1. Nick C. Polfer, Jos Oomens FOM-Institute for Plasmaphysics Robert C. Dunbar Case Western Reserve University Conformations of Metal Cation/Tryptophan Complexes From IRMPD Spectroscopy with the Free Electron Laser $$$ PRF, FOM, NHMFL, NSF Columbus Spectroscopy Meeting 2006

2. Overview M + Tryptophan Tryptophan  6 Metal ions (Alkalis and Ag + )  3 Principal binding motifs  Distinguish by IR spectra  Compare with DFT

3. Binding Motifs Two structure questions:  Zwitterion or charge-solvated structures?  Charge solvated structures in N/O/Ring chelation or O/Ring chelation ?

4. Zwitterion features  Stable solution form, might be electrosprayed  Stabilized by salt bridge and by cation-  + ¯ +  ¯

5. CS Chelation modes  CS is favored form of gas-phase neutral Advantage: 3-fold chelation Advantage: Sweet spot over ring O/Ring N/O/Ring

6. DFT Expectations Alkalis – Solid symbols Silver – Open symbols

7. Prior Expectations (Thermodynamic Control)  N/O/Ring for small metal ions, O/Ring for large ones  ZW is disfavored by 15 kJ/mol  Crossover at K +

8. Action Spectroscopy and IRMPD Infrared Multiple Photon Dissociation IR photon typically 0.1 eV Dissociation energy typically 3 eV Many photons delivered by an intense, short laser pulse (IRMPD) M + Trp M + + Trp

9. Light Source The Free Electron Laser gives  Convenient sweep across the chemically informative IR spectrum  High intensity and energy per pulse  Tight collimation of beam FELIX Free Electron Laser for Infared EXperiments (FOM Institute for Plasma Physics, Netherlands)

11. Spectra N/O/Ring O/Ring ZW

12. Li + * Peak not matched. Overtone? Fermi Resonance?

13. Na + * Peak not matched. Overtone? Fermi Resonance?

14. K+K+ * Peak not matched. Overtone? Fermi Resonance?

15. “Quantitation” DFT calculation of relative peak heights for IRMPD only semi-quantitative  Computational uncertainty  Inexact relation of IRMPD to IR intensity

16. Isomer Distribution  No detectable zwitterion components  Approximate CS compositions like this: Percent N/O/Ring

17. Results  No ZW  All N/O/Ring for smallest (tightest) metal ions  Majority O/Ring for largest metal ions  Crossover from N/O/Ring to O/Ring at K +  Excellent accord with thermochemistry

19. Zwitterion (L Form)