Left-handed alpha helices
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We all know (or do we?;) that α-helices that are encountered in
proteins are right-handed. And are there any left-handed helices in
the proteins?

Questions:

- are there any left-handed α-helices found in protein structures
  described in the PDB archive? If not, why? If so, where and how many
  are found, why?

- are left-handed ones found in natural proteins/peptides α-helices?
  If not, why? If so, where and why?

Program:

This study requires a program that reads PDB archive files, finds
α-helices in them and identifies and deduces the "handedness" of the
helix (i.e. whether the helix is left-handed or right-handed). Can we
trust PDB file annotations? If so, how to check it? If not, what to
do?