Small-Angle X-ray Scattering of the Interaction of Poly(ethylene oxide)-b-Poly(propylene oxide)-b-

Poly(ethylene oxide)Triblock Copolymers with Lipid Bilayers

Millicient A. Firestone, Amanda C. Wolf, and Sonke Seifert

Chris Bianchi

4/30/12

Background

• Polymer-a large molecule composed of repeating structural units.

• Copolymer-a polymer derived from two or more monomeric species.

• Lipid bilayer-cellular structure that acts as a barrier separating the outside of the cell from the inside.

Background

eastman.com

http://en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Lipid_Bilayer

What Was Studied?

• Mechanistic study of a commercial surfactant known as Pluronic on model lipid bilayers.

• Specifically the interaction of structural variants of the triblock copolymer PEO-PP0-PEO with dimyristol-sn-glycero-3-phosphocholine (DMPC) lipid bilayers.

Firestone, et al. 2003

Why Study This?

• Potential pharmaceutical application by replacing lipid grafted polymers in liposomes.

• In addition for sealing damaged cell membranes in injuries

such as heat shock, high dose radiation, and neurotoxic events.

http://en.wikipedia.org/wiki/Liposome

Techniques

• Small-Angle X-ray Scattering at undulator 12ID-C of the Advanced Photon Source at Argonne National Laboratory were done on samples sealed in 1.5mm quartz capillaries.

• The sample-to-detector distance was such as to provide scattering vector range of 0.0025/Å < q < 0.6/Å

• Thermal analysis was done using differential scanning calorimetry (DSC).

SAXS Theory

Nielsen & McMorrow, 2010

SAXS Theory Sphere

Long Wavelength Limit: QR<<1

Short Wavelength Limit: QR >>1

Results

• They first studied the affect of PPO chain length.

• Compared to SAXS profiles to two well characterized systems

1. Pure DMPC in solution

2. PEGlyated DMPE, DMPC, LDAO

Results

• The DMPC bilayer has two equally spaced Bragg peaks at q = 0.099/Å and q = 0.199/Å and d = 63 Å.

• PEG5000-DMPE has five distinct Bragg peaks at q = 0.0244, 0.0448, 0.0732, 0.0976, and 0.123 Å and d = 257 Å . This is a swollen lamellar structure.

Results DMPC-Water PEG5000-DMPC

Firestone, et al. 2003

Results

Firestone, et al. 2003

PO Chain Length Effect

• Determined the effects of Pluronic F38, F68, F88, F98, and P85*

*This is a different physical form and was not used in first round of experiments

Firestone, et al. 2003

PO Chain Length Effect SAXS Pluronic # PO

Repeat Units

Rg (Å) # Bragg Peaks

q-Positions (1/Å)

d-spacing (Å)

Summary

F38 15 7 2 0.111, 0.219

57 shallow insertion

F68 30 - 2 0.111, 0.223

57 shallow insertion

F88 39 15 4 0.0467, 0.094, 0.142, 0.188

134 increased insertion from F38 F68

F98 47 - 5 0.0433, 0.0867, 0.133, 0.174, 0.216

145 increased insertion from F38, F68, F88

*Bilayer hydrophobic moieties Rg is 20 Å

PO Chain Length Effect SAXS

DMPC-Water PEG5000-DMPC

F38 F88

F68 F98

Firestone, et al. 2003

PO Chain Length Effect

F38 and F68 F88 and F98

Firestone, et al. 2003

EO Chain Length Effect

• The authors hypothesize the PEO chain length and concentration are contributing factors to the structural properties of the bilayer.

• While the PPO chains integrate into the tail region of the bilayer the PEO moieties aggregate on the polar head region which produces a surface tension and leads to area expansion.

EO Chain Length Effect Pluronic # PO

repeat units

Rg (Å) # EO repeat units

# Bragg Peaks

q-Positions ( 1/Å)

d-spacing (Å)

Summary

P85 40 - 25 3 0.061, 0.122, 0.183

103 Swollen Lamellar Structure

F88 39 15 103 5 0.047, 0.094, 0.142, 0.189, 0.227

134 Increase in long range ordering

P85 F88

Firestone, et al. 2003

EO Concentration Effect F98

Concentration d-spacing (Å)

4 mol % 120

8 mol % 128

10 mol % 145

12 mol % 112

20 mol % Not sated (hexagonal phase emerges)

*Reminder F98: #PO=47, #E0=117 http://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Micelle_scheme-en.svg/250px-Micelle_scheme-en.svg.png

EO Concentration Effect F98

4 mol % 8 mol %

12 mol % 20 mol %

Firestone, et al. 2003

EO Concentration Effect F68

• Insufficient number of PO molecular repeat units to span the bilayer.

• SAXS patterns were invariant under changes in concentration of F68.

• This suggest an interplay between both PO and EO chain length for d-spacing tuning and long range ordering of the bilayer.

Effect of Temperature F98 SAXS

• PPO block is more water soluble at lower temperatures.

• As temperature increases hydrophobicity increases and thus insertion of PPO block into the bilayer increases.

Effect of Temperature F98 SAXS

Firestone, et al. 2003

Conclusions

• PEO-PPO-PEO can be used as alternatives to more costly and limited PEGlyated biomimetic nanostructures.

• When PPO block is less than bilayer thickness insertion is poor, but PEO chains adopt more of a flat orientation.

• If EO concentration is grater than 10 mol % a saturation limit is reached and micelles begin to form.

• Hydrophobicity of PPO block is dependent upon temperature.