Mw 800

Poly(ethyleneimine) (Sigma Aldrich, MW 800, cat. no. 408719) was dissolved in water to prepare a stock solution (90 mg ml -1 ). Aqueous PEI was sonicated in a bath for 15 min. FND stock solution (2 mg ml -1 ) was added to the PEI solution (1 : 1 vol : vol) and sonicated with a probe (400 W, 15 min) under water cooling. The solution was washed with deionised water and centrifuged at 14 500g for 15 min (3 times) to remove residual PEI. The pellet was then redispersed in deionised water to a final concentration of 0.48 mg per ml of FND-PEI complex (based on FND concentration).
DNA (160 ng) was dissolved in 5 µl water and sonicated in a bath cooled with ice for 1 h. This solution was then added to a 25 µl FND-PEI complex (0.48 mg ml -1 , containing 12 µg FND) followed by sonication for 2 h. The FND-PEI-DNA complex was isolated by centrifugation at 14 000g for 15 min, and the pellet was resuspended in an appropriate amount of DNAsefree water (Qiagen) to obtain a stock solution of 2 mg per ml FND-PEI-DNA.