Cholesteric liquid crystalline phases given by three helical biological polymers : DNA, PBLG and xanthan. A comparative analysis of their textures

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Abstract
The cholesteric liquid crystalline phases of three polymers of biological interest have been investigated: PBLG (a polypeptide), DNA (a polynucleotide) and xanthan (a polysaccharide). The textures (and the defects which they contain) of these three mesophases are analysed and compared The main difference concerns focal lines which apparently do not occur in PBLG, are rare in DNA but occur frequently in xanthan. The frequency of occurrence of the different types of rotation and translation dislocations were measured : - π disclinations are always of the λ- type and + π disclinations are mainly λ +. Among polymers, differences exist in the distribution of translation dislocations but in these three cases, when only one layer is added, the defect may be either τ- λ+ or λ- τ + which distinguishes polymer mesophases from those of small molecules previously studied The main textures found in classical (small molecule) cholesteric phases were also obtained (i.e. planar textures, textures showing numerous disclinations and polygonal textures). However, they are more difficult to analyse especially for DNA and xanthan mesophases where the helicoidal pitches are small and may not be resolvable in an optical microscope