What are the psi and phi backbone conformational angles in a protein?

Question

I just know that Phi and psi angles are used in the classification of some secondary structure elements such as beta turns.

But I want to know the basics.

what are these angles?

are they present only in the secondary proteins?

how they help in the classification of secondary proteins?

Answer 1

In a polypeptide the main chain N-Calpha and Calpha-C bonds relatively are free to rotate. These rotations are represented by the torsion angles phi and psi, respectively.

Repeating values of phi and psi along the chain result in regular structure. For example, repeating values of phi ~-57o and psi ~-47o give a right-handed helical fold (the alpha-helix).  (see link 2).

Similarly, repetitive values in the region of phi = -110 to -140 and psi = +110 to +135 give extended chains with conformations that allow interactions between closely folded parallel segments (beta sheet structures).  (se link 2).