Nematic behavior of reversibly polymerizing proteins

The nematic behavior exhibited by a variety of amphiphiles is driven by the coupling between the growth and alignment of elongated aggregates. We have previously studied these systems using a model derived from lattice combinatorics, with attendant restriction of particle orientations to the three mutually orthogonal lattice axes. In this work we present a generalization of the above model for rod-like aggregates with a continuum of orientations. Overall, the calculated phase diagrams are similar to those obtained earlier for discrete orientations. This includes the second order isotropic-to-nematic transition in the weak aggregation regime. However, the nematic-to-nematic transition, obtained previously for intermediate aggregation strength, is not reproduced. We also consider the consequences of solvent filled spaces in the aggregates. Increasing the effective excluded volume of the solute by this mechanism causes the isotropic-to-nematic transition to shift to lower concentrations. This response of the phase behavior to the incorporation of solvent in the aggregate is important for the interpretation of experimental data.