PEI reagent
10% polyethylenimine (w/v)
Add 10 ml of polyethylenimine (Polymin P, BASF or BRL, molecular-weight range 30,000-40,000) to 70 ml of H2O. Titrate the solution to pH 7.9 with concentrated HCl, and adjust the final volume to 100 ml with H2O. Filter the PEI solution through a 0.22-μm nitrocellulose filter (e.g., GS filter, Millipore) and store the solution at 4°C. This 10% (w/v) solution is the working PEI reagent and appears to be stable indefinitely. To make 6% (w/v) PEI, add 6 ml of 10% PEI to 4 ml of H2O.
