Self-Folding Molecules: A Well Defined, Stable Loop Formed by a Carboxylate−Guanidinium Zwitterion in DMSO

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Abstract
Two zwitterions 1a,b have been synthesized, in which a carboxylate group is attached via a flexible alkyl chain of different length (butylene and ethylene, respectively) to a guanidiniocarbonyl pyrrole cation moiety. For 1b, no signs for either an intra- or intermolecular association between these two groups in polar solution (DMSO) could be found. In contrast to this, the 1H NMR spectrum of 1a shows clear evidence for a strong interaction between the carboxylate and the guanidiniocarbonyl pyrrole cation. According to variable-temperature and concentration-dependent NMR studies, this interaction stems from an intramolecular complexation. It was shown by ROESY and H/D-solvent exchange experiments that 1a, even in DMSO, folds into a well-defined intramolecular loop conformation held together by multiple weak interactions.