Peptide tau angles
==================

The configuration of peptides in proteins can be described to a first
approximation by angles φ, ψ and ω, where the angles φ and ψ can have
values from continuous range, the angle ω is either 0 or 180 degrees
(for cis- and trans- peptides, respectively) and all other backbone
angles are fixed, as are the bond lengths. But this is only the first
approximation – in real life all bond parameters are variable.

One of the angles whose variation can be interesting is the τ angle,
which formed by chemical bonds between N-Cα and Cα-C atoms.

Questions:

- explore how this angle varies in different structures. What kind
  typical value of this angle? Why? What are the values of this angle
  distribution? Is it symmetrical? What is its mean value? What is the
  physical meaning of the calculated "typical" value? What statistical
  parameter (mode, median, sample mean, variance, standard deviation,
  ...) best represents the important properties of the τ angle
  distribution?

- in which structures, or in which amino acids (a.r.), τ angle
  significantly different from "typical" values? Why? Are these
  differences structure resolution errors, do they make biological
  sense?

Program:

This research requires a program that reads PDB archive files and
determines and outputs the values of the τ angle for all
residues. Don't forget that it will be necessary to trace for which
protein and which of its residues each τ value is computed.