Polypeptide Conformation

(Cooper, 1997 p. )

The angle conformations of the bonds around the alpha carbon are critical in determining the structure of the polypeptide and ultimately its function (Fig. 1).

Bonds

The bond between the alpha-carbon and the nitrogen of the peptide bond assumes an angle called Phi. This angle can range from -180 to 180 degrees.
The bond between the slpha-carbon and the carbon attached to the oxygen in the peptide plane assumes an angle called Psi. This angle can also range from -180 to 180 degrees.
When each of these angles are set to 0, the peptide planes are oriented as shown in Fig. 2. Notice that you are looking down the plane which includes the alpha-carbon, the H and the R-group.

Angle Combinations

The conformation of the phi and psi angles shown in Fig. 2 obviously does not exist because of the steric hinderance between the H and O groups from the adjacent planes. When angles of phi and psi are monitored in natural polypeptides and proteins, it is found that these two angles tend to be restricted to certain combinations of angles. Fig. 3 shows a plot of the phi and psi angles from over 300 residues naturally occuring in polypeptides.

Summary:

Even though the peptide planes have the potential to assume any angle around the alpha carbon, they do not because certain combinations of Phi and Psi angles

References:
Cooper, Geoffrey M. (1997) The Cell: A Molecular Approach; ASM Press, Washington, D.C. / Sinauer Associates, Inc., Sunderland, MA.