Gang Ma, Gang Ma, Xiangke ChenXiangke Chen and Heather Allen and Heather Allen

The Ohio State UniversityThe Ohio State University

June 21June 21stst 2007 2007

Interfacial water structure of phospholipid Interfacial water structure of phospholipid Langmuir monolayers investigated by Langmuir monolayers investigated by vibrational sum frequency generation vibrational sum frequency generation

spectroscopyspectroscopy

SFG

Lung SurfactantLung SurfactantAn essential component for our An essential component for our breathingbreathing

FunctionFunction

1.1. Reach near-zero surface tensionReach near-zero surface tension

2.2. Prevent lung from collapsePrevent lung from collapse

3.3. Ease work of breathingEase work of breathing

DPPC: Dipalmitoyl-phosphatidyl-choline

DPPE: Dipalmitoyl-phosphatidyl-ethanolamine

Broad Bandwidth Sum Frequency Broad Bandwidth Sum Frequency Generation &Generation &

Langmuir troughLangmuir trough

sample

vis

IR

vis + IR= sum

~100fs

SFG is a powerful technique providing vibrational spectra of interfaces.

Simultaneous acquisition of thermodynamic and spectroscopic data when combined with Langmuir trough.

Trough

0

10

20

30

40

50

60

70

80

10 20 30 40 50 60 70 80 90 100 110

mean molecular area (Å2/molecule)

su

rfa

ce

pre

ss

ure

(m

N/m

)

DPPC-d62

DPPE-d62

PA-d31

IsothermsIsotherms

LE

Condensed

LC

Gas

Free OD of neat DFree OD of neat D22OO

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2550 2600 2650 2700 2750 2800 2850 2900

wavenumber (cm-1)

SF

In

ten

sity

(a.

u.)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2550 2600 2650 2700 2750 2800 2850 2900

SF

In

ten

sity

(a.

u.)

2740 cm-1

free OD of d62-DPPC

0

0.2

0.4

0.6

0.8

1

1.2

2550 2600 2650 2700 2750 2800 2850 2900

wavenumber (cm -1)

SF

Inte

nsi

ty (

a.u

.)DPPC-d62 free OD DPPC-d62 free OD

evolution evolution

Neat D2O

Π=0mN/m (G)

Π=0.5mN/m

Π=1mN/m (LE)

Π=8mN/m (LE)

Π=42mN/m (LC)

2740 cm-1

~2720 cm-1

~2680 cm-1

• Upon compression, two new bands at ~2680 cm-1 and 2720 cm-1 appear

- Free OD peak gradually disappears

DPPE-d62 free OD DPPE-d62 free OD evolution evolution

free OD of d62-DPPE

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2550 2600 2650 2700 2750 2800 2850 2900

wavenumber (cm-1)

SF

In

ten

sity

(a.

u.)

Neat D2O

Π=1mN/m

Π=2mN/m (LC)

Π=42mN/m (LC)

~2670 cm-1

2740 cm-1

Which part of the lipid is perturbing the free OD slightlyslightly?

A comprehensive view

• Tail Hydrocarbon Chain?

Hydrophobic perturbation

• Head Phosphate, Choline or -C=O group?

H-bond interaction

Devlin P. has demonstrated that CH4 causes ice free OD to red shift!

Rowland B., Fisher M., Devlin P. J. Chem. Phys. 95(2), 1991

free OD of d31-PA

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2550 2600 2650 2700 2750 2800 2850 2900

wavenumber (cm-1)

SF

In

ten

sity

(a.

u.)

How about PA-d31 (chain How about PA-d31 (chain only)?only)?

~2720 cm-1

~2670 cm-1

Π=0mN/m

Π=1mN/m

Π=3mN/m (LC)

Π=25mN/m (LC)

2740 cm-1

free OD of d31-PA (25mN/m) @26 C & 11C

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2550 2600 2650 2700 2750 2800 2850 2900

wavenumber (cm-1)

SF

In

ten

sity

(a.

u.)

26 C

11 C

~2670 cm-1

Lowering temperature can reduce tail influence

from hydrogen bonding region

Distance-dependent hydrophobic perturbation

I~2670 cm

I I~2720 cm

I I I2740 cm-1

I~2670 cm-1

I I~2720 cm -1

I I I2740 cm-1

Distance-dependent hydrophobic perturbation

D2O

PA monolayer

~2670 cm-1 2740 cm-1 ~2720 cm-1

Air

ConclusionConclusion

Free OD/OH still persists at the interface even Free OD/OH still persists at the interface even under full monolayer coverage.under full monolayer coverage.

Due to a distance dependent chain Due to a distance dependent chain perturbation, the vibrational frequency of free perturbation, the vibrational frequency of free OD/OH red-shifts as the Langmuir monolayers OD/OH red-shifts as the Langmuir monolayers are packed more and more tightly.are packed more and more tightly.

Acknowledgements

Prof. H. Allen

Dr. Gang Maand

All Allen group members!

Thanks for your attention!!!