"Knock-out" analogues as a tool to quantify supramolecular processes: a theoretical study of molecular interactions in guanidiniocarbonyl pyrrole carboxylate dimers

It was recently shown experimentally that 5-(guanidiniocarbonyl)-1H-pyrrole-2-carboxylate 1, a self-complementary zwitterion, dimerizes even in water with an unprecedented high association constant of K = 170 M(-1) (J. Am. Chem. Soc. 2003, 125, 452-459). To get an insight into the importance of the various noncovalent binding interactions and of their interplay (electrostatic interactions, hydrogen binding, cooperative effects), we employ density functional theory to study the stability of several "knock-out" analogues in which single hydrogen bonds within these multiple point binding motif are switched off by replacing N-H hydrogen-donor groups with either methylene groups or an oxygen ether bridge. The influence of a highly polar solvent on the dimer stabilities is also examined. These calculations reproduce the experimental data for zwitterion 1. A comparison of 1 with the arginine dimer shows that the energy contents of the monomers also significantly influence the dimer stabilities. The analysis of the various "knock-out" analogues reveals as a main conclusion that simple models either based just on hydrogen-bond counting or on the assumption that the charge interaction by itself is the main and dominant factor fail to explain the stability of such self-assembled dimers. Our computations show that the hydrogen-bond network, the electrostatic attraction, and also their mutual interactions are responsible for the high stability of zwitterion 1.