DIRECT MEASUREMENT OF HYDROGEN-BONDING IN DNA NUCLEOTIDE BASES BY ATOMIC-FORCE MICROSCOPY

We have used self-assembled purines and pyrimidines on planar gold sur faces and on gold-coated atomic force microscope (AFM) tips to directl y probe intermolecular hydrogen bonds. Electron spectroscopy for chemi cal analysis (ESCA) and thermal programmed desorption (TPD) measuremen ts of the molecular layers suggested mono-layer coverage and a desorpt ion energy of about 25 kcal/mol. Experiments were performed under wate r, with all four DNA bases immobilized on AFM tips and flat surfaces. Directional hydrogen-bonding interaction between the tip molecules and the surface molecules could be measured only when opposite base-pair coatings were used. The directional interactions were inhibited by exc ess nucleotide base in solution. Nondirectional van der Waals forces w ere present in all other cases. Forces as low as two interacting base pairs have been measured. With coated AFM tips, surface chemistry-sens itive recognition atomic force microscopy can be performed.