AFM phase lag mapping for protein–DNA oligonucleotide complexes

Atomic force microscope phase lag imaging of protein–DNA oligonucleotide complexes has been performed to visualize the immobilized oligonucleotides on the protein surface. In normal sample conditions, neither the topographic nor phase lag images show any discriminate signals for the immobilized oligonucleotides. Use of a highly humid incubator, controls the surface humidity of the sample. Thereby, the phase lag image reveals the oligonucleotide location by the local difference of tip adhesion distribution. The resultant phase lag image shows extremely strong signals in the center of the protein surface, indicating the location of the oligonucleotides with resolution better than 20 nm. The signal frequency was strongly influenced by the used oligonucleotide concentration in the range 5 nM–50 (mu)M.