Pacific Northwest National Laboratory,

Environmental Molecular

Sciences Laboratory,

Richland, Washington

Biomolecular ions Soft-Landing on Surfaces:

First Observation of Charge Loss and Desorption Kinetics

AVSSeattle 2007

Omar HadjarJ. H. Futrell, J. Laskin

Motivation

mass-selected ions

Soft-Landing (SL)

purification of

material using ion

deposition

fundamental

understanding of

charge retention,

charge loss,

desorption kinetics

Specific and prompt

surface modification

with minimum

amount of material

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

SH

(CH2)n

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

the terminal group responsible for modified surface

reactivity towards the soft landed ionic peptide.

Used here are CF3 , CH3 and COOH terminal groups

10-1 TorrIon

Funnel

Collision

energy

7*10-10 Torr

Flight Tube

Front

Trap

Back

Trap

Movable

Surface

for SIMS

subsequent

Soft Landing

6T

Field

40 by 40 mm cell

8 Segments Ring

ICR Cell

5

5*10-5 Torr Resolving Quadrupole

5*10-8 Torr

Electrostatic

Ion Guide 4

Gas cell Collision

Octopole

Ar Gas

line

3

2*10-2 TorrCollision Quadrupole 1

2Conductance Limit

-V

8 keV Cs+

Gun

Electrospray

+(2-3) kV20l/h of 0.1

mM peptide

solution+15V

C.L.

+26V

C.Q.

1

2

-250V

Ion Guide 4

SIMS

Soft Landing

Ring Front

Trap

Back

Trap

+

surface

-30V -30V

0V

+20V+20V

0V

5

-45V

Trap2

-5V

C.O.

-45V

Trap1

3

FT-ICR-SIMS Instrument Schematic

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

101

102

103

104

105

TO

F S

IMS

Sig

nal

Line Scan (mm)

Cs+

Peptide

Au+

Experiment Principles: Ion Deposition &

Surface Analysis

ex situ

TOF-SIMS

Line Scan

200 400 600 800 1000 1200

m/z

Au2SH+

Au2+

Au3+

Au3S+

AuCF2+

571.0 571.5 572.0 572.5 573.0 573.5

m/z

(GS+2H)2+

Ion Beam

8 keV

Cs+

8 keV

Cs+

Alternating exposure of the

surface to both beams

real time SIMS

during and after Soft-Landing

200 400 600 800 1000 1200

m/z

(GS+Au)+

(GS+H)+(GS/2+H)+

PVO+(GS+2H)2+

0 50 100 150 200 250 300 350 400 450 500 5500.0

0.2

0.4

0.6

0.8

1.0

1.2

0

100

200

300

400

500

Time (min)

Surface peak: Au2SH+

(GS+2H)2+

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

time (min)

571.5

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

time (min)

572

(0GS/2)+ 1GS2+

0 100 200 300 400 5000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

time (min)

813

0 100 200 300 400 5000

1

2

3

4

5

6

time (min)

1142

GS+

0 100 200 300 400 5000.00

0.05

0.10

0.15

0.20

0.25

0.30

time (min)

1338

(GS+Au)+

0 100 200 300 400 5000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

time (min)

0 100 200 300 400 5000.00

0.02

0.04

0.06

0.08

0.10

0.12

time (min)

70 115

0 100 200 300 400 5000.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

time (min)

169

0 100 200 300 400 5000.00

0.01

0.02

0.03

time (min)

192

0 100 200 300 400 5000.00

0.01

0.02

0.03

time (min)

233

P+ O+ (PV-28)+

(LF-28)+

0 100 200 300 400 5000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

time (min)

261

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

time (min)

311

0 100 200 300 400 5000.00

0.02

0.04

0.06

0.08

0.10

0.12

time (min)

429

0 100 200 300 400 5000.00

0.01

0.02

0.03

0.04

time (min)

441

0 100 200 300 400 5000.0

0.1

0.2

0.3

0.4

time (min)

457

PVO+

LF+

(LFPV-28)+

(FPVO-NH3)+

(LFPV)+

Kinetics of Peptide related peaks after S.L.

Surface

Population

SIMS

Population

B

z=1

A

z=2

C

z=0

A*

z=2

B*

z=1

C’Fragments

from neutrals

charge lossk1

charge lossk3

k2 k4k5

FIB FIC

and neutralizationFC

dA/dt = -(k1+k2)A + R dB/dt = -(k3+k4)B + k1A + FBR dC/dt = -k5C + k3B + FCR

R

S.L. induced sudden charge lossFB

Kinetics Model during and after S.L.

A* = SA A +FIB B B* = SB B + FIC C C’ = SC C

0 100 200 300 400 500 6000.0

0.3

0.6

0.9

FT

-IC

R-S

IMS

sig

nal

(arb

. unit

s)

0 100 200 300 400 500 6000

1

2

3

0 100 200 300 400 500 6000.0

0.2

0.4

Time (min)

GS0

(GS+2H)2+

(GS+H)+

Experimental Results & Kinetics Model Fit

End

of S.L.

Best simultaneous fit

of the three populations

k1 ~ 10-2

k3 ~ 2*10-5

Charge

Reduction:

(min-1)

Desorption:

k2 10-4

k4 ~ 6*10-4

k5 ~ 10-3

(min-1)

300 400 500 600 700 800 900 1000 11000

300

600

900

m/z

FS

AM

300 400 500 600 700 800 900 1000 11000

50

100

m/z

HS

AM

300 400 500 600 700 800 900 1000 11000

50

100

m/z

CO

OH

SA

M

t0= end of

Soft-Landing

Surface Effect on Charge Retention

0 100 200 300 400 500 600

0

2

4

6

Time (min)

0.0

0.5

1.0

1.5

0.0

0.5

1.0

0

1

2

3

4

0.0

0.3

0.6

0.9

0.0

0.3

0.6

0.9

GS+

m/z=1141

PVO+

m/z=311FT-ICR-SIMS signal

GS+

m/z=1141

PVO+

m/z=311

Snapshot @ t0

GS+/PVO+

571 572 573 574

(GS/2)+

m/z=571

571 572 573 574

(GS/2)+

m/z=571

GS2+

571 572 573 574

(GS/2)+

m/z=571

0.30

1.11

6.26

-100 0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (min)

F

T-I

CR

-SIM

S (

no

rmali

zed

GS

+ s

ign

al)

0 100 200 300 400 500 600 700

0.0

0.5

1.0

1.5

2.0

2.5

S.L.GS2+ vs 1+

SIMSGS1+ t0 end of soft

Landing

Effect of the Charge State on the Kinetics

200 400 600 800 1000 1200 1400

20

40

60

80

100

m/z

200 400 600 800 1000 1200 1400

20

40

60

80

100

120

140

160

180

m/z

GS2+ GS+

♣ FSAM retains more charges than H & COOH-SAM

♣ Proton loss governs GS2+ signal decay

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

♣ First experimental values of rate constants produced

♣ Excellent agreement between experiments & a simple kinetic model

What have we learned:

♣ Desorption governs GS+ signal decay

♣ Sudden neutralization governs GS0 formation

Thanks:

Zhibo YangPeng Wang

Julia Laskin

• 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.