Supercoiled and linear plasmid DNAs interactions with methylene blue

• DNA accumulation on glassy carbon from pg/mL solutions results in stable layers.
• Linear (lin) plasmids are oxidized differently from their supercoiled (sc) counterparts.
• DNA-methylene blue interactions also reveal differences between lin and sc plasmids.
• Enhancement of nucleic acid oxidation is observed due to methylene blue intercalation.

Slow accumulation of plasmids from their diluted, pg/mL solutions (pH 4.7) on a well defined glassy carbon (GC) electrode surface allowed for the formation of stable electrode layers consisting of two types of plasmid DNAs — pUC19 and pGEX-4T-2, in two forms — supercoiled circular (sc) and linear (lin). In the presence of methylene blue (MB), a typical redox indicator, the oxidation signals of nucleic acid bases are significantly enhanced. The interactions of the plasmids with MB are tested and used to distinguish between various types of plasmids. Instead of an MB reduction signal at ca. − 0.2 V vs. SCE, typically used to study MB interactions with DNAs, we have used the corresponding oxidation signal at ca. − 0.2 V, MB(I), as well as another oxidation signal at 1.05 V, MB(III). On a bare GC electrode, the MB(III) and MB(I) signals are proportional to each other, while in the presence of the plasmid DNAs the relations between MB(III) and MB(I) depend on the type of plasmid. The plots: MB(III)/MB(I) vs. [MB] and MB(I) potential shift vs. [MB] are used to distinguish between the supercoiled and linear forms of the pUC19 and pGEX-4T-2 plasmids.