Spectroscopic signatures and structural motifs in isolated and hydrated serotonin: a computational study

Abstract

The conformational landscapes of neutral serotonin and its hydrated complex were characterized by MP2, CC2 and DFT methods. The ground state geometry optimization of the twenty three lowest energy structures of serotonin have been performed employing higher basis sets. The MP2, CC2 and DFT (M06-2X, ωB97X-D and B3LYP-D3) calculations predict that the Gph-out/anti conformation of serotonin is the most stable which is in agreement with the experimental rotational spectroscopy (Cabezas et al., Phys. Chem. Chem. Phys., 2012, 14, 13618) and is in contrast to the resonance-enhanced two photon ionization (R2PI) and UV–UV hole burning (UVHB) spectroscopy results. Computed wave-numbers and intensities of the observed conformers are found in consonance with the experiment (LeGreve et al., J. Am. Chem. Soc., 2007, 129, 4028). The predicted intensity of the OH bending fundamental provides a useful diagnostic for the 5-OH anti and syn conformations. The computed hydrogen bond geometries of the experimentally observed Sero₁–(H₂O)₁ and Sero₁–(H₂O)₂ clusters are found in remarkable agreement with the experiment. The Sero₁–(H₂O)₂ involving the Gph(out) conformation is used to form a strong water dimer bridge to the 5-OH with a binding energy of 104 kJ mol⁻¹. The low-lying excited states of each experimentally observed conformer of serotonin have been determined by means of coupled cluster singles and approximate doubles (CC2) and TDDFT methods and a satisfactory interpretation of the electronic absorption spectra is obtained. One striking feature is the coexistence of the blue and red shift of the vertical excitation energies of the ¹L_b (ππ*) and the ¹L_a (ππ*) state upon forming a complex with water. The effect of hydration on the lowest ¹L_b (ππ*) excited state due to the bulk water environment was mimicked by a combination of a polarizable continuum solvent model (PCM) and conductor like screening model (COSMO), for the different serotonin conformations which shows a red shift. The lowest excited state (¹L_b) of the most stable Sero₁–(H₂O)₁ structure shows a significant shift of 1.15 Å for a water molecule towards the 5-OH group due to the S₀–S₁ electronic excitation.