Scanning probe microscopy of collagen I and pN-collagen I assemblies and the relevance to scanning tunnelling microscopy contrast generation in proteins

Abstract

Scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have been carried out on hydrated fibrous assemblies of collagen I and pN-collagen I. STM of calf-skin collagen I that had been allowed to assemble on a graphite substrate showed the predicted 8 nm diameter microfibrils, arrays of which showed a 67 nm repeat. A detailed contrast variation along the microfilbrils was observed, corresponding to the established stain banding observed by transmission electron microscopy (TEM) and associated with hydrophilic, charged amino acid side chains along the staggered-molecule microfibril. This indicates that adsorbed water is an important mediator of STM imaging in insulators such as proteins. AFM images were obtained of D-periodic assemblies of pN-collagen deposited on carbon-coated copper TEM grids. The use of carbon-coated TEM grids as an SPM substrate allows experiments to be performed on the same samples, and correlation made between TEM and SPM images.