Presented in Biophysical Society meeting held in 2008

1. Effect of Spontaneous Curvature on
the Adsorption of Lipids to the
Air/Water Interface
Samares C. Biswas1, Shankar B. Rananavare2,
Stephen B. Hall1.
1Oregon Health & Science University, Portland, OR,
USA, 2Portland State University, Portland, OR, USA.

2. Pulmonary surfactant
Pulmonary Surfactant:
•Produced in alveolar type- II pneumocytes
•Secreted into the alveolar hypophase as
multilamellar vesicles
•Adsorbs to the air/liquid interface
•Forms film that lowers surface tension.
Lipids Proteins
SP-A, SP-D
SP-B, SP-C
Pulmonary Surfactant
Contains phospholipids , cholesterol
and specific proteins.
Alveolus in the Lung
Lipid film
Water
Air

3. Rapid adsorption of pulmonary surfactant is
essential
In a normal lung:
• Surfactant adsorbs rapidly, during first breath.
• Rapid adsorption requires hydrophobic surfactant proteins (SP-B/-C).
• Lack of hydrophobic surfactant proteins is lethal
CLSE: Calf lung surfactant extract
N&PL: Neutral & phospholipids
(Total surfactant lipids without proteins)
N&PL:
•Reduces surface tension slowly
•Adsorption is incomplete
Hydrophobic surfactant proteins accelerate adsorption

4. Fundamental observation
Rate-limiting structure is equally accessible from both locations
SP-B/-C in bulk
Without protein
With protein
Without protein
With protein
Proteins (SP-B/-C) accelerate adsorption whether in vesicles or at the interface.
N&PL vesicles
below prespread film

5. Proposed model: Stalk Intermediate
•Stalk bridges the gap
between the vesicle and the
interface.
•Stalk equally accessible
from both locations.
•SP-B/-C promote adsorption by stabilizing stalk.
•Bending energy is major component of activation barrier
Negative curvature

6. co= spontaneous curvature in the absence of applied force
c1 and c2 = curvatures along the principal radii of the actual structure
k and kG = curvature-elastic moduli of splay and saddle-splay, respectively
Bending Energy- Helfrich Equation
Two mechanisms for lowering bending energy:
1. Produce tight negative spontaneous curvature
2. Reduce k and kG (Lower rigidity)
f = 1/2·k(c1 + c2 - co)2 + kGc1c2

7. Hypothesis
Flexibility
Spontaneous negative curvature
should both enhance adsorption kinetics
DEPE = 1,2-Dielaidoyl (18:1(delta 9-trans) PE:
undergoes Lb-La-HII phase transitions
DPPC = 1,2-Dipalmitoyl (16:0) PC :
undergoes Lb-La phase transitions
•Spontaneous curvature (C0):
DEPE forms HII phase → C0
DEPE < 0
DPPC does not form HII → C0
DPPC ≥ 0
•Flexibility:
La should adsorb faster than Lb phase (lower k, kG)
DEPE should adsorb faster

8. Phase behavior of DEPE and DPPC
DEPE:
•Lb-La Transition at 38C
•Lb-HII Transition ~67C
•More flexible at higher temperature (La phase)
Lb La HII
DPPC (published data):
•Lb-La Transition at 41C
•No HII phase detected
DEPE
DEPE

9. Adsorption of DEPE and DPPC
DEPE adsorbs faster than DPPC
•negative spontaneous curvature
▪faster adsorption
La phase adsorbs faster than Lb phase
•Lower membrane rigidity
▪faster adsorption

10. CONCLUSIONS
These results fit with the stalk model
•Rate of adsorption is limited by bending energy
•Factors that lower bending energy
produce faster adsorption

11. Acknowledgements
SSRL (BL 1-4 & BL 4-2)
NIH (HL 54209)

14. Microstructure of CLSE dispersion
Both 31P -NMR and SAXD showed only the presence of
lamellar structure in CLSE dispersion