Molecular-scale visualization and surface charge density measurement of Z-DNA in aqueous solution

The DNA in the left-handed conformation (Z-conformation) was first discovered by A. Rich, who revealed the crystalline structure of a DNA oligomer d(GC)₃ by X-ray diffraction method. Later it was also found that DNA molecules change their conformations from typical right-handed form (B-DNA) to the left-handed form (Z-DNA) under specific conditions (B-Z transition). Furthermore, the detailed structures of the interface between B- and Z-DNAs, B-Z junction, was also determined with an atomic resolution. Recently it was found that some proteins have the Z-DNA binding domains, but the biological functions of Z-DNA are not well understood yet. Therefore the investigation of Z-DNA under physiological conditions is highly essential. In this study, we demonstrated the high-resolution real-space imaging of DNA molecules having the Z- and B-form conformations by frequency-modulation atomic force microscopy (FM-AFM), that has made a great progress in recent years, in an aqueous solution. The major and minor grooves of both DNA conformations were clearly visualized. Furthermore, the surface charge density was measured by three-dimensional (3D) force mapping method. We found that Z-form region was less negatively charged than the B-form region.