Strings and stripes formed by a protein system interacting via a single-patch attraction

The phase behavior of lactoferrin has been studied as a function of concentration at a pH and ionic strength where lactoferrin is known to interact effectively via a patch-patch attraction. In contrast to isotropic attractive potentials, the directional attraction gives rise to a different phase or solution behavior. At low concentrations, the protein dimerizes. As the concentration is increased, the protein self-assembles into elongated, stripe-like structures at intermediate protein concentrations, a behavior which has been predicted for the case of attractive one-patch colloids. The stripe phase is surprisingly difficult to detect using conventional techniques, i.e. small-angle X-ray scattering, since only a small fraction of the proteins participate in the stripes combined with sedimentation due to micron-sized entities. This is circumvented by monitoring the change in the overall protein concentration by static light scattering and the stripe formation can be followed. For visualization of the structures cryo-TEM is used.