Download OSU Spectr 08

IRMPD Spectroscopy of Doubly
Charged Metal Ion/Tryptophan
Complexes
Robert C. Dunbar
Case Western Reserve University
Nicolas Polfer
University of Florida
Jeffrey Steill, Jos Oomens
FOM Institute for Plasma Physics
$$$
FOM, NHMFL, NSF
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+2Trp2 are readily
formed in the electrospray. What the
conformation of the second ligand in the
Ba+2Trp2 case, and what is the
conformation of both ligands in the general
dimer case?
Ion Spectroscopy for Structure Study
Conventional absorption spectroscopy
Sample
Photon detector
Problem: Too little light absorbed by ions
Action Spectroscopy (consequence spectroscopy)
Fragment
molecule
detector
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
  
Many IR photons
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
Duration
 Electrosprayed M+L accumulates, cools
in hexapole
500 msec
 Transfer M+L to ICR, radiative cooling
1 sec
 FTICR eject sequence isolates M+L in
cell
100 msec
 ~20 FELIX macropulses pass through
ion cloud
 FTICR detect sequence measures
relative abundance of M+L ions and
photofragment ions
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+2Tryptophan (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)
O
O
M+
Charge Solvated (CS)
OH
O M+
[COO- +NH3+] 1600-1700 cm-1
C=O 1700-1775 cm-1
NH3+ 1430 cm-1
OH 1150 cm-1
Ba+2Trp Monomer – An Authentic ZW Ligand
+2
Ba Trp
Experiment
ZW
IR Intensity
Zwitterion
Charge solvated
ZW Ligand Signatures:
[COO- stretch+NH3 bend]
(1600 cm-1)
NH3 umbrella (1450
cm-1)
600
800
1000
1200
1400
-1
Wavenumber (cm )
1600
1800
CdClTrp+ -- An Authentic CS Ligand
+
CdClTrp
Ba is the only +2 monomer
complex we have been able
to spray. (Zwitterion)
The spectrum shows that the
Trp is CS.
Dissociation Yield
The Cd+2Cl-1Trp complex
gives a good model for a
single Trp ligand complexed
with a +2 metal ion.
Experiment
CS
CS
Zwitterion
Charge Solvated
CS Ligand Signatures:
C=O stretch (1730 cm-1)
COH bend (1170 cm-1)
600
800
1000
1200
1400
1600
-1
Wavenumber (cm )
1800
Expected Conformations Based on Monomers
For monomer complexes, ZW
(SB) conformations are only
predicted for weakly binding +2
metal ions Ba+2, Sr+2 and Ca+2
+1 complexes
+2 complexes
Naïve expectation would be
ZW ligands for Ba, Sr and
Ca, and CS ligands for all
other metal ions in this set
Most of the +1 complexes are confirmed CS by experiment. No
+2 monomer conformations have been checked by experiment
except Ba+2
Spectra of Dimer Complexes
Alkaline Earth Dimers
Transition Metal Dimers
Mn
Co
Sr
Dissociation Yield
Dissociation Yield
Ca
Ba
600
800
1000
1200
1400
1600
Ni
Zn
1800
-1
Wavenumber (cm )
Cd
All dimer complexes show CS signatures
600
First conclusion: all dimers
(including Ba) show a CS ligand
800
1000
1200
1400
-1
Wavenumber (cm )
1600
1800
Dimer vs Monomer
The multiple solvation of Ba+2 in the
dimer complex reduces ZW preference
One CS ligand
This is the only known
occurrence of the CS
conformation for an amino
acid bound to Ba+2
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:
NH3 bend at 1600 cm-1
Strong NH3 umbrella mode at 1450 cm-1
Spectra of Dimer Complexes
Alkaline Earth Dimers
Transition Metal Dimers
Mn
Co
Sr
Dissociation Yield
Dissociation Yield
Ca
Ba
600
800
1000
1200
1400
1600
Ni
Zn
1800
-1
Wavenumber (cm )
Cd
All dimer complexes except Ni
show ZW signatures
600
800
1000
1200
1400
-1
Wavenumber (cm )
1600
1800
Alkaline Earth Spectra
+2
+2
Ba Trp2
CaTrp2
Experiment
Experiment
CS/ZW(extended)
CS/ZW(extended)
0 kJ/mol
0 kJ/mol
IR Intensity
IR Intensity
CS/CS
+4 kJ/mol
CS/CS
+9 kJ/mol
ZW/ZW
ZW/ZW
+23 kJ/mol
+25 kJ/mol
CS/ZW(compact)
CS/ZW(compact)
+18 kJ/mol
+21 kJ/mol
1000
1200
1400
1600
-1
Wavenumber (cm )
1800
1000
1200
1400
1600
Wavenumber / cm
-1
1800
Cd: A Typical Transition Metal Dimer Spectrum
+2
Trp2Cd
Experiment
CS2/ZW(extended)
IR Intensity
0 kJ/mol
CS2/CS2
+12 kJ/mol
CS2/ZW(compact)
+26 kJ/mol
600
800
1000
1200
1400
Wavenumber / cm
1600
-1
1800
Second Ligand
Conclusion: Alkaline earth
dimers and transition metal
dimers except Ni are CS/ZW
Ni Appears to be Exceptional
Transition Metal Dimers
+2
Ni Trp2
Mn
IR Intensity
Experiment
Dissociation Yield
Co
CS/CS
Ni
Most stable isomer
600
Zn
800
1000
1200
1400
1600
1800
-1
Wavenumber (cm )
Cd
600
800
Conclusion: The Ni dimer is
most likely CS/CS
1000
1200
1400
-1
Wavenumber (cm )
1600
1800
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