Structural Mechanisms of Forced Unfolding of Double-Stranded Fibrin Oligomers.

Fibrin forms a polymeric scaffold of blood clots, which are subjected to deformation in their dynamic environment. The extensible fibrin network allows fibers to stretch without breaking, but the mechanisms of their forced elongation are not understood. We combined atomic force microscopy, computer simulations, and Machine Learning to explore the nanomechanics of double-stranded cross-linked fibrin oligomers (FO). From the experimental force-extension profiles, the median 63 pN unfolding force and median 8.1 nm peak-to-peak distance with corresponding 56 pN and 11.4 nm interquartile ranges indicate substantial scatter due to ∼3-5 nm extension fluctuation of the triple α-helical coiled-coils. From simulations, unraveling of FO is determined by coupled dissociation of the D:D interface, γ-nodules unfolding, and reversible unfolding-refolding of the coiled-coils. These can occur as single structural transitions (60% of the time) or mixed transitions (40% of the time), with an alternating order of strands in which unfolding transitions occur, i.e., if the previous transition takes place in one strand, the next transition occurs in the other strand. The double-stranded FO are less extensible but stiffer and more stable compared with the single-stranded oligomers. These findings provide important insights into the biomechanics and dynamic structural properties of fibrin necessary to understand the (sub)molecular origin of fibrin extensibility.