Imaging isolated strands of DNA molecules by atomic force microscopy

Area of Science:

• Biophysics
• Materials Science

Background:

• Atomic force microscopy (AFM) is a powerful tool for visualizing biological molecules at the nanoscale.
• Understanding DNA structure and behavior in different environments is crucial for molecular biology and nanotechnology.

Purpose of the Study:

• To investigate the effect of humidity on the atomic force microscopy (AFM) imaging of plasmid DNA.
• To explore the relationship between relative humidity and the contrast and apparent width of DNA strands observed via AFM.

Main Methods:

• Adsorption of pBS+ plasmid DNA onto freshly cleaved mica surfaces.
• Imaging of DNA samples in air using atomic force microscopy (AFM).
• Controlled manipulation of relative humidity within the AFM environment.

Main Results:

• Isolated DNA strands were observed at low concentrations (<0.3 µg/ml), while interconnected networks formed at higher concentrations (>3 µg/ml).
• DNA imaging contrast and width were found to be highly dependent on relative humidity.
• Negative contrast images (strand widths ~20x DNA) were observed above 60% relative humidity.
• Positive contrast images (strand widths 7-10x DNA) were observed below 30% relative humidity.

Conclusions:

• Relative humidity significantly influences AFM imaging of plasmid DNA, affecting contrast and apparent width.
• Controlled humidity allows for switching between positive and negative contrast imaging modes.
• These findings are important for accurate AFM-based structural analysis of DNA and other biomolecules.