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| - Highly Stable Self-Assembly in Water: Ion Pair DrivenDimerization of a Guanidiniocarbonyl Pyrrole CarboxylateZwitterion
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| - The synthesis of a novel water-soluble guanidiniocarbonyl pyrrole carboxylate zwitterion 2 isdescribed, and its self-association in aqueous solutions is studied. Zwitterion 2 forms extremely stable 1:1dimers which are held together by an extensive hydrogen bonding network in combination with two mutualinteracting ion pairs as could be shown by ESI MS and X-ray structure determination. NMR dilution studiesin different highly polar solvents showed that dimerization is fast on the NMR time scale with associationconstants ranging from an estimated 1010 M-1 in DMSO to a surprisingly high 170 M-1 in water. Hence,zwitterion 2 belongs to the most efficient self-assembling systems solely on the basis of electrostaticinteractions reported so far. Furthermore, an amidopyridine pyrrole carboxylic acid 10 was developed asa neutral analogue of zwitterion 2, which also dimerizes with an essentially identical hydrogen bondingpattern (according to ESI MS and X-ray structure determination) but lacking the ionic interactions. NMRbinding studies demonstrated that the solely hydrogen-bonded neutral dimer of 10 is stable only in organicsolvents of low polarity (K> 104 M-1 in CDCl3 but <10 M-1 in 5% DMSO in CDCl3). The comparison ofboth systems impressively underlines the importance of ion pair interactions for stable self-association ofsuch H-bonded binding motifs in water.
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