The supercoiled plasmid (also referred to as covalently closed circular or CCC form) is the topological isoform of plasmid DNA in which the double-stranded circular DNA molecule is further twisted on itself, resulting in a compact, tightly coiled structure. It is the biologically active and therapeutically preferred isoform for gene therapy, viral vector production, and transfection applications, as it exhibits superior transfection efficiency, stability, and biological activity compared to the relaxed (open circular) or linear forms. The proportion of supercoiled isoform relative to other forms is a critical quality attribute (CQA) in plasmid DNA manufacturing and must be controlled and quantified — typically by agarose gel electrophoresis or HPLC — as part of batch release testing and regulatory submissions.
