Transcript OSU 12.pptx

OSU Conference 2012
METAL ION BINDING TO POLYPEPTIDES
CHARACTERIZED BY IRMPD SPECTROSCOPY
METAL-AMIDE NITROGEN BINDING AND THE IMINOL
TAUTOMERIZATION
Robert C. Dunbar
Case Western Reserve University
Nicolas Polfer
University of Florida
Giel Berden
FOM Institute for Plasma Physics
Jos Oomens
FOM Institute for Plasma Physics
and University of Amsterdam
Introduction
Metal ions are often bound by peptides
 Ion channels
 Metal transport and storage
 Active sites
Gas-phase study of small model peptides can
elucidate types of binding sites favored by different
metal ions and different side-chain interactions
Metal Ions Bound to Peptide Chains
Two basic modes of backbone binding:
Amide carbonyl oxygens
“Charge-solvated” (CS)
• Highly coordinated (e.g. octahedral)
• Alkali metals and Ca2+, Mg2+
Amide nitrogens
“Deprotonated” or “Iminol”
• Square planar (may be capped)
• Ni2+, Cu2+
Normal Condensed Phase Behavior
Examples of deprotonated amide binding
P
Ex
Cu2+, Ni2+
Oxytocin: Wyttenbach, Liu and
Bowers, JACS 130, 5993 (2008)
Prion Protein: Guerrieri et al., J.
Biol. Inorg. Chem. 14, 361 (2009)
Examples of carbonyl oxygen binding
Mg2+
Ca2+
Bound
oxygens
Magnesium transporter protein: Hattori, Tanaka,
Fukai, Ishitani and Nureki, Nature 448, 30 (2007)
Calmodulin: Chattopadhyaya et
al., J. Mol. Biol. 228,1177 (1992)
Working with the Complexes
The Metal-Ion/Peptide complexes are readily introduced
into the mass spectrometer by electrospray of metal salt
plus peptide from solution.
How to probe structures?
Computation (DFT spectrum simulation)
Infrared spectroscopy
Spectroscopy – Can’t do direct absorption spectroscopy,
so must resort to some form of action spectroscopy.
Photodissociation spectroscopy – plot extent of
dissociation vs IR wavelength.
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) 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)
Instrumentation
Gas-Phase Dipeptides
OH bend
Amide II
2
DFT
Charge Solvated
DFT
Iminol Tautomer
Two Things About Dipeptide Complexes
 Metal ions less active than Ca2+ show CS binding
Metal ions more active than Mg2+ show iminol binding
Break between calcium and magnesium

Magnesium anomaly – Condensed-phase peptide binding
of Mg2+ is CS, but gas-phase dipeptides show iminol
binding
Try larger peptide ligands?!
Gas-Phase Dipeptides
OH bend
Amide II
DFT
Charge Solvated
DFT
Iminol Tautomer
Larger Peptides
00
Mg2+FGG: CS Diastereomers
2+
Mg FGG Expt
1000
1200
1400
1600
1800
-
DFT OOOP1
DFT OOOP2
-1
0 kJ mol
+25 kJ mol
1200
-
1400
1600
1800 1000
1200
-1
1400
“Diastereomers” -- Almost identical spectra,
but OOOP2 is substantially lower in energy
1600
1800
Conclusions:The Magnesium Story

The Magnesium anomaly: magnesium ion commonly
binds condensed-phase peptides in the CS (oxygen)
mode. But with gas-phase dipeptides we have observed
iminol (deprotonated nitrogen) binding.
 The present new results with larger peptides FGG and
FGGF show CS binding, which resolves the anomaly
Conclusions: Binding to Larger Peptides
 Metal ions less active than Mg2+ show CS binding
K+, Ba2+, Ca2+
 Metal ions more active than Mg2+ show iminol binding
Ni2+
 The Amide II band at 1500-1550 cm-1 is a good diagnostic
for the existence of CS binding