A line group is a mathematical way of describing symmetries associated with moving along a line. These symmetries may include repeating along that line, making that line a one-dimensional lattice. Most line groups have more than one dimension, and involve those dimensions in its isometries or symmetry transformations. One constructs a line group by taking a point group in the full dimensions of the space and adding translations (sometimes with a twist) along the line to each of the point group's elements, in the fashion of constructing a space group. Although a point group by definition holds at least one point stationary, this is not necessarily true of a line group because of the translations.
One-dimensional There are 2 one-dimensional line groups. They are the infinite limits of the discrete two-dimensional point groups Cn and Dn:
Two-dimensional There are 7 frieze groups, which involve reflections along the line, reflections perpendicular to the line, and 180° rotations in the two dimensions.
Three-dimensional There are 13 infinite families of three-dimensional line groups, derived from the 7 infinite families of axial three-dimensional point groups. As with space groups in general, line groups with the same point group may have different patterns of offsets (two or even three), giving line groups in different families. Each of the families is based on a group of rotations around the axis with order n, where n can be any positive integer including 1. As mentioned earlier, there is not necessarily any point where the point group applies. For example, in the line group P21/m, associated with the point group C2h (called 2/m in the H-M notation), there is no point with C2h symmetry, because the two-fold rotation is converted into a screw displacement. The groups are listed in Hermann-Mauguin notation, and for the point groups, Schönflies notation. There appears to be no comparable notation for the line groups. These groups can also be interpreted as patterns of wallpaper groups wrapped around a cylinder n times and infinitely repeating along the cylinder's axis, much like the three-dimensional point groups and the frieze groups. A table of these groups:
The offset types are:
None. Offsets along the axis include no offsets around it to within repeats of the unit cell around the axis. Helical offset with helicity q. For a unit offset along the axis, there is an offset of q around it. A point that has repeated offsets will trace out a helix. Zigzag offset. Helical offset of 1/2 relative to the unit cell around the axis. Note that the wallpaper groups pm, pg, cm, and pmg appear twice. Each appearance has a different orientation relative to the line-group axis; reflection parallel (h) or perpendicular (v). The other groups have no such orientation: p1, p2, pmm, pgg, cmm. If the point group is constrained to be a crystallographic point group, a symmetry of some three-dimensional lattice, then the resulting line group is called a rod group. There are 75 rod groups.
The Coxeter notation is based on the rectangular wallpaper groups, with the vertical axis wrapped into a cylinder of symmetry order n or 2n. Going to the continuum limit, with n to ∞, the possible point groups become C∞, C∞h, C∞v, D∞, and D∞h, and the line groups have the appropriate possible offsets, with the exception of zigzag.
Helical symmetry
The groups Cn(q) and Dn(q) (those based on the point groups Cn and Dn) express the symmetries of helical objects. Cn(q) is for n helices oriented in the same direction, while Dn(q) is for n non-oriented helices or n pairs helices with alternating orientations. Reversing the sign of q creates a mirror image, reversing the helices' chirality or handedness.
Nucleic acids (DNA and RNA) are well known for their helical symmetry. Single strands of nucleic acids have a well-defined direction, with line group C1(q) if we ignore the identities of the nucleic bases. If we do not ignore the bases, then the strand must consist of some sequence of bases that repeats to have this symmetry. Double stranded nucleic acid has two strands of opposite directions, but not on opposite sides of the helix axis because the line group is D1(q) (if we ignore the bases) rather than D2(q). If we do not ignore the bases, then to have this symmetry it must consist of repeats of a section, such as "TA" or "TACTAGTA", made up of two complementary halves with opposite order. There are three forms of DNA, with different details but the same line group. Microtubules are made up of helical chains of alternating α-tubulin and β-tubulin, called protofilaments. In the usual form, thirteen such chains wind together around a common axis, touching one another, and going from a dimer in one protofilament to the dimer in the next and so on to get back to the first protofilament results in being three dimers further along it. The line group is C1(q). A dimer of α-tubulin and β-tubulin can be carried by a rotation of around 4/13 of 360° and a displacement in the direction of the axis to coincide with the position of another such dimer. This operation is a generator for the group. Three repetitions of this generating element comes to the neighboring dimer to the first one, in the next protofilament, and 13 repetitions arrives at the next dimer in the original protofilament. Another example having the line group C1(q) is the rod-like structures found in many viruses. For example, the tobacco mosaic virus has a helix of protein surrounding the RNA. The generating element combines a rotation of around 1/17 of 360° with a small displacement in the direction of the axis.
See also Point group Space group One-dimensional symmetry group Frieze group Rod group
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