| Abstract
| - It was recently shown experimentally that 5-(guanidiniocarbonyl)-1H-pyrrole-2-carboxylate 1, aself-complementary zwitterion, dimerizes even in water with an unprecedented high association constantof K = 170 M-1 (J. Am. Chem. Soc.2003, 125, 452−459). To get an insight into the importance of thevarious 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 byreplacing N−H hydrogen-donor groups with either methylene groups or an oxygen ether bridge. Theinfluence of a highly polar solvent on the dimer stabilities is also examined. These calculations reproducethe experimental data for zwitterion 1. A comparison of 1 with the arginine dimer shows that the energycontents 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 factorfail to explain the stability of such self-assembled dimers. Our computations show that the hydrogen-bondnetwork, the electrostatic attraction, and also their mutual interactions are responsible for the high stabilityof zwitterion 1.
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