Harvard Medical School
Brigham and Women's Hospital
Boston, MA Harvard-MIT Division of Health Sciences and Technology
Cambridge MA 02138
Many biopolymers share the properties of being only slightly flexible and having a high negative surface charge. These general properties of the single filaments have many implications for the structure and mechanical properties of the macroscopic networks they form in physiologic settings. Two biochemically distinct polymers, F-actin and the filamentous viruses fD, M13, and pf1 exhibit similar behavior in forming viscoelastic networks and filament bundles driven by counterion condensation. Among these properties are the relatively large elastic moduli of semi-flexible polymer networks and a high degree of strain hardening that networks of flexible polymers do not exhibit. The viscoelasticity of actin networks contributes to the mechanics of the cellular cytoskeleton, and transitions from isotropic to laterally aligned domains are implicated in a variety of cellular processes. In pathological settings such as the abnormally stiff sputum of cystic fibrosis patients, poly-cation driven bundling of both actin filaments and DNA within the normal sputum matrix can be relieved in vitro by manipulation of the ionic composition. These examples and others suggest areas where a quantitative understanding of biopolymer assembly and self organization may elucidate a variety of biological processes and possibly contribute to treatments of abnormal biopolymer organization and accumulation.