Optical isomers of the side chain of the Thr residue
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Threonine (Thr, https://en.wikipedia.org/wiki/Threonine) is one from
those amino acids (a.a.) that have an asymmetric atom not only in the
main chain (Cα) but also in the side chain (where?). Thus, if most
a.a., such as leucine (Leu), have only two optical isomers (L-Leu and
D-Leu), Thr has more (how many?).

Questions:

- how many protein structures with various optical isomers Thr are in
  the PDB archive?

- does the optical configuration of the Thr Cα atom correlates with
  the side chain optical configuration, or are they "combined"
  randomly? Why?

- What are the *rare* optical isomers of Thr found in the PDB archive?
  Why they appeared in these PDB structures? Are these PDB errors or
  interesting natural phenomena? Will these isomers be found in the
  wild?

Program:

This research requires a program that reads PDB archive files and
determines and derives the optical configuration of Thr residues.