Definition of Φ and Ψ (phi, psi) and the Ramachandran plot

Once upon a time, many many years ago in a far away country called India a bioinformatician avant la lettre called Ramachandran worked without a computer but nevertheless computed nearly correct what torsion angles of amino acids should be.

In this plot you find the combinations of Phi and Psi that are favourable for helix, strand, and left handed helix or some residues in a β-turn:

• φ-ψ ~ (-60,-40) Helix
• φ-ψ ~ (-150,150) Strand
• φ-ψ ~ (40,40) Turn (and left handed helix, but those are very rare)

Definition of φ-ψ (phi-psi), atoms involved

Now that you know the values for φ and ψ in, for example, a helix, and you know the handedness of residues, you can look at any helix (take for example one helix from crambin) and figure out which atoms define φ and which define ψ.