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.