The TRA (3-[2-aminoethyl]indole) is an important neurotransmitter with a close structural and chemical similarity to the neurotransmitter serotonin (5-hydroxytryptamine), and to melatonin (5-methoxy-N-acetyltryptamine), which plays a key role in daily human behavior. Moreover, TRA, and other indolic compounds are very efficient antioxidants. In this work the conformational space of TRA was scanned in aqueous solution, simulating the solvent by the polarizable continuum model. Geometry optimizations were performed at B3LYP/6-31+G** level. Electronic distributions were analyzed at a better calculation level, thus improving the basis set (6-311++G**). A topological study based on Bader's theory (atoms in molecules) and natural bond orbital (NBO) framework was performed. Structural changes found in solution were related with charge delocalization mechanisms, which explained the changes in the conformational relative population in aqueous phase. Solvent effects on molecular electrostatic potential (MEPs) were also quantified and rationalized through charge delocalization mechanisms, thus connecting changes in MEPs with changes in structure, bond polarization, orbital bonding populations, natural charges, and bond topological properties. Moreover, polarizabilities and dipolar moments were calculated. All conformers were taken into account. Our results are the first prediction of TRA polarizabilities. The results reported contribute to the understanding of the structure, stability and reactivity of TRA and other indole derivatives.

atoms in molecules) and natural bond orbital (NBO) framework was performed. The study was enriched by a deep analysis of maps of molecular electrostatic potential (MEP) through a coordinated NBO/AIM analysis. The conformational preferences were explained by hyperconjugative interactions, which were revealed by NBO data. Because radical scavenging by indolic compounds is strongly modulated by their functional residues our study was related to similar analysis done previously on Indole and 1H-indole-3-acetic acid (IAA). Therefore, the conformational space of TRA was studied from a new perspective focusing on a deep analysis of the geometric and electronic properties of TRA conformers. The changes of the electronic distribution introduced by the substituent and the conformational flexibility of the side chain were addressed. The results reported contribute to the understanding of the structure, stability and reactivity of TRA and others indole derivatives.

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Theoretical studies on 1H-indole-3-acetic acid (IAA) were performed to investigate the conformational properties of dimeric species and vibrational spectra. Experimental infrared spectra at 100 K and 297 K and Raman spectrum at 297 K were analyzed and compared against calculations performed at B3LYP/6-31G** level. A exploratory study of the conformational space of dimeric species was performed. Our analysis showed that dimeric forms predicted theoretically contribute distinctively to the assignments of experimental results. These structures are defined by the orientation of the acetyl moieties with respect to the plane of indole ring. The dimers are formed by two symmetrical IAA monomers (one of them with the acetyl moiety upward oriented, Re-face, and the other isomer having the acetyl moiety downward oriented, Si-face) in tail-to-tail way. The X-ray geometry and FTIR vibrational frequencies were compared with the results of DFT calculations. A conformational equilibrium involving the non-equivalent IAA dimers: CCT-CCT, A(+)A(+)T-A(-)A(-)T, A(+)A(-)T-A(-)A(+)T, and A(+)CT-A(-)CT was found. The relation of the conformational properties of the IAA molecule with the features of the vibrational spectra was described in detail. The band assignments were discussed as related to the conformations properties. Our analysis shows the significance of the theoretical study of the conformational space of the monomeric molecule in the rationalization of experimental results.

atoms in molecules) and natural bond orbital (NBO) framework performed with the aim to analyze the stability and reactivity of the conformers allowed the understanding of electronic aspects relevant in the study of the antioxidant properties of IAA. Intramolecular hydrogen bonds were found and were characterized as blue-shifting hydrogen bonding interactions. Furthermore, molecular electrostatic potential maps (MEPs) were obtained and analyzed in the light of AIM and NBO results, thus showing subtle but essential features related not only to reactivity but also with intramolecular weak interactions, charge delocalization and structure stabilization.