Real crystals of macromolecules are not arranged perfectly.

Think about the temperature factors of the atoms. Even if the average atom positions would be identical, the atomic displacement factors can vary drastically in all three spatial dimensions.

Thank you Stefan,

But it would be better if you elaborate your answer than everyone can have a clear concept about anisotropy of the materials.

Best wishes,

Abhay Dasadia..

Anisotropy is the non-equivalence of space directions. The fact that a crystal is periodic in the 3 directions does not at all insure the equivalence of those directions. If you get a triclinic P1 crystal, then nothing is equivalent and you'll have anisotropic properties. Please refer to the international tables of Crystallography, vol A, chapter 10.2 p804 in the 2005 edition where the link between physical properties and the crystal point group is explicited for various physical properties.

Abhay,

Real crystals of molecules are not perfectly periodic in all three dimensions.

Even if the molecules pack well in one or two directions doesn't mean the other direction is equally well defined.

If you have disorder in all packing interactions your crystal will diffract roughly isotropic but to lower resolution.

If you have disorder only in one direction the resolution drops only in the dimension the molecules pack poorly. Then your diffraction pattern will become elliptical and you speak of anisotropic diffraction.

huh? even if we assume a perfect arrangement, we can easily understand that there must be an anisotropy in the properties. This is simply due to the bonds: we don't have an isotropic distribution of atoms around a given one so we have properties that depend on the direction of the bonds (even a fully ionic bond that is typically non directional, lies along a direction once the atoms are put in the lattice)