| Abstract
| - Poly[2-(dimethylamino)ethyl methacrylate-b-2-methacryloyloxyethyl phosphorylcholine] (DMA-MPC)is currently under investigation as a new vector candidate for gene therapy. The DMA block has beenpreviously demonstrated to condense DNA effectively. The MPC block contains a phosphorylcholine (PC)headgroup, which can be found naturally in the outside of the cell membrane. This PC-based polymer isextremely hydrophilic and acts as a biocompatible steric stabilizer. In this study, we assess in detail themorphologies of DNA complexes obtained using the diblock copolymer series DMAxMPC30 (where the meandegree of polymerization of the MPC block was fixed at 30 and the DMA block length was systematicallyvaried) using transmission electron microscopy (TEM) and liquid atomic force microscopy (AFM). Bothtechniques indicate more compact complex morphologies (more efficient condensation) as the length of thecationic DMA block increases. However, the detailed morphologies of the DMAxMPC30−DNA complexesobserved by TEM in vacuo and by AFM in aqueous medium are different. This phenomena is believed tobe related to the highly hydrophilic nature of the MPC block. TEM studies revealed that the morphologyof the complexes changes from loosely condensed structures to highly condensed rods, toroids, and oval-shaped particles as the DMA moiety increases. In contrast, morphological changes from plectonemic loopsto flowerlike and rectangular blocklike structures, with an increase in highly condensed central regions,are observed by in situ AFM studies. The relative population of each structure is clearly dependent on thepolymer molecular composition. Enzymatic degradation assays revealed that only the DMA homopolymerprovided effective DNA protection against DNase I degradation, while other highly condensed copolymercomplexes, as judged from TEM and gel electrophoresis, only partially protected the DNA. However, AFMimages indicated that the same highly condensed complexes have less condensed regions, which we believeto be the initiation sites for enzymatic attack. This indicates that the open structures observed by AFMof the DNA complexation by the DMAxMPC30 copolymer series are closer to in vivo morphology whencompared to TEM.
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