1. Robert C. Dunbar Case Western Reserve University Jeffrey D. Steill FOM Institute for Plasma Physics Jos Oomens FOM Institute for Plasma Physics and University of Amsterdam HOW CATION-PI INTERACTIONS ENHANCE AND STRUCTURE THE BINDING OF METAL IONS TO AMINO ACIDS AND PEPTIDES M + /DIPHENYLALANINE PROBED BY IRMPD SPECTROSCOPY OSU Conference 2010: Symposium on Metal Containing Molecules

2. Encapsulation of Metal Ions by PhePhe The metal ion “intercalates” between the two parallel benzene rings of PhePhe in a very stable encapsulated conformation.

3. Ca 2+ (Benzene) 2 Inter-ring distance 5.1 Å Calcium graphite (CaC 6 ) Inter-plane distance 4.5 Å Ca 2+ (PhePhe) Inter-ring distance 5.5 Å Ca Intercalation Compounds

4. Metal-ion complexes of amino acids and peptides are readily introduced into the FT-ICR mass spectrometer by electrospray of metal salt plus peptide from solution. How to probe structures? Computation Infrared spectroscopy Working with the Complexes

5. Solving the Intensity Problem for Trapped Ions Conventional absorption spectroscopy Action Spectroscopy (consequence spectroscopy) Sample Photon detector Fragment molecule detector Problem: Too little light absorbed by ions Plot the extent of ion dissociation versus wavelength

6. 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

7. Light Source The Free Electron Laser (FELIX) gives  Convenient sweep across the chemically informative IR spectrum  High intensity and energy per pulse  Tight collimation of beam Downside:  Big (very big)  Expensive (very expensive)

8. Instrumentation

9. The Question: Zwitterion or Charge Solvated  Fundamental choice between two metal-binding motifs R + + + _ R Zwitterion (Salt Bridge) ZW Charge Solvated CS

10. The Role of Cation-Pi Interactions  The Premise: Normal state (inactive side chains) is ZW  Stabiliization by cation-pi “internal solvation” interactions can stabilize CS The elegant Ba 2+ PhePhe cage complex is the last chapter in a nice story For a start, Ba 2+ complexes.of amino acids and small peptides are closely balanced CS vs ZW

11. Ba 2+ Progression (Amino Acids) Ba 2+ ValineNo RingsZW Ba2+TryptophanOne Ring (Strained)ZW Ba2+PhenylalanineOne RingCS+ZW

12. Ba 2+ Progression (Dipeptides) Ba 2+ AlaAlaNo RingsZW Ba 2+ AlaPhe Ba 2+ PheAla Ba 2+ PhePheTwo RingsCS One RingZW or CS }{

13. Ba 2+ Valine: No Rings DF T Z IRMPD DFT ZW DFT CS A Simple Zwitterion Taken from Bush, Oomens, Saykally and Williams, JACS 103, 6563, 2002

14. Charge Solvated Zwitterion Phe and Trp Are Usually Very Similar. But…  Tryptophan makes simple zwitterion  But Phenylalanine makes a mixture with charge-solvated ions

15. 2.606 2.624 Ba 2+ Phe Ba 2+ Trp Trp solvates the ion worse than Phe:  Metal-Oxygen bond is stretched  Metal displaced from “sweet spot” over 6-membered ring Why Do Phe and Trp Give Different Conformations?

16. AlaAla: No Rings A simple zwitterion! Experiment DFT: Best ZW DFT: Best CS

17. The Curious Case of PheAla and AlaPhe PheAla = Charge solvated AlaPhe = Zwitterion Very slight differences in interactions and steric strain effects make the difference here Sequence dependent!

18. 2+ Metal ions Rigid, nearly symmetric structure gives simple spectrum, sharp peaks. 1+ Metal ions Looser structure, more conformers, gives less simple spectrum. M 2+ PhePhe and M + PhePhe Spectra Wavenumber cm -1

19. Caged Ba 2+ Gives Only Charge Solvated Ions  Fine fit to predicted CS (OORR)  No sign of any zwitterion contribution

20. Alkali Structures (Na + ) CS NORCS OORR +13 kJ/mol CS OOR 0 kJ/mol +7 kJ/mol The cage structure is calculated less stable than two open-structure motifs Spectra can distinguish NOR, but are similar for OOR and OORR

21. The Chirality Experiment The alkali complexes are compared for LL-PhePhe and DL-PhePhe As shown below for Na + the contribution of NOR complex disappears for the DL isomer Found true for Cs, K, Na and Li

22. The Chiral Experiment with Na +

23. Closeup of the Interesting Region

24. DL-FF-Na + +19 kJ/mol LL-FF-Na + +7 kJ/mol Chiral Isomers of the NOR conformation The NOR conformer for DL is much less favorable than for LL. This explains the spectroscopic contrast in the NOR intensities at 1780 cm -1

25. Other Metals Give More CS Ca 2+ and Sr 2+ show a mixture of CS and ZW. Smaller ions with high charge are more effectively solvated than Ba 2+. Alkalis (K + ) give CS. Lower charge gives weaker salt bridge. CS ZW

26. Conclusions  Ba 2+ complexes are near the balance of CS vs ZW  Inert side chains lead to ZW, but increasing cation- pi interactions enhance CS  Residue sequence is important (PheAla vs AlaPhe)  PhePhe gives CS with a pleasing and strong encapsulation motif for alkaline earth ions  Chiral PhePhe can give distinctly different conformations for alkali complexes

28. CS ZW Ba +2 Trp Spectrum Success! The zwitterion is favored

29. The Intensity Problem Conventional absorption spectroscopy Sample Photon detector Problem: Too little light absorbed by ions Heroic solution: cavity ringdown spectroscopy

30. Zwitterion Features  Normal solution-phase form, might expect to obtain it from electrospray  Stabilized by salt bridge + ¯ +  ¯ Tryptophan (Trp)

31. Does the Phe/Ala order matter? With K + the order makes almost no difference. With Ba +2 the order is crucial! What are these structures, and why are they so different??