Thermal denaturation of fibrinogen visualized by single-molecule atomic force microscopy

Fibrinogen denaturation is an important phenomenon in biology and medicine. It has been previously investigated with bulk methods and characterized by parameters, which refer to big protein ensembles. Here we provide a new insight into fibrinogen denaturation with a high-resolution single-molecule atomic force microscopy (AFM). The ultrastructure of individual fibrinogen molecules was studied after heating or extended contact with the highly oriented pyrolytic graphite surface (HOPG) modified with oligoglycine-hydrocarbon graphite modifier (GM). Fibrinogen heating to 65 °C and 90 °C resulted in the formation of various shapes containing fibrillar and globular structures, which were attributed to the monomers and small aggregates of fibrinogen. Fibrinogen unfolded by the extended (10 min) incubation on GM-HOPG surface in water revealed a different morphology. It contained fibrillar structures only, and their organization reflected the initial native structure of fibrinogen: typically, six polypeptide chains connected by multiple disulfide bonds were seen. A combination of two morphologies - globular aggregates with dense fibrillar networks - has been revealed for thermally denatured protein adsorbed on a GM-HOPG surface with extended (10 min) rinsing with water. The obtained results provide better understanding of fibrinogen unfolding induced by different factors and are important for improvement of biomedical applications, such as fibrinogen-based protein matrixes and carbon-based biomaterials.