Fluctuations in self-assembled sickle hemoglobin fibers

We discuss microscopic measurements of thermal fluctuations in self-assembled sickle hemoglobin fibers. These permit the measurement of various mechanical moduli that control the rheology of the fiber gel responsible for the pathology of the disease. Differential interference contrast microscopy of isolated fibers undergoing equilibrium thermal bending fluctuations leads to estimates of thermal persistence lengths of between 0.23 and 10.4 mm for the most flexible and stiffest fibers, respectively. We argue that this large range may reflect the formation of fiber bundles. If the most flexible fibers are single fibers, then the stiffest measured object is consistent with a "closed shell" hexagonal bundle of 7 single fibers. We estimate the spectrum of persistence lengths associated with bundles consisting of different numbers of single fibers and compare this with the experimental data. The equivalent Young'S modulus for the material is 0.1 GPa, less than for structural proteins but much larger than for extensible proteins.