On Thu, 18 Nov 2004 16:28:35 +0000, jodhbir wrote:
> Hi,
> For non crystallographic structures 5 fold symmetry is allowed but it
> is not allowed for crystallographic structures. Can someone please
> shed some light on this?
>> Thanks,
> Jodhbir
5-fold symmetry does not fill up space in a way that i necessary to create
crystal planes. When a set of pentagrams are joined together they will
form a dodecahedron, a ball. Only "infinitely long" planes act as
mirrors and diffract incoming radiation to create a diffraction pattern.
And only such very long planes create the straight edges which defines
the crystal.
However, the ability to create balls is abused by viruses to envelope its
DNA or RNA code with protein. Were it not possible to form balls out of
identical subunits, it would be impossible to envelope DNA with protein,
since any nucleic acid takes up more space than a single protein encoded
out of it. A virus would then be an impossible concept. :)
