Chiral Nematic Phase of Suspensions of Rodlike Viruses: Left-Handed Phase Helicity from a Right-Handed Molecular Helix

We report a study on charged, filamentous virus called M13, whose suspensions in water exhibit a chiral nematic (cholesteric) phase. In spite of the right-handed helicity of the virus, a left-handed phase helicity is found, with a cholesteric pitch which increases with temperature and ionic strength. Several sources of chirality can be devised in the system, ranging from the subnanometer to the micrometer length scale. Here an explanation is proposed for the microscopic origin of the cholesteric organization, which arises from the helical arrangement of coat proteins on the virus surface. The phase organization is explained as the result of the competition between contributions of opposite handedness, deriving from best packing of viral particles and electrostatic interparticle repulsions. This hypothesis is supported by calculations based on a coarse-grained representation of the virus.

Figure 1

Surface of the M13 virus [24], generated from the 2C0X.pdb capsid protein structure [8]. (a) All-atom representation; proteins with a given color are related by a fivefold symmetry axis. The distance of 3.3 nm corresponds to the periodicity, i.e., the separation between pairs of proteins with the same coordinates in the plane perpendicular to this symmetry axis. (b) Surface accessible to a sphere of radius 0.5 nm, rolling on the all-atom representation. Colors correspond to the electrostatic potential (red: negative, blue: positive). (c) Coarse-grained representation. Stacked light gray spheres (radius of 2.5 nm) represent the core of the virus, while dark gray spheres (radius of 0.6 nm) represent protruding amino acids (each sphere corresponds to 4 amino acids). Red spots correspond to point charges (located at 0.1 nm distance from the virus surface). Black lines show the tangent to a right-handed molecular helix identified on the virus surface ($\phi \sim 41°$).

Figure 2

Direct visualization by confocal fluorescence microscopy of the left-handed cholesteric helix in M13 virus suspensions, at concentration $C_{\mathrm{M}13}=49\mathrm{mg}/\mathrm{ml}$ and ionic strength $I=110\mathrm{m}M$. The sequence of images follows the virus director, represented by red arrows, through the sample thickness ($Z$). One M13 virus out of ${10}^4$ has been labeled with a fluorescein dye. The scale bar indicates $50\mu \mathrm{m}$.