Mimicking trimeric interactions in the aromatic side chains of the proteins: A gas phase study of indole…(pyrrole)2 heterotrimer

Abstract

Aromatic trimeric interactions are extremely significant in the stabilization of the specific structures of the proteins as well as protein-protein, and protein-ligand interactions. Here we have reported a direct evidence of the observation of a cyclic asymmetric structure of indole…(pyrrole)2 trimer bound by three N–H…π hydrogen bonding interactions in a supersonic jet. The experiment has been performed by using resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectroscopic techniques. Density functional theory (DFT) calculations nicely corroborate the experimental results showing one weakly allowed IR-active band due to symmetric stretch of the N–H bonds and two strongly allowed IR-active bands due to two types of asymmetric stretches of the N–H bonds in the trimer. The present spectroscopic investigation demonstrates that the strength of the three N–H…π bound intermolecular interactions in the cyclic asymmetric trimer is quite different unlike the corresponding interactions of similar strength in a cyclic symmetric trimer.