Depending on the sequence, siRNA can have 40-42 phosphate atoms per
molecule. The no. of protonable nitrogens in PEI monomer can be counted
and depending on its molecular weight, the nitrogens in one PEI chain
can be calculated. N/P ratio is just the ratio of total nitrogens to
total phosphates taken in solution.
Example: For the siRNA with Sense Sequence
(G.G.U.G.A.A.G.G.U.A.G.A.U.C.A.A.A.G.A.dT.dT), and Antisense Seq.:
U.C.U.U.U.G.A.U.C.U.A.C.C.U.U.C.A.C.C.dT.dT ; there are 40 phosphate atoms
per molecule (each strand has 20 phosphates indicated by dot representing
a phosphodiester linkage). So, the total moles phosphates atoms can be
calculated as (n*40), where n is the total no. of moles of siRNA in
solution.
For the PEI, the total moles of nitrogen atoms can be calculated as
(M*V*N), where M is the molarity of PEI stock solution based on repeat
unit, V is the volume of PEI (of molarity M) taken in solution, and N is
the no. of nitrogens in one monomer unit of PEI. As example, for LPEI N
=1.
Thus, N/P ratio can be calculated as: (M*V*N)/(n*40). If N/P is known, V
can be calculated to make solution with desired N/P ratio.