乙醇能够消除核酸的水化层，使带负电荷的磷酸基团暴露出来。Na＋之类的平衡离子能够与这些带电基团结合，在沉淀形成部位降低多核苷酸链之间的排斥作用。因此只有在阳离子的量足以中和暴露的磷酸残基的电荷时才会发生乙醇沉淀。
最常用的阳离子：
（1）醋酸铵 ：贮存液 10。0mol/L 终浓度 2。0－2。5mol/L
常用于减少多余的杂质（如DNTP及多糖）与核酸的共沉淀。例如在2mol/L醋酸铵存在的情况下连续两次DNA沉淀可从DNA制品中除去>99％的dNTP。在通过琼脂糖酶消化琼脂糖之后沉淀核酸时，使用醋酸铵也是最佳选择，这种阳离子可以减少寡糖消化产物共沉淀的可能性。然而若用沉淀的核酸进行磷酸化时，就不要用醋酸铵沉淀核酸，因为铵离子能够抑制T4噬菌体多核苷酸激酶。
（2）氯化锂：贮存液 8。0mol/L 终浓度 0。8mol/L
常用于需高浓度乙醇进行的沉淀（如沉淀RNA）。LiCl在乙醇溶液中的溶解度很高而且不随核酸共沉淀。小分子RNA（如tRNA及5S RNA）在高离子强度下（没有乙醇时）是可溶的，
而大分子RNA则不溶。可以利用这个差异在高浓度LiCl（0。8mol/L)中纯化大分子RNA。
（3）氯化钠：贮存液 5。0mol/L 终浓度 0。2 mol/L
（0。2 mol/L）用于DNA样品中存在SDS时。这种去污剂在70％乙醇中仍为可溶。
（4）醋酸钠：贮存液 3。0mol/L（ph5。2） 终浓度 0。3 mol/L
（0。3mol/L，ph5。2）在DNA和RNA的常规沉淀中最为常用。
Dave's EtOH Precipitation Procedure
网易晕*** Jongmin's comment
You cannot get rid of toxic phenol by EtOH prep!! You got to do Gel purification after Phenol/Chloroform extraction.
***
Usage: For parating DNA from the solution it is currently in.
Uful for removing salts, stains, dyes, etc. Not uful for removing
proteins and not uful for parating DNA/RNAs of different lengths.
Basic Theory: DNA is relatively insoluble in a solution of 70% EtOH,
while most salts, stains, and dyes are. Thus, upon adding in 2X pure
EtOH to a salt adjusted aqueous solution of DNA, the DNA will become
supersaturated, and precipitate out upon centrifugation.
Efficiencies and Loss: This procedure should remove more than 99% of
impurities soluble in 70% EtOH in solution, and provide 80% DNA yield
网络营销经典for medium to high concentrations of DNA. For DNA concentrations of
less than 0.1 uM, this procedure is not very efficient. While testing
this procedure at 0.03 uM, efficiency has been en to be as low as
自我介绍英语作文20%. The possibly predominant reason for this loss is becau in the
equation DNA(aq) <-> DNA(s), Ksp = [DNA(aq)], with no dependence on
DNA(s), which means that you lo a constant amount of DNA given a
constant conditions of aqueous and EtOH volume, no matter how much or
little total DNA you have, provided it is more than the prequisite
amount to get any precipitation at all. Erik suspects that we're also
losing a fair bit to the sides of the centrifuge tubes, so try to
conduct this entire procedure in one single tube.
Procedure: First, find the volume of the solution of DNA you are
working with. To enhance the efficiency of this process, it is
recommended that you Vacu-fuge the sample for an hour at 60 degrees
Celcius first to decrea volume. However, you should not decrea
the volume to below 40uL. If you inadvertently do so, add water until
volume is above 40uL. If the post-Vacufuge volume of your sample is
more than 1/5 the total volume of the tube you're performing this
reaction in, divide your sample into veral tubes, such that each
tube is no more than 1/5 full. For our standard 1.7mL tubes, I like
to put in 300uL of DNA solution at this step.
Next, we have to adjust salt concentrations. This can be done veral
个体工商户条例ways, but for this procedure I recommend using 2/3 X of 7.5 M ammonium
acetate (NH4)C2H3O2 (e.g. add om 200uL of 7.5 M ammonium acetate for
300uL of DNA solution). *Important Note* You should not skip this
稀浆封层
step even if you already have a concentrated salt solution in your DNA
(possibly due to previous Vacu-fuging). Extra salts here do not
really matter much, as generally less than 1 part in 750 of salts will
remain at the end of this procedure.
Now, add in 2X of your current volume of EtOH (e.g. if we started with
300uL DNA solution, our volume now is 500uL, so we add 1000uL of
年假计算方法属猪的出生年份表EtOH). You may add a little more EtOH, but do not add less. Nick
古开来recommends using cold (freezer temp) 'molecular biology grade' 200 proof
EtOH. Then put your samples intothe freezer for 1 to 2 hours (official
number here is 1 hour, but I like to do at least 2; go out to dinner or
something). Start the "Fast-Cool" on the centrifuge, adjusting temperature
to 3 degrees Celsius.
When you're back from dinner, remove your samples from the freezer and
place in the centrifuge in a way that's balanced. *Important Note*
You MUST balance centrifuge It will be very unhappy if you don't,
and that will cau the rest of the lab to be unhappy. The centrifuge
has 30 slots, which means most number of samples can be easily
