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. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).