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Scanning tunneling microscopic study of osmium-impregnated collagen.

The vapor-phase osmium impregnation method was applied to increase the conductivity of the collagen specimens used in scanning tunneling microscopy (STM), since the application of STM to biological materials has been limited by the poor conductivity, irregular sample geometry, and instability of these materials. The impregnation method was applied to collagen fibrils attached to highly oriented pyrolytic graphite, and the STM images obtained were compared with those of routinely processed uncoated specimens. The osmium-impregnated collagen fibrils showed a periodicity of about 67 nm, which was also observed on the negative-stained specimens by transmission electron microscopy. In addition, the periodicity of 3.1 nm, which corresponded to the helical cycle of the 3 alpha-chains within collagen, was clearly demonstrated by STM. Moreover, the comparative observations of collagen specimens without osmium impregnation disclosed the left-handed helical structure with a periodicity of 3.1 nm in a single collagen filament.