Infrared and Ultraviolet Spectral Signatures and Conformational Preferences of Jet-Cooled Serotonin

Abstract

The ultraviolet and infrared spectroscopy of single conformations of neutral serotonin (5-hydroxytryptamine) have been studied in the gas phase using a combination of methods including laser-induced fluorescence, resonance-enhanced two-photon ionization, UV-UV hole-burning spectroscopy, and resonant ion-dip infrared spectroscopy. By comparison to its close analogue tryptamine, for which firm assignments to seven low-energy conformations have been made, UV and IR transitions due to eight conformations of serotonin are observed and assigned. The ultraviolet spectrum divides into two subsets of transitions separated from one another by approximately 230 cm-1 ascribable to syn and anti conformations of the 5-OH group. These two subsets are also distinguishable via their 5-OH stretch fundamentals, with the anti-OH subset shifted by approximately 4-5 cm-1 to lower frequency than those due to syn-OH conformers. The existing firm assignments for tryptamine play a decisive role in assignments in serotonin, where the alkyl CH stretch infrared spectrum is diagnostic of the conformation of the ethylamine side chain. Conformer A of serotonin (SERO(A)), with S1 <-- S0 origin transition at 32584 cm-1, is assigned to Gpy(out)/anti-OH, SERO(B) at 32548 cm-1 to Gpy(up)/anti, SERO(C) at 32545 cm-1 to Gph(out)/anti, SERO(D) at 32560 cm-1 to Anti(py)/anti, SERO(E) at 32537 cm-1 to Anti(up)/anti, SERO(F) at 32353 cm-1 to Gpy(out)/syn, SERO(G) at 32313 cm-1 to Gpy(up)/syn, and SERO(H) at 32282 cm-1 to Gph(out)/syn. The conformational preferences of serotonin differ from those of tryptamine most notably in the selective stabilization observed for the Gph(out)/anti-OH conformer SERO(C), which makes it the second-most intense transition in the ultraviolet spectrum, surpassed only by the Gpy(out)/anti-OH conformer SERO(A).

Structural and vibrational investigations of a neurotransmitter molecule: Serotonin (5-hydroxy tryptamine)

“…LeGreve et al [26] have proposed eight conformational isomers of serotonin while the same authors [34] have suggested that serotonin exist in 13 conformations. Cabezas et al [35] have found only three conformers from microwave spectroscopy.…”

“…LeGreve et al [26] have studied the ultraviolet and IR spectra of single conformation of neutral serotonin in the gas phase. Mouric and Emson [24] have carried out theoretical conformational study of serotonin and assigned calculated frequencies for the conformers only in the region 2800e3000 cm À1 .…”

“…4 eOH)(19)þa 2 (R 2 )(12)-F 2 (R 2 )(11)-F 2 (R 1 )(11)þ F 1 (R 1 )(8)þa 3 (R 1 )(5)-y(C 1 eC 6 )(5)þb(C 4 eOH) (4)þ F 3 (R 1 )(4) þ g(C 6 eH 11 )(4)-t(C 1 eN 16 )R 2 )(29)þF 1 (R 1 )(23)þg(C 4 eOH)(18)-g(C 8 eC 17 ) (10) þy(C 8 eC 17 ) (8)-a 1 (R 2 )(7)þy(C 4 eOH)(5)-a(C 17 eC 20 eN 23 ) (5)-a(C 8 eC 17 eC 20 )(4) …”

Effect of pyridyl nitrogen on the conformational landscape of 7-azaserotonin: A computational study

“…There have been a lot of experimental and theoretical studies on multiconformer biomolecular systems in gas phase [1][2][3][4][5]. Among all the neurotransmitters serotonin has been studied extensively whose flexible ethylamine chain has attracted the most for the quantum chemical investigations.…”

“…The stability of different conformers of serotonin is determined by the interaction of ethylamine chain with aromatic p-electron density [4,5]. The interest in the conformational studies on these large flexible biomolecules has been enhanced by the growing accuracy of quantum mechanical calculations.…”