Highly stable self-association of 5-(guanidiniocarbonyl)-1H-pyrrole-2-carboxylate in DMSO -: The importance of electrostatic interactions

The zwitterionic compound 5-(guanidiniocarbonyl)-1H-pyrrole-2-carboxylate (1) self-assembles to form dimers, which are completely stable in DMSO even at 170 degrees C or at concentrations of 0.001 mM. This high stability stems from a combination of multiple weak interactions. NMR titrations of 1H-pyrrole-2-carboxylate (5) with ( 1H-pyrrole-2-carbonyl)-guanidinium (6) and [5-(methoxycarbonyl)-1H-pyrrole-2-carbonyl]guanidinium (8) led to binding constants of K approximate to 10(6) mol(-1) in DMSO and K approximate to 10(3) mol(-1) in 40% water/DMSO for carboxylate binding by the 2-(guanidiniocarbonyl)pyrrole moiety. The stability constant for the dimer 1, in DMSO could therefore be estimated as K approximate to 10(12) mol(-1). In solution, the self-association process of 1 can be completely disrupted by protonation of the carboxylate group. In the solid state, however, the hydrochloride salt 1(+) also exists as a similar but only very weakly hydrogen-bonded dimer.