A 40 year old women presents with a several year history of progressive abdominal colic and constipation. Colonic biopsy stained with hematoxylin and eosin demonstrates deposition of a pink acellular material in the submucosa and around blood vessels. When sections from this biopsy are stained with Congo red, the acellular material exhibits green birefringence. The birefringence is thought to be most closely related to which of the following protein propeies?

The patient has amyloidosis, which can be present in the manner illustrated. Originally, it was thought that amyloid represented a single protein. However, as the various settings in which amyloidosis occurs were fuher explored, it was discovered that a variety of clearly distinct proteins could produce the material. Eventually, it was observed that what these diverse proteins had in common was a beta-pleated sheet teiary structure that apparently is reflected in the apple-green birefringence. None of the other propeies are thought to contribute to birefringence when viewed with polarized light. The ability to bind oxygen is impoant to hemoglobin. Electrophoretic mobility is used to separate serum proteins. Hydroxyproline content is impoant to collagen structure.