Lipoplexes are complexes formed between cationic liposomes (L(+)) and polyanionic nucleic acids (P(-)). They are commonly used in vitro and in vivo as a nucleic acid delivery system. Our study aims are to investigate how DOTAP-based cationic liposomes, which vary in their helper lipid (cholesterol or DOPE) and in media of different ionic strengths affect the degree, mode of association and degree of condensation of pDNA. This was determined by ultracentrifugation and gel electrophoresis, methods based on different physical principles. In addition, the degree of pDNA condensation was also determined using the ethidium bromide (EtBr) intercalation assay. The results suggest that for cationic lipid compositions (DOTAP/DOPE and DOTAP/cholesterol), 1.5 M NaCl, but not 0.15 M NaCl, both prevent lipoplex formation and/or induce partial dissociation between lipid and DNA of preformed lipoplexes. The higher the salt concentration the greater is the similarity of DNA condensation (monitored by EtBr intercalation) between lipoplex DNA and free DNA. As determined by ultracentrifugation and agarose gel electrophoresis, 30-90% of the DNA is uncondensed. SDS below its critical micellar concentration (CMC) induced "de-condensation" of DNA without its physical release (assessed by ultracentrifugation) for both DOTAP/DOPE and DOTAP/cholesterol lipoplexes. As was assessed by agarose gel electrophoresis SDS induced release of 50-60% of DNA from the DOTAP/cholesterol lipoplex but not from the DOTAP/DOPE lipoplex. This study shows that there are conditions under which DNA is still physically associated with the cationic lipids but undergoes unwinding to become less condensed. We also proved that the helper lipid affects level and strength of the L(+) and DNA(-) electrostatic association; these interactions are weaker for DOTAP/cholesterol than for DOTAP/DOPE, despite the fact that the positive charge and surface pH of DOTAP/cholesterol and DOTAP/DOPE are similar.

中文翻译：

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影响DNA结合和与阳离子脂质体缔合稳定性的因素。

脂质复合物是在阳离子脂质体（L（+））和聚阴离子核酸（P（-））之间形成的复合物。它们通常在体外和体内用作核酸递送系统。我们的研究目的是研究基于DOTAP的阳离子脂质体（其辅助脂质（胆固醇或DOPE）和不同离子强度的介质中的差异）如何影响pDNA的程度，缔合方式和缩合程度。这是通过基于不同物理原理的超速离心和凝胶电泳法确定的。此外，还使用溴化乙锭（EtBr）插入测定法确定pDNA的缩合度。结果表明，对于阳离子脂质组合物（DOTAP / DOPE和DOTAP /胆固醇），使用1.5 M NaCl而不是0.15 M NaCl，二者均防止脂质复合物的形成和/或诱导脂质与预先形成的脂质复合物的DNA之间的部分解离。盐浓度越高，脂质复合物DNA和游离DNA之间的DNA缩合（通过EtBr嵌入监测）的相似性就越大。通过超速离心和琼脂糖凝胶电泳确定，30-90％的DNA未缩合。低于其临界胶束浓度（CMC）的SDS不会对DOTAP / DOPE和DOTAP /胆固醇脂复合物产生物理释放（通过超速离心评估）而导致DNA的“去缩合”。如通过琼脂糖凝胶电泳所评估的，SDS诱导从DOTAP /胆固醇脂复合物而非从DOTAP / DOPE脂复合物释放50-60％的DNA。这项研究表明，在某些情况下，DNA仍与阳离子脂质物理结合，但展开后变得不那么凝聚。我们还证明了辅助脂质会影响L（+）和DNA（-）静电缔合的水平和强度。尽管DOTAP /胆固醇和DOTAP / DOPE的正电荷和表面pH值相似，但DOTAP /胆固醇的这些相互作用弱于DOTAP / DOPE。