Biomolecular ions Soft-Landing on Surfaces with first observation of charge transfer

1. Biomolecular ions Soft-Landing on Surfaces:
First Observation of Charge Loss and Desorption Kinetics
Omar Hadjar
J. H. Futrell, J. Laskin
AVS
Seattle 2007
Pacific Northwest National Laboratory,
Environmental Molecular
Sciences Laboratory,
Richland, Washington

2. Motivation
mass-selected ions
Soft-Landing (SL)
Specific and prompt
surface modification
with minimum
purification of amount of material
material using ion
deposition
fundamental
understanding of
charge retention,
charge loss,
desorption kinetics

3. Systems
•Cyclic Gramicidin S (GS), M=1141 amu
Left: view perpendicular to the plane of the ring, illustrating
the peptide backbone structure. The antiparallel  -sheet
region is stabilized by hydrogen bonds.
Right: side-view, indicating the disposition in space of the
hydrophobic Val and Leu residues (left) and the basic Orn
(right) relative to the peptide ring.
•Protonation state: 1, 2
•Soft Landing energy: 1 to 100 eV
Terminal
Group
The Alkyl thiol based SAMs consist of three parts: namely,
the thiol group (SH) which covalently bonds the two
dimensional crystal to the Au by loss of a hydrogen atom, a
spacer group (CH2)n defining the length of the molecule and
(CH2)n the terminal group responsible for modified surface
reactivity towards the soft landed ionic peptide.
Used here are CF3 , CH3 and COOH terminal groups
SH

4. FT-ICR-SIMS Instrument Schematic
+26V 20l/h of 0.1 +(2-3) kV
C.Q. mM peptide
+20V +20V
Collision +15V 1 solution Electrospray
energy C.L.
0V 0V
2 Ion
Back -5V 10-1 Torr
C.O. Funnel
Trap
+ -30V -30V
3
surface Ring Front -45V -45V
Trap Trap2 Trap1 Collision Quadrupole 1
5 2*10-2 Torr
SIMS Ar Gas Conductance Limit 2
-250V
Soft Landing line
Ion Guide 4
5*10-5 Torr Resolving Quadrupole
Back Front
Trap Trap
Gas cell Collision 3
6T Octopole
Field 5*10-8 Torr
Movable
7*10-10 Torr 8 keV Cs+
8 Segments Ring Gun
Surface
for SIMS
subsequent -V
Soft Landing Electrostatic
ICR Cell Flight Tube Ion Guide 4
40 by 40 mm cell
5

5. Experiment Principles: Ion Deposition &
Surface Analysis
Au2SH+
Alternating exposure of the
Au3+
surface to both beams
Au3S+
Au2+
AuCF2+
200 400 600 800 1000 1200
m/z real time SIMS
during and after Soft-Landing
(GS+2H)2+
500
Ion Beam
400
8 keV 8 keV
300
Cs+ Cs+
200 Surface peak: Au2SH+
ex situ 571.0 571.5 572.0 572.5 573.0 573.5 100
m/z
TOF-SIMS 0
Line Scan (GS/2+H)+ (GS+H)+
10
5 1.2
4
Au+ 1.0
10 0.8
(GS+2H)2+
TOF SIMS Signal
10
3 Peptide (GS+2H)2+ 0.6
PVO+ 0.4
10
2
(GS+Au)+ 0.2
1
Cs+ 0.0
10
200 400 600 800 1000 1200 0 50 100 150 200 250 300 350 400 450 500 550
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 m/z Time (min)
Line Scan (mm)

6. Kinetics of Peptide related peaks after S.L.
0.08
0.24
0.07 70 0.12
115 0.21 169 0.03 192 0.03 233
0.06 0.10
0.18
0.05 0.08 0.15
0.04 0.06 0.12
0.02 0.02
(LF-28)+
0.03 0.09
0.04 0.01 0.01
0.02 0.06
0.02
0.01
0.00
P+ 0.00
O+ 0.03
0.00
(PV-28)+ 0.00 0.00
0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500
time (min) time (min) time (min) time (min) time (min)
0.12
0.08
0.07 261 0.8 311 0.10 429 0.04 441 0.4 457
0.06 0.08 0.03
0.6 0.3
0.05
0.04 LF+ 0.4
0.06
0.02
(FPVO-NH3)+
0.2
0.03 0.04
0.02 0.2 0.01 0.1
0.02
0.01
0.00 0.0
PVO+ 0.00
(LFPV-28)+ 0.00 (LFPV)+
0.0
0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500
time (min) time (min) time (min) time (min) time (min)
0.08 6
1.4
1.2
571.5 1.2
572 0.07 813 5 1142 0.30 1338
1.0 0.06 0.25
1.0 0.05 4
0.8 0.20
0.8 0.04 3
0.6 0.15
0.6 0.03
0.4 2
0.10
0.4
0.2
(0GS/2)+ 0.2
1GS2+ 0.02
0.01 1 GS+ 0.05
(GS+Au)+
0.0 0.0 0.00 0 0.00
0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500
time (min) time (min) time (min) time (min) time (min)

7. Kinetics Model during and after S.L.
A* = SA A +FIB B B* = SB B + FIC C C’ = SC C
A* B* C’ SIMS
Fragments
Population
z=2 FIB z=1 FIC from neutrals
k2 k4 k5
R
A charge loss
B charge loss
C Surface
z=2 k1 z=1 k3 z=0 Population
S.L. induced sudden charge loss
FB FC
and neutralization
dA/dt = -(k1+k2)A + R dB/dt = -(k3+k4)B + k1A + FBR dC/dt = -k5C + k3B + FCR

8. Experimental Results & Kinetics Model Fit
Charge
0.9
Best simultaneous fit
Reduction: Desorption:
of the three populations (min-1) (min-1)
0.6
(GS+2H)2+ k1 ~ 10-2 k2  10-4
0.3
0.0
0 100 200 300 400 500 600
3
2
FT-ICR-SIMS signal (arb. units)
1 (GS+H)+ k3 ~ 2*10-5 k4 ~ 6*10-4
0
0 100 200 300 400 500 600
0.4
End
of S.L. 0.2
GS0 k5 ~ 10-3
Time (min) 0.0
0 100 200 300 400 500 600

9. Surface Effect on Charge Retention
PVO+ GS+
m/z=311 m/z=1141
0.30 (GS/2)+ GS+ PVO+
100 m/z=571 m/z=1141
FT-ICR-SIMS signal m/z=311
COOHSAM
GS+/PVO+
50
4
571 572 573 574
1.0 3
2
0 0.5
300 400 500 600 700 800 900 1000 1100 1
m/z
0.0 0
(GS/2)+
1.5
100
m/z=571
1.11 0.9
HSAM
1.0
0.6
50 571 572 573 574 0.5
0.3
0.0 0.0
0 6 0.9
300 400 500 600 700 800 900 1000 1100
m/z
FSAM
4 0.6
900
(GS/2)+
m/z=571 2 0.3
GS2+
600
0 0.0
0 100 200 300 400 500 600
571 572 573 574
300 6.26 Time (min)
t0= end of
0
300 400 500 600 700 800 900 1000 1100
m/z
Soft-Landing
Snapshot @ t0

10. Effect of the Charge State on the Kinetics
180
160 GS2+ 100
GS+
140
80
120
100 60
80
40
60
40
20
20
200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400
m/z m/z
0 100 200 300 400 500 600 700
FT-ICR-SIMS (normalized GS signal) 3.0
2.5
GS2+ vs 1+ S.L.
+
2.5
2.0
2.0
1.5
1.5
1.0
1.0
GS1+ SIMS 0.5 t0 end of soft 0.5
Landing
0.0 0.0
-100 0 100 200 300 400 500 600
Time (min)

11. Conclusion
♣ S.L.-SIMS: New tool for fundamental understanding of ion-surface interactions
♣ First observation of charge loss & desorption of soft landed ions in real time
♣ Excellent agreement between experiments & a simple kinetic model
♣ First experimental values of rate constants produced
What have we learned:
♣ Proton loss governs GS2+ signal decay
♣ Desorption governs GS+ signal decay
♣ Sudden neutralization governs GS0 formation
♣ FSAM retains more charges than H & COOH-SAM

12. Thanks:
Julia Laskin
Peng Wang Zhibo Yang
• Chemical Sciences Division (CSD)
• Office of Basic Energy Sciences (BES) of the US Department of Energy.
• Laboratory Directed Research and Development (LDRD) Program at PNNL.