The phrase "a two fold axis" is not correct in standard written English; it should be "a twofold axis." You can use it when describing an axis that has two distinct aspects or dimensions, often in a mathematical or scientific context.
Example: "The study focused on a twofold axis of analysis, examining both the quantitative and qualitative data."
Alternatives: "a dual axis" or "a bifold axis."
As expected, the quality of the electron density map in these stretches is much better for monomer A than B. Monomer A is rotated nearly through 180° with respect to monomer B but due to the differences in local structure and B-factors between the two monomers, the two monomers are not related by a crystallographic two fold axis as in case of BsuL.
The same applies to the "barns", which experimentally exhibit a fourfold in-plane symmetry, whereas a barn has only a two fold symmetry axis.
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Pfu-SRP54 crystallized in the tetragonal space group P42212 with two monomers in the asymmetric unit resulting in a fairly high solvent content of 69%; the two monomers are related by a non-crystallographic two fold symmetry axis (Figure 3A).
Owing to the two fold crystallographic axis that runs through this channel there are two symmetry related positions for the ligand at both ends of the channel.
The piezoelectric material is polarized when the six fold symmetry axis is put along the propagation direction x1.
The protein forms a windmill-like pentamer along a five-fold axis, forming five highly similar ligand-binding sites between two adjacent protomers (Fig. 1).
This revealed the existence of a 2-fold axis between its two rings resulting in two homotypic subunit interactions across the rings.
A crystallographic 2-fold axis brings two ValA chains together to form a dimer that, according to the PISA server, buries 4220 Å of surface area (i.e., 2110 Å per monomer).
