A Molecular Oyster: A Neutral Anion Receptor Containing Two Cyclopeptide Subunits with a Remarkable Sulfate Affinity in Aqueous Solution
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- 作者：Stefan Kubik ; R. Kirchner ; D. Nolting ; and J. Seidel
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：October 30, 2002
- 年：2002
- 卷：124
- 期：43
- 页码：12752 - 12760
- 全文大小：171K
- 年卷期：v.124,no.43(October 30, 2002)
- ISSN：1520-5126
文摘
An artificial anion receptor is presented, in which two cyclohexapeptide subunits containingL-proline and 6-aminopicolinic acid subunits in an alternating sequence are connected via an adipinic acidspacer. This compound was devised to stabilize the 2:1 sandwich-type anion complexes that are observedwhen the two cyclopeptide moieties are not covalently connected and to obtain a 1:1 stoichiometry forthese aggregates. Electrospray ionization mass spectrometry and NMR spectroscopic investigations showedthat the bridged bis(cyclopeptide) does indeed form defined 1:1 complexes with halides, sulfate, and nitrate.ROESY NMR spectroscopy and molecular modeling allowed a structural assignment of the sulfate complexin solution. The stabilities of various anion complexes were determined by means of NMR titrations andisothermal titration microcalorimetry in 50% water/methanol. Both methods gave essentially the samequantitative results, namely stability constants that varied in the range 105-102 M-1 and decreased in theorder SO42- > I- > Br- > Cl- > NO3-. This order was rationalized in terms of the size of the anions withthe larger anions forming the more stable complexes because they better fit into the cavity of the host. Theability of sulfate to form stronger hydrogen bonds to the NH groups of the receptor, in addition to its slightlylarger ionic radius with respect to iodide, causes the higher stability of the sulfate complex. No significanteffect of the countercation on complex stability was observed. Furthermore, complex stability is enthalpicallyas well as entropically favored. A comparison of the iodide and sulfate complex stabilities of the ditopicreceptor with those of a cyclopeptide that forms 1:1 anion complexes in solution showed that the presenceof a second binding site increases complex stability by a factor of 100-350.