Molecular structure of organic compounds observed by high resolution scanning tunneling microscopy

Uncoated DNA molecules marked with an activated tris (1-aziridinyl) phosphine oxide (TAPO) solution were deposited on gold substrates and imaged in air with the use of a high-resolution scanning tunneling microscope (STM). Constant-current and gap-modulated STM images show a clear evidence of the helicity of the DNA structure; pitch periodicity ranges from 2.5 to 3.5 nm whereas the average diameter is 2.0 nm. Molecular structure within a single helix turn was also observed. TAPO molecules have been imaged alone and appear as a crystalline structure of rows of molecules with a side-by-side spacing of 0.3 nm.