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

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 center dot center dot center dot pi 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 center dot center dot center dot pi bound intermolecular interactions in the cyclic asymmetric trimer is quite different unlike the corresponding interactions of similar strength in a cyclic symmetric trimer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4706517]