Robert C. Dunbar Case Western Reserve University Nicolas Polfer University of Florida Jeffrey Steill, Jos Oomens FOM Institute for Plasma Physics $$$ FOM, NHMFL, NSF IRMPD Spectroscopy of Doubly Charged Metal Ion/Tryptophan Complexes

Background  Last year we showed that the Ba +2 monomer complex has a zwitterion (ZW) ligand. Subsequently a number of Ba +2 complexes with other amino acids have also been shown to be ZW.  Many dimer complexes M +2 Trp 2 are readily formed in the electrospray. What the conformation of the second ligand in the Ba +2 Trp 2 case, and what is the conformation of both ligands in the general dimer case?

Ion Spectroscopy for Structure Study Conventional absorption spectroscopy Action Spectroscopy (consequence spectroscopy) Sample Photon detector Fragment molecule detector Problem: Too little light absorbed by ions

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

Light Source The Free Electron Laser FELIX at the FOM Institute (Utrecht) 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)

Experimental Protocol  Electrosprayed M + L accumulates, cools in hexapole  Transfer M + L to ICR, radiative cooling  FTICR eject sequence isolates M + L in cell  ~20 FELIX macropulses pass through ion cloud  FTICR detect sequence measures relative abundance of M + L ions and photofragment ions Duration 500 msec 1 sec 100 msec 4 sec 100 msec

Zwitterion (ZW) Structure  Normal solution-phase form, might expect it to be electrosprayed  Stabilized by salt bridge, cation-pi interaction ++ ¯ +  ¯ M +2 Tryptophan (Trp) + ¯ +  ¯ ZW (compact) ZW (extended)

Charge-Solvated (CS) Structure  Normal gas-phase form  Stabilized by chelation, cation-pi interaction CS N/O/Ring CS O/Ring

Identifying Zwitterions Spectroscopic signatures for amino acid conformation Zwitterion (ZW) [COO - +NH 3 + ] cm -1 NH cm -1 Charge Solvated (CS) C=O cm -1 OH 1150 cm -1

Ba +2 Trp Monomer – An Authentic ZW Ligand ZW Ligand Signatures: [COO - stretch+NH 3 bend] (1600 cm -1 ) NH 3 umbrella (1450 cm -1 ) ZW

CdClTrp + -- An Authentic CS Ligand Ba is the only +2 monomer complex we have been able to spray. (Zwitterion) The Cd +2 Cl -1 Trp complex gives a good model for a single Trp ligand complexed with a +2 metal ion. The spectrum shows that the Trp is CS. CS CS Ligand Signatures: C=O stretch (1730 cm -1 ) COH bend (1170 cm -1 )

Expected Conformations Based on Monomers Most of the +1 complexes are confirmed CS by experiment. No +2 monomer conformations have been checked by experiment except Ba complexes +2 complexes For monomer complexes, ZW (SB) conformations are only predicted for weakly binding +2 metal ions Ba +2, Sr +2 and Ca +2 Naïve expectation would be ZW ligands for Ba, Sr and Ca, and CS ligands for all other metal ions in this set

Spectra of Dimer Complexes First conclusion: all dimers (including Ba) show a CS ligand All dimer complexes show CS signatures

Dimer vs Monomer The multiple solvation of Ba +2 in the dimer complex reduces ZW preference This is the only known occurrence of the CS conformation for an amino acid bound to Ba +2 One CS ligand ZW ligand

Harder Question: What is the Other Ligand? Have shown that one ligand is CS. Is the other ligand CS or ZW? From the computed graph, might expect Ba, Sr, Ca to have ZW ligand(s). Other metals less likely to have any ZW ligand

Spectroscopic Tools for this Question  CS easy to recognize.  ZW has no prominent, sure-fire diagnostic features. Possibly useful are: NH 3 bend at 1600 cm -1 Strong NH 3 umbrella mode at 1450 cm -1

Spectra of Dimer Complexes All dimer complexes except Ni show ZW signatures

Alkaline Earth Spectra

Cd: A Typical Transition Metal Dimer Spectrum

Second Ligand Conclusion: Alkaline earth dimers and transition metal dimers except Ni are CS/ZW

Ni Appears to be Exceptional Conclusion: The Ni dimer is most likely CS/CS Most stable isomer

Conclusions All dimers have at least one CS ligand The CdClTrp +1 complex is a CS ligand Ba +2, Sr +2 and Ca +2 have one ZW ligand in extended conformation (CS/ZW) Similar conclusion for Zn +2, Cd +2, Mn +2, and Co +2 Ni +2 (smallest and most tightly binding transition metal) is probably CS/CS