Collagen structure: Evidence for a helical organization of the collagen fibril

The collagen fibrils of human or guinea pig dermis when exposed to the denaturing agents, urea or guanidine—HCl, dissociated into smaller, disparate subunits, probably aggregates of microfibrils. The process of dissociation demonstrates that the fibrils are assembled helically. Initially, diagonal clefts appear on the surface of the fibril. These clefts are surface manifestations of a spirally oriented, internal space. Continued exposure to these denaturants resulted in progressive dissociation of the fibril into helically oriented subunits. It is suggested that water-miscible compounds such as glycols or hydroxypropyl methacrylate, in addition to the urea—guanidinium class of denaturants used in this study, affect the observed fbrillar changes through the disruption of hydrogen bonds between the microfibrils making up the fibril. Such a mode of action may explain why freeze-fractured or “inert embedded” collagen demonstrates helical organization while other, more conventional methods of tissue processing do not. Further support for the proposed mode of action of these dissociative agents was provided by the observation that mature collagen, in which extensive intra- and intermolecular covalent crosslinks are present, is more resistant to dissociation than newly formed collagen.