Figure 3.

Images of T4 lysozyme molecules obtained with the SFM. The overall size of each image field is 500 nm × 500 nm. (A) Polymers of T4 lysozyme molecules from dissolved crystals (exposed to oxygen for 30 days). The dimensions of the extended features indicate that they are polymers with lengths up to 30 monomers. (B) Monomeric T4 lysozyme molecules. The globular features have typical diameters of ≈8 nm and heights of ≈2 nm, which are consistent with that of a folded T4 lysozyme molecule because the lateral dimensions of the molecule are overestimated in the image because of the finite size of the SFM tip. If a T4 molecule is approximated as a sphere with a radius of 1.8 nm, its image will have radii of 4 nm in the lateral dimensions when imaged with a tip with a radius of curvature of 10 nm. In these experiments, 20 μl of protein solution (in PBS buffer) was deposited on freshly cleaved mica and allowed to adsorb for 10 min. The sample then was washed with the same buffer and dried with nitrogen gas. Imaging was performed in air by using a Nanoscope SFM (Digital Instruments, Santa Barbara, CA) operating in the tapping mode.