Modeling of macromolecular alignment in nematic virus suspensions. Application to the prediction of NMR residual dipolar couplings

The alignment of macromolecules in dilute suspensions of filamentous phages is described in terms of steric and electrostatic contributions; the former are modeled as excluded volume interactions between a viral particle and a macromolecule, and the latter are treated at the mean field level, through the Poisson-Boltzmann equation for the virus surrounded by an ion density. The virus is represented as a uniformly charged rod, whereas the relevant features of the macromolecule, i.e., shape and charge distribution, are explicitly taken into account. As an application, the residual dipolar couplings between N-15 and H-1 nuclear spins in the Ig-binding domain of streptococcal protein G are calculated, and their dependence on ionic strength and virus dimension or concentration is analyzed. The theoretical predictions enable us to explain the NMR observations reported for this protein.