Characterization of the cholesteric phase of filamentous bacteriophage fd for molecular alignment

Residual dipolar couplings arise from small degrees of alignment of molecules in a magnetic field and have proven to provide valuable structural information. Colloidal suspensions of rod-shaped viruses and bacteriophages constitute a frequently employed medium for imparting such alignment onto biomolecules. The stability and behavior of the liquid crystalline phases with respect to solution conditions such as pH, ionic strength, and temperature vary, and characterization should benefit practical applications as well as theoretical understanding. In this Communication we describe the pH dependence of the cholesteric liquid crystalline phase of the filamentous bacteriophage fd and demonstrate that the alignment tensor of the solute protein is modulated by pH. We also report the interesting observation that the relative sign of the residual dipolar coupling changes at low pH values. In addition, we demonstrate that the degree of alignment inversely scales with the lengths of the phage particles for phages with identical mass and charge per unit length.