We compare the phase behavior of a mutant filamentous virus, fd Y21M, to that of a conventional fd wild-type (wt). We find significantly different macroscopic phase behavior despite the only microscopic difference between the two viruses being in a single amino acid of the major coat protein pVIII. Compared to fd wt, the location of the isotropic-cholesteric phase transition for fd Y21M shifts to lower densities. This is attributable to a significant difference in the flexibility of the two viruses. The persistence length of fd wt is 2.8 +/- 0.7 mu m, whereas the persistence length of fd Y21M is 9.9 +/- 1.6 mu m. The large persistence length of fd Y21M makes it an essentially rigid rod, thus allowing for the first time a quantitative test of the Onsager theory for the isotropic-nematic phase transition. Even more striking, is the difference in the chiral phase behavior of the two viruses. Both viruses form cholesteric phases, with the fd wt forming a left-handed cholesteric helix, and the fd Y21M forming a right-handed one. At a given density, the magnitude of the cholesteric pitch between the two systems is different by fivefold. Using mixtures of the two viruses, we create a liquid crystalline system with a tunable control over its macroscopic chirality.
