Infrared Spectra of K+(Tryptamine)(H2O)n and K+(Tryptamine)(H2O)nAr Cluster Ions Amy L. Nicely and James M. Lisy OSU International Symposium on Molecular Spectroscopy June 16, 2008 *IR study

Outline Motivation Apparatus and formation of cluster ions Supporting calculations K+(Tryptamine)(H2O)n vs K+(Tryptamine)(H2O)nAr experimental and calculated IR spectra

Motivation Extension of previous studies Biological significance K+(H2O)n K+(Indole)(H2O)n Biological significance Neurotransmitters (serotonin) Amino acids (tryptophan)

Triple quadrupole mass spectrometer LaserVision OPO/A Tunable: 1.35-10 µm Neutral clusters are formed via a supersonic expansion Tryptamine in sample heater (~115 °C) Fully expanded neutral clusters collide with alkali cations produced via thermionic emission MS-MS method: select ion cluster, dissociate with IR laser, detect fragment ion Nd3+:YAG Laser (1064 nm) 10 Hz, 10 ns pulse width *Mixture of argon/water passes over heated sample holder before reaching nozzle *YAG “pumps” a tunable OPO *RRKM theory: unimolecular dissociation information allows us to determine rates of dissociation, which is directly related to the total energy of the cluster. Once the total average energy [ <E>=integral(E*P(E)) ] is determined, it can be divided among all of the available degrees of freedom. Temperature can be estimated by solving for T in the Boltzman factors

Terminal temperature ~40-100 K Terminal temperature ~300-400 K Evaporative Cooling Cooling efficiency determined by the evaporating ligands’ binding energy Most weakly bound ligand evaporates to cool cluster, removes both mass and energy Terminal temperature ~40-100 K H2O evap. = larger energy loss Terminal temperature ~300-400 K Ar evap. = smaller energy loss Efinal [K+(Tryp)(H2O)2] Energy ΔE ≈ BE H2O En [K+(Tryp)(H2O)n] 0- ΔE ≈ BE Ar Efinal [K+(Tryp)(H2O)2Ar] En [K+(Tryp)(H2O)2Arn] *large excess of internal energy from impact of collision and stabilization *different ratio of Ar/H2O for warm vs cold experiments… harsher expansion conditions for cold clusters (higher BP, more argon)

Calculation details Preliminary structures generated using SPARTAN 02 Geometries optimized, frequencies and energies calculated at B3LYP/6-31+G* level with GAUSSIAN 03 SWIZARD used to apply Gaussian lineshape with 5-100 cm-1 peak width to scaled calculated frequencies Thermodynamics data obtained using THERMO.PL perl script *thermo.pl uses the output from the Gaussian frequency calculations to determine the thermodynamics values… the script itself does not do the calculations

Hydrated Biomolecules Zwier, T.S., et. al., Science 2004, 303, 1169-1173. Tryptamine has nine conformers which differ in side chain orientation and lone pair position Favored conformer in neutral gas-phase experiments Not observed in neutral gas-phase experiments Gph(in) and Gpy(in) are the highest-energy structures; energetically less favorable due to electron-electron interaction

K+(Tryptamine) spectra AGpy(in) AGph(in) NH AGph(in) NH2 asym NH2 sym In the presence of K+, the two lowest-energy K+(Tryptamine) isomers are built from those not seen in neutral experiments AGpy(in) Simulated Experimental spectrum shows presence of both isomers, in good agreement with the calculated spectra K+(Tryp)Ar3 Experimental

Temperature Dependence “Tagging” the cluster ions with an argon atom reduces the internal energy By changing the effective temperature of the cluster ions, different isomers may be thermodynamically favored, resulting in different spectral features

Identifying the OH and NH features K+(Tryp)(H2O) NH Vaden, T.D., Weinheimer, C.J. and Lisy, J.M., J. Chem. Phys. 2004, 121, 3102-3107. OH νasym OH νsym *K+(Indole)W2 spectrum explains ALL of the features (except NH2) in the K+(Tryp)W1 spectrum  must be multiple isomers present NH2 asym

Identifying the OH and NH features K+(Tryp)(H2O) NH OH νasym/ νfree OH νsym OH π-hydrogen bond *K+(Indole)W2 spectrum explains ALL of the features (except NH2) in the K+(Tryp)W1 spectrum  must be multiple isomers present Miller, D.J., Lisy, J.M., J. Chem. Phys. 2006, 124, 184301. NH2 asym

Identifying the OH and NH features OH νsym ??? OH π-hydrogen bond OH νfree NH OH νasym K+(Tryp)(H2O)Ar

Relative Free Energies *we use this as a GUIDE only

1E 1E 1B 1B 1A 1C 1A 1D 1C 1D K+(Tryp)(H2O)Ar K+(Tryp)(H2O) The relative intensities of the features in the experimental spectrum provide information about the abundances of each of the calculated isomers 1D

Simulated Spectra K+(Tryp)(H2O) K+(Tryp)(H2O)Ar 1D 1A 1E 1C 1C ~87% ~50% ~35% ~13% 1C *1E is not predicted by thermodynamics  trapping process ~15%

K+(Tryp)(H2O)2 Spectra Significant differences observed again between the warm and cold spectra No new features compared with n=1 spectra, but there is some additional splitting and broadening

K+(Tryp)(H2O)2 2A 2B ~43% ~30% 2C ~27%

K+(Tryp)(H2O)2Ar 2A 2B ~37% ~37% 2E 2F ~7% 2G ~3% ~17%

Conclusions K+ stabilizes the high-energy tryptamine conformers K+…Tryp and K+…OH2 interactions favored over Tryp…OH2 interactions Temperature dependence High temperatures  favors “free” water molecules and π-hydrogen bonds Low temperatures  favors hydrogen-bonded water molecules, traps higher-energy isomers

Acknowledgements Dr. Jim Lisy Dr. Dotti Miller Lisy Group Members Mr. Jason Rodriguez Mr. Jordan Beck Mr. Oscar Rodriguez, Jr. Mr. Brian E. Nicely Funding NSF CHE-0415859 NSF CRIF-0541659 UIUC Department of Chemistry UIUC Graduate College Block Grant

Competition between interactions vs. (Tryptamine)(H2O) K+(Tryptamine) Meerts, W.L., et. al., J. Am. Chem. Soc. 2005, 127, 10356-10364. K+(Tryptamine)(H2O) structure maximizes potassium interactions