Direct observation of human liver ferritin by scanning tunneling microscopy

We report here on some scanning tunneling microscopy (STM) observations on iron-loaded (I 200 atoms/molecule) human liver ferritin (HLF) chemically bound to gold surfaces activated with ~A' -dipyridil disulfide. The samples were prepared by immersing the activated gold foils in 10-500 ,ug/mL ferritin solutions in 0.3 mmollL tris(hydroxymethyl)-aminomethan (TRIS), pH 7.4. After a 30 min incubation period the foils were washed first in water. then in ethanol, air dried, and analyzed with a STM microscope. Individual ferritin molecules were resolved under the STM as spheroidal objects of approximately 10 nm diameter. The HLF molecules appeared as positive bumps above the bare substrates both in constant current and gap-modulated images, with a nonrandom internal structure possibly related to their quaternary structure. After the observation the HLF was extracted and quantified by an immunoenzymatic test. In conclusion, the results obtained by the STM analysis were found to concur with the information provided by x-ray crystallography.