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Yol. 56were designed, however, for a different purpose, and are not as well adapted for extrapolation, 0.0 0.2 reaction V02+ + 2H+ + e = VO + + + H20 has been determined as 0.9996 volt. Berkeley, Calif.

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The differences in the extractability of the various mercaptans from an oil phase by an aqueous alkaline solution are due almost entirely to differences in the solubility of the unneutralized mercaptan in the aqueous phase, rather than to differences in the acidity of the various mercaptans.

In the past decade, the role of complementary size, shape, and functionality in the molecular recognition of neutral molecules has been pointed out. Artificial receptor molecules have been designed with multiple recognition sites and a well-defined geometry, capable of strong and selective binding of the appropriate substrates by hydrogen bonding, ionic interaction, and/or hydrophobic interactions .2-8 Rebek et aL2 have used molecular clefts with acidic functions for the complexation of diamines, such as imidazole. Our group has shown that additional hydrogen bond donorsg or electrophilic metal centers,l0 incorporated in the cavities of macrocyclic polyethers, improve strongly the complexation of urea. This type of cocom-plexation of a neutral guest by a host molecule either by hydrogen bonding or by coordination with a metal ion, is frequently observed in (metal1o)enzymes. Imidazole and guanidine are two biologically important organic bases. These base residues are part of the essential amino acids histidine and arginine, and occur in many natural po1ypeptides.l' Guanidine is one of the strongest organic bases (pK, = 13.6),12 and it will predominantly be present as the conjugated acid. On the other hand, im-idazole is present as a free base in the pH range at which most enzymes function (pK, = 6.99)." This may contribute to the unique properties of histidine residues in the active site of most enzymes where they can act either as proton donor or proton acceptor. Imidazole Histidine For the optimal complexation of polyfunctional cations such as uronium, and guanidinium salts, it has been shown that crown ethers with at least 27 ring atoms are needed for the formation of encapsulated c o m p l e ~ e s. l ~-~ ~ Oth-(1) Grootenhuis, P. D. J.; van der Wal, P. D.; Reinhoudt, D. N. Tet-(2) Rebek, J., Jr.; Askew, B.; Killoran, M.; Nemeth, D.; Lin, F.-T. van Staveren, C. J.; Grootenhuis, P. D. J.; van Eerden, J.; Harkema, S.; Reinhoudt, D. N. J. Am. Chem. SOC. 1986,108, 5035. (10) (a) van Staveren, C. J.; Fenton, D. E.; Reinhoudt, D. N.; van Eerden, J.; Harkema, S. 2, n = 7: B27C9 3, n = 8: B30C10 4, n = 9: B33Cll 5, n = 1, m = 5 [3,7]DB30C10 6, n = 0, m = 6: [2,8]DB30C10 7, n = 2, m = 5: [4,7]DB33Cll "The following code was used in the abbreviated names: C = crown, B = benzo, D = di (or bis), [,I = code for the amount of hetero atoms between the functionalities. Table I. Extraction Efficiency of Benzo and Dibenzo Crown Ethers for Imidazolium Perchlorate extraction crown ether ring size [CE],"," M [CEImt,b M efficiencv 1 24 0.20 0.19 <0.05 2 27 0.12 0.12 <0.05 3 30 0.20 0.18 0.62 4 33 0.18 0.18 0.65 5 30 0.15 0.15 0.56 6 30 0.11 0.11 0.21 7 33 0.20 0.20 <0.05 The initial crown ether concentration in the organic phase. bThe total crown ether concentration in the organic phase at equilibrium. The ratio of the imidazolium perchlorate concentration and the total crown ether concentration in the organic phase at equilibrium; Standard deviation *lo%. erwise, relatively weak perching complexes are formed.21 Many X-ray structur...