Geometry Dynamics of α-Helices in Different Class I Major Histocompatibility Complexes

Figure 5

Surface torsion of the interhelical surface. Front view (left) and top-down view (right) on MHC α-helices G-ALPHA1 and G-ALPHA2 whose axes are modelled by second-degree polynomials (blue and red). Lines (in grey, called “rulings”) between the polynomials span a ruled surface. Taking the mean coordinates of the lines coloured in blue and red results in the centre line (green). When moving over the surface along the centre line we see that rulings change direction, which can be quantified by a parameter called “surface torsion.” Surface torsion describes the extent and orientation of twist of a surface along a given line, which is the centre line in our case. The surface torsion describes important aspects of the relative orientation of the two helix axes towards each other.

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