In peptides the values of dihedral angle phi (D) is based on rotation around
(1) N-Ca bond
(2) Ca -C’ bond
(3) C’-N bond
(4) N-H bond
The phi (φ) angle in peptides is defined as the dihedral angle around the N–Cα bond. This angle is measured between the plane formed by the C’ (of the previous residue) – N – Cα and the plane formed by the N – Cα – C’ (of the current residue).
Thus, the correct answer is:
(1) N-Cα bond.