【求助】siRNA阳离子载体达人入·求助N/P计算

查阅了很多文献，结果把自己越搞越糊涂了

因为是做阳离子的复合载体，主要是用PEI进行siRNA的静电吸附，PEI经过修饰之后，氨基有所变化，已经通过化学的比色法进行残余氨基测定，可以确定氨基残基（N）的mol数

但是每个mol的21nt的双链siRNA（19对RNA，undefined2个嵌合DNA，很经典的搭配）中磷酸集团（P）应该算成几个mol呢？查阅了许多类似的siRNA文献，都没有正面说这个问题。DNA的东西貌似无法完全作为参考，而且现在大家都是做质粒，质量和mol乱的一塌糊涂。

更是让人晕的是，不少类似的复合载体文章中，作者干脆只算PEI的质量转化为mol数，理由是，只有PEI有N原子。。。

因为N/P是一个重要的参数，文章中实在是不能回避，而临床背景半路出家，很多分子生物学的基础很差，经过几个礼拜的苦苦折磨之后，实在是没有办法了，到这里来抓稻草了，希望RNA载体达人予以指点一二。

在这种情况下，我一般都是直接写信给作者。并告诉他/她：愿意以后引用其文章。一般都回信。

我请教了一个作者,他答复如下:

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 (undefined40), 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
(undefinedundefinedN), 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: (undefinedundefinedN)/(undefined40). If N/P is known, V
can be calculated to make solution with desired N/P ratio.

超赞的回答，多谢。您给我帮了大忙了！

很好的解释，支持一下！