1. 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

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

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

4. Triple quadrupole mass spectrometer
LaserVision OPO/A
Tunable: 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

5. 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 ~ K
H2O evap. =
larger energy loss
Terminal temperature ~ 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)

6. 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 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

7. Hydrated Biomolecules
Zwier, T.S., et. al., Science 2004, 303,
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

8. 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

9. 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

10. 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,
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

11. 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,
NH2 asym

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

13. Relative Free Energies
*we use this as a GUIDE only

14. 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

15. 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%

16. 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

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

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

19. 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

20. 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
NSF CRIF
UIUC Department of Chemistry
UIUC Graduate College Block Grant

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