Highly Stable Self-Assembly in Water:  Ion Pair Driven Dimerization of a Guanidiniocarbonyl Pyrrole Carboxylate Zwitterion

The synthesis of a novel water-soluble guanidiniocarbonyl pyrrole carboxylate zwitterion 2 is described, and its self-association in aqueous solutions is studied. Zwitterion 2 forms extremely stable 1:1 dimers which are held together by an extensive hydrogen bonding network in combination with two mutual interacting ion pairs as could be shown by ESI MS and X-ray structure determination. NMR dilution studies in different highly polar solvents showed that dimerization is fast on the NMR time scale with association constants ranging from an estimated 10(10) 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 electrostatic interactions reported so far. Furthermore, an amidopyridine pyrrole carboxylic acid 10 was developed as a neutral analogue of zwitterion 2, which also dimerizes with an essentially identical hydrogen bonding pattern (according to ESI MS and X-ray structure determination) but lacking the ionic interactions. NMR binding studies demonstrated that the solely hydrogen-bonded neutral dimer of 10 is stable only in organic solvents of low polarity (K > 10(4) M(-1) in CDCl3 but <10 M(-1) in 5% DMSO in CDCl3). The comparison of both systems impressively underlines the importance of ion pair interactions for stable self-association of such H-bonded binding motifs in water.