Does the Macrostructure of Deoxyribonucleic Acid Molecules Adsorbed on Substrates by the Pulse Injection Method Reflect That in Solution

In order to investigate whether the concentration-dependent macrostructure of Deoxyribonucleic acid (DNA) molecules can be retained after pulse injection, as compared to that originally present in the aqueous solution, we performed scanning tunneling microscopy (STM) imaging of plasmid DNA pulse-injected from an aqueous solution containing different DNA concentrations. The observed STM images revealed the presence of a relaxed circular macrostructure at low concentrations (1.2 µg/ml of DNA and 0.3 mM of Na+) whereas an entangled circular macrostructure was observed at high concentration (60 µg/ml of DNA and 15 mM of Na+). We conclude that plasmid DNA molecules retain the macrostructures possessed in the aqueous solution even after pulse injection onto a solid surface under vacuum environment.