Experimental Determination of Conformational Isomerization Energy Thresholds in Serotonin

Stimulated emission pumping-population transfer (SEP-PT) and hole-filling (SEP-HF) spectroscopies were used to determine the energy thresholds to isomerization between thirteen reactant−product conformer pairs in the biomolecule serotonin (SERO). Serotonin is a close structural analog of tryptamine (TRA), differing in having a hydroxyl group in the 5 position of the indole ring. A previous spectroscopic study (LeGreve; et al. J. Am. Chem. Soc. 2007, 129 (13), 4028) identified eight conformational isomers of SERO, whose interconversion involves motion of the 3-ethylamine side chain, the 5-OH group, or both. In the cases in which only an ethylamine side chain reorientation occurred, the barriers were found to be similar to, but systematically somewhat smaller than, those in TRA, which has been studied by similar methods (Dian; et al. Science 2004, 303 (5661), 1169; Clarkson; et al. J. Chem. Phys. 2005, 122 (21), Art. No. 214311). In most cases, the experimental thresholds are well reproduced by calculated t...