Department of Chemistry, The University of Memphis, Memphis, Tennessee 38154, USA.
Biomacromolecules. 2010 Jan 11;11(1):29-38. doi: 10.1021/bm900842d.
The success of polyethyleneimine (PEI) as a nonviral-based gene delivery vector has been attributed to its proton buffering capacity. Despite the great interest in PEI for its use in nonviral-based gene delivery, the protonation behavior of PEI in solution is not well understood. Earlier experimental studies have reported inconsistent values of the protonation state of PEI. In this work, we report our investigation of the protonation behavior of a realistic linear PEI (lPEI) with computational approaches. Reported experimental pK(a) values of several diamine compounds are first examined. A screened Coulombic interaction with a distance dependence dielectric is shown to reproduce the shifted pK(a) values of the model diamine compounds. Then atomistic molecular dynamic simulations of lPEI chain with 20 repeating units are performed and the results are used to provide parameters for a coarse-grained polyamine model. The screened Coulombic interaction is then incorporated in the coarse-grained lPEI chain and computational titrations are performed. The obtained computational titration curves of lPEI in solutions were found to be in best agreement with experimental results by Smits et al., but the computational titration curves have too strong of a dependence on salt concentration compared to the experimental results by Smits et al. Disregarding the discrepancy in the salt dependence, our computational titrations reveal that approximately 55% of the lPEI amine groups are protonated under physiological conditions in solution with a nearly alternating arrangement of protonated and nonprotonated amines. Titrations of lPEI in the presence of a polyanion are also performed to determine how the charge state of lPEI could be affected by complexation with DNA in gene therapy preparations. While the presence of the polyanion increases the degree of protonation of the PEI, many of PEI amines remain unprotonated under physiological conditions, providing evidence that PEI complexed with DNA could still have proton buffering capacity. Potential sources of error that have resulted in the inconsistency of previously reported protonation states of PEI were also discussed.
聚亚乙基亚胺（PEI）作为一种非病毒基因传递载体的成功归因于其质子缓冲能力。尽管人们对 PEI 在非病毒基因传递中的应用非常感兴趣，但 PEI 在溶液中的质子化行为仍未得到很好的理解。早期的实验研究报告了 PEI 质子化状态的不一致值。在这项工作中，我们使用计算方法报告了对真实线性 PEI（lPEI）质子化行为的研究。首先检查了几种二胺化合物的实验报告 pK(a) 值。结果表明，具有距离依赖性介电常数的屏蔽库仑相互作用可以重现模型二胺化合物的移位 pK(a) 值。然后对具有 20 个重复单元的 lPEI 链进行原子分子动力学模拟，并将结果用于提供粗粒化聚胺模型的参数。然后将屏蔽库仑相互作用纳入粗粒化 lPEI 链中并进行计算滴定。结果发现，获得的 lPEI 在溶液中的计算滴定曲线与 Smits 等人的实验结果最为吻合，但与 Smits 等人的实验结果相比，计算滴定曲线对盐浓度的依赖性太强。忽略盐依赖性的差异，我们的计算滴定表明，在生理条件下，溶液中约 55%的 lPEI 胺基被质子化，质子化和非质子化胺基呈近乎交替排列。还进行了 lPEI 在多阴离子存在下的滴定，以确定 lPEI 的电荷状态如何可能会受到基因治疗制剂中与 DNA 络合的影响。虽然多阴离子的存在增加了 PEI 的质子化程度，但在生理条件下，许多 PEI 胺基仍未质子化，这表明与 DNA 络合的 PEI 仍可能具有质子缓冲能力。还讨论了导致以前报告的 PEI 质子化状态不一致的潜在误差源。
