Thursday, October 20, 2011

Lipid movements in liposomes

Liposomes are dynamic systems. The lipids in the liposome bilayers move in various ways. 

Lateral diffusion refers to the lateral movement of lipids in the membrane. Lipids are generally free to move laterally if they are not restricted by certain interactions. Lateral diffusion is a fairly quick and spontaneous process.  The first part of the animation shows the lateral movement of a red color lipid.

Lipid rotation around its axis:  The whole lipid molecule is free to rotate around its vertical axis. This motion is slower: it takes few microseconds to complete one rotation.

Lipid tail wagging: The non-polar tails undergo a "wagging" motion due to the rotation around the C-C single bonds. These motions are rapid (several times in a nanosecond) because the barrier for internal rotation around the C-C bond is low. However, the configuration around the cisdouble bond remains unchanged. 

Flip-Flop from one half of the bilayer to the other half of the bilayer is normally very slow. Flip-flop would require the polar head-group of a lipid to traverse the hydrophobic core of the membrane. The last part of the animation shows the flip-flop of a red color lipid from one layer to another.