chi1_selection() fails for some residues

[orbeckst]

Expected behavior

ag.chi1_selection() should produce a selection for all standard protein residues (except GLY and PRO).

Actual behavior

The following residues return None instead of a valid selection:

• ALA
• CYSH
• GLY
• ILE
• SER
• THR
• VAL

for the example TPR/XTC. (Others might also be affected.)

The problem is that chi1_selection() checks for CG but these residues can contain CG1, CG2 etc.

Code to reproduce the behavior

import MDAnalysis as mda; from MDAnalysis.tests import datafiles as data
u = mda.Universe(data.TPR, data.XTC)

for resname in sorted(set(u.residues.resnames)):
    res = u.select_atoms(f"resname {resname}").residues
    # get middle one
    r = res[len(res)//2]
    #print(r, r.chi1_selection() is None)
    if r.chi1_selection() is None:
        print(r.resname)

Current version of MDAnalysis

• Which version are you using? (run python -c "import MDAnalysis as mda; print(mda.__version__)") 1.0.1 (also 1.0.0)
• Which version of Python (python -V)? 3.8.5
• Which operating system? macOS 10.15.7

[orbeckst]

This appears to be complementary to #2044.

[orbeckst]

duplicate #2044