METAL ION BINDING TO POLYPEPTIDES

unkindnesskindΠολεοδομικά Έργα

15 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

58 εμφανίσεις

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



OSU Conference
2012

METAL ION BINDING TO POLYPEPTIDES
CHARACTERIZED BY IRMPD SPECTROSCOPY

METAL
-
AMIDE NITROGEN BINDING AND THE IMINOL
TAUTOMERIZATION

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 Ca
2+
, Mg
2+

Amide nitrogens

“Deprotonated” or “Iminol”


Square planar (may be capped)


Ni
2+
, Cu
2+

Normal Condensed Phase Behavior

Ex






Oxytocin:
Wyttenbach, Liu and
Bowers, JACS 130, 5993 (2008)

Prion Protein:
Guerrieri et al., J.
Biol. Inorg. Chem. 14, 361 (2009)

Examples of deprotonated amide binding

Cu
2+,
Ni
2+

Magnesium transporter protein:
Hattori, Tanaka,
Fukai, Ishitani and Nureki, Nature 448, 30 (2007)

Calmodulin:
Chattopadhyaya et
al., J. Mol. Biol. 228,1177 (1992)

Mg
2+

Ca
2+

Bound
oxygens

Prion Protein

Examples of carbonyl oxygen binding

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.

Working with the Complexes

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








photons
IR
Many
Light Source

The

Free Electron Laser

(FELIX)

gives


Convenient s
weep
across the chemically
informative IR spectrum


High
intensity
and energy per pulse


Tight c
ollimation
of beam


Downside:


Big (
very

big)


Expensive (
very

expensive)

Instrumentation

Gas
-
Phase Dipeptides

Charge Solvated

Iminol Tautomer

DFT

DFT

OH bend

Amide II

2

Two Things About Dipeptide Complexes


Metal ions less active than Ca
2+

show CS binding


Metal ions more active than Mg
2+

show iminol binding


Break between calcium and magnesium






Magnesium anomaly



Condensed
-
phase peptide binding
of Mg
2+

is CS, but gas
-
phase dipeptides show iminol
binding

Try larger peptide ligands?!

Gas
-
Phase Dipeptides

Charge Solvated

Iminol Tautomer

DFT

DFT

OH bend

Amide II

Larger Peptides

-

-

Mg
2+
FGG: CS Diastereomers

1000
1200
1400
1600
1800
Mg
2+
FGG Expt
1000
1200
1400
1600
1800
DFT OOOP2
0 kJ mol
-1
1000
1200
1400
1600
1800
DFT OOOP1
+25 kJ mol
-1
“Diastereomers”
--

Almost identical spectra,
but OOOP2 is substantially lower in energy

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 Mg
2+

show
CS binding


K
+
, Ba
2+
, Ca
2+


Metal ions more active than Mg
2+

show
iminol binding



Ni
2+


The Amide II band at 1500
-
1550 cm
-
1

is a good
diagnostic
for the existence of CS binding