Solvent Effects on the Conformational Preferences of Serotonin: Serotonin-(H2O)n, n=1,2

Neutral serotonin-(H2O)(n) clusters with n = 1,2 have been studied under jet-cooled conditions using a combination of resonant two-photon ionization (R2PI), UV-UV hole-burning (UVHB), and resonant iondip infrared (RIDIR) spectroscopy. Serotonin (5-hydroxytryptamine, SERO) is a close analogue of tryptamine, differing by the addition of an OH substituent in the 5-position on the indole ring, but sharing the same ethylamine side chain in the 3-position. Three conformational isomers of SERO-(H2O)(1) were observed via UVHB, with S-0-S-1 origins at 32 671 (A), 32 454 (B), and 32 188 cm(-1) (C). RIDIR spectroscopy provided infrared spectra in the hydride stretch region that reflected the hydrogen-bonding arrangement of each conformer. Two of the three SERO-(H2O)(1) conformers have RIDIR spectra nearly identical to that of the only observed conformer of tryptamine-(H2O)(1), differing only in the orientation of the 5-OH group (syn vs anti). In this structure, the H2O molecule acts as H-bond donor to the NH2 group on the ethylamine side chain, which is configured in the Gpy(out) conformation that is the global minimum in the absence of water. Comparison of the OH stretch RIDIR spectrum of the third SERO-(H2O)(1) conformer with calculation leads to its assignment to a structure in which the water molecule forms a H-bonded bridge between the amino group and the 5-OH group of SERO, with the ethylamine side chain in the Gph(out) conformation that facilitates bridge formation, corresponding to the second most populated conformer in the isolated SERO monomer. The OH and CH stretch infrared absorptions for the single observed conformer of SERO-(H2O)(2) indicate that it is also a bridge structure linking the NH2 and OH groups of SERO, retaining the same Gph(out) ethylamine conformation as in conformer C of SERO-(H2O)(1). The ultraviolet and infrared spectroscopy reflect the fact that the single-water bridge cannot optimally span the gap between the 5-OH and NH2 groups, while the water dimer bridge forms a set of three strong H-bonds that lock in the Gph(out) ethylamine and anti 5-OH orientations in a near-optimal configuration.