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?