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+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

Action Spectroscopy (consequence spectroscopy)

Sample Photon detector

Fragmentmolecule 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 photonsIRMany

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+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+

[COO- +NH3+] 1600-1700 cm-1

NH3+ 1430 cm-1

OH

O M+

Charge Solvated (CS)

C=O 1700-1775 cm-1

OH 1150 cm-1

Ba+2Trp Monomer – An Authentic ZW Ligand

ZW Ligand Signatures:

[COO- stretch+NH3 bend] (1600 cm-1)

NH3 umbrella (1450 cm-1)

600 800 1000 1200 1400 1600 1800

IR In

ten

sity

Charge solvated

Ba+2Trp

Wavenumber (cm-1)

Experiment

Zwitterion

ZW

CdClTrp+ -- An Authentic CS Ligand

600 800 1000 1200 1400 1600 1800

Dis

soci

atio

n Y

ield

Charge Solvated

Wavenumber (cm-1)

Experiment

Zwitterion

CdClTrp+

Ba is the only +2 monomer complex we have been able to spray. (Zwitterion)

The Cd+2Cl-1Trp 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

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+2

+1 complexes +2 complexesFor 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

600 800 1000 1200 1400 1600 1800

Dis

soci

atio

n Y

ield

Alkaline Earth Dimers

Wavenumber (cm-1)

Ba

Sr

Ca

600 800 1000 1200 1400 1600 1800

Dis

soci

atio

n Y

ield

Transition Metal Dimers

Wavenumber (cm-1)

Cd

Zn

Ni

Co

Mn

First conclusion: all dimers (including Ba) show a CS ligand

All dimer complexes show CS signatures

Dimer vs MonomerThe 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:

NH3 bend at 1600 cm-1

Strong NH3 umbrella mode at 1450 cm-1

Spectra of Dimer Complexes

600 800 1000 1200 1400 1600 1800

Dis

soci

atio

n Y

ield

Alkaline Earth Dimers

Wavenumber (cm-1)

Ba

Sr

Ca

600 800 1000 1200 1400 1600 1800

Dis

soci

atio

n Y

ield

Transition Metal Dimers

Wavenumber (cm-1)

Cd

Zn

Ni

Co

Mn

All dimer complexes except Ni show ZW signatures

Alkaline Earth Spectra

1000 1200 1400 1600 1800

+21 kJ/mol

IR In

ten

sity

Wavenumber (cm-1)

CS/ZW(compact)

+25 kJ/mol

Experiment

CS/CS

+4 kJ/mol

ZW/ZW

CaTrp2

+2

CS/ZW(extended)0 kJ/mol

1000 1200 1400 1600 1800

Wavenumber / cm-1

Ba+2Trp2

CS/ZW(extended)

CS/ZW(compact)

CS/CS

+18 kJ/mol

ZW/ZW+23 kJ/mol

IR In

ten

sity

+9 kJ/mol

Experiment

0 kJ/mol

600 800 1000 1200 1400 1600 1800

+26 kJ/mol

Wavenumber / cm-1

Trp2Cd+2

CS2/ZW(compact)

CS2/ZW(extended)

+12 kJ/molCS2/CS2IR

Inte

nsi

ty

0 kJ/mol

Experiment

Cd: A Typical Transition Metal Dimer Spectrum

Second Ligand

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

600 800 1000 1200 1400 1600 1800

IR In

ten

sity

Ni+2Trp2

Wavenumber (cm-1)

CS/CS

Experiment

Experiment

Ni Appears to be Exceptional

600 800 1000 1200 1400 1600 1800

Dis

soci

atio

n Y

ield

Transition Metal Dimers

Wavenumber (cm-1)

Cd

Zn

Ni

Co

Mn

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