Ion Pairing Between the Chain Ends Induces Folding of a Flexible Zwitterion in Methanol

A well defined folded loop structure can be induced in a flexible zwitterion 10 in the polar and protic solvent methanol by charge interactions between the two termini of the zwitterion. In 10 a (guanidiniocarbonyl)pyrrole moiety, a highly efficient oxoanion binding site, and a pyrrole‐2‐carboxylate unit serve as complementary binding sites at the ends of a flexible strand in which two amino acids, alanine and valine, are linked via a butylene spacer in a head‐to‐head orientation. Intramolecular ion pair formation between the carboxylate and the (guanidiniocarbonyl)pyrrole cation leads to the formation of a well defined loop as could be shown by NMR analysis (NOESY and H/D‐exchange experiments) as well as molecular modelling calculations. Without this intramolecular charge interaction, as in the protected and hence uncharged precursor 9 , no loop is formed but rather weak intermolecular dimerization is observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)