The possible symmetries of arrays extending in one direction with a fixed
repeating distance.
Linear polymer chains in the crystalline state must belong to one of the line repetition groups.
Permitted symmetry elements are: the identity operation (symbol l); the translation
along the chain axis (symbol t); the mirror plane orthogonal to the chain axis (symbol
) and that containing the chain axis (symbol
); the glide plane containing the chain axis (symbol
); the
inversion centre, placed on the chain axis (symbol
); the two-fold axis orthogonal to the chain axis (symbol 2); the helical, or screw,
symmetry where the axis of the
helix coincides with the chain axis. In the latter case, the symbol is
,
where
stands for the screw axis,
is the class of the
helix, * and / are separators, and
is the integral number of residues contained in
turns, corresponding to the identity period
(
and
must be prime to each other). The class index
may be dropped if deemed unnecessary, so that the
helix may also be simply denoted as
.
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