The 5-hydroxytryptamine receptors (5HTn) are optimized for 5-hydrotryptamine molecules, resulting in a significantly enhanced psychoactive response compared with the 4-, 6-, 7-isomers. This is despite their relatively similar energetic stabilities, excited state lifetimes and emission characteristics. In this work we investigate the conformational space of serotonin (5-hydroxytryptamine) using a combination of rotationally resolved electronic spectroscopy and ab initio calculations. The geometries of the four most abundant conformers are assigned from their molecular parameters in the electronic ground and excited state. We find a conformer-dependent competition between two polar groups trying to establish a hydrogen bond with the same H-atom in the most stable conformer of serotonin. The result explains some remarkable deviations with respect to the conformational space of the closely related neurotransmitter tryptamine. Based on the comparison to other 5-substituted indoles we propose to generalize this finding to explain the conformational preferences of indole-based neurotransmitters.