnati, A.; Contino, A.; Gulino, F. G.; Sciotto, D.; Ungaro, R. J. Chem. Soc., Perkin Trans. 2 2000, 419-423. (18) Hu¨nenberger, P. H.; Granwehr, J. K.; Aebischer, J.-N.; Ghoneim, N.; Haselbach, E.; vanGunsteren, W. F. J. Am. Chem. Soc. 1997, 119, 7533-7544. Zimmerman, S. C.; Kwan, W.-S. Angew. Chem., Int. Ed. Engl. 1995, 34, 2404-2406. Inoue, Y.; Liu, Y.; Tong, L.-H.; Shen, B.-J.; Jin, D.-S. J. Am. Chem. Soc. 1993, 115, 10637-10644. Cromwell, W. C.; Bystrom, K.; Eftink, M. R. J. Phys. Chem. 1985, 89, 326-332.
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Table 2. Association Constants at Various Temperaturesa host guest 10 °C 2 3
4 5
6 7 6 7 6 7 6 7
600 580 720 1000
20 °C
40 °C
50 °C
∆H
∆S
d6-DMSO 790 620 510 1950 1560 1150 11 000 8500 6300 22 000 19 800 16 100
30 °C
420 910 5200 12 900
-4.0 -4.9 -4.8 -3.4
+0.3 -1.5 +2.0 +8.3
25% D2O/MeOD 720 1010 780 1550 840 1070 1160 1570
1430 2250 1280 2200
+3.8 +6.2 +2.5 +3.5
+26 +34 +22 +26
a Measured by NMR titration with bis-TBA salts of listed dicarboxylates. Errors in Ka
