Thursday, January 22, 2009

The Number of Lipid Molecules per Liposome

Based on the request of many scientists, we will explain the concept of particle number in this blog entry.

How can one calculate the number of lipid molecules in a liposome if the size and lamellarity of liposomes are known?

One way to measure the lamellarity of liposomes is 31P-NMR spectroscopy.

We can also assume that 100 nm liposomes are unilamellar.

This equation is used to calculate the number of lipid molecules in a unilamellar liposome.


is the surface area of one of the "monolayers" of the liposome. (d is the diameter of the liposome),

h is the thickness of the bilayer and it is about 5 nm,

a is the lipid head group area. The headgroup area of phosphatidylcholine is about 0.71 nm square,

In the above equation the surface area of both monolayers in a unilamellar liposome are added together. Then the total lipid area is divided to the head group area of one lipid molecule.

For a unilamlellar liposome composed of phosphatidylcholine the above equation is simplified to:

As an example the number of lipids in a 100 nm size liposome is about 80047.

If the concentration of lipids in the solution is known then the total number of particles per ml can easily be calculated.

The following formulation is used for calculation of number of liposome particles per milliliter of liposome solution.

where N(a) is the Avogadro number and it is equal to 6.02E23,

M (lipid) is the molar concentration of lipid,

and N(tot) is the total number of lipids per liposome.

As an example the number of liposomes (assuming that liposomes are unilamellar and 100 nm in size) in 1 milliliter of solution that has a lipid concentration of 3 micro Molar is 22.6 billion.