For each trajectory structure, x, y, and z field components at the substrate proline N and preceding C atom were evaluated directly in the Cartesian coordinate frame. The field at each atom was also projected along a vector perpendicular to the plane of the substrate proline ring, calculated individually for each frame as the unit-length cross product of the N-Ca and N-Cd bond vectors. This vector approximates the proposed orientation of the peptidyl carbonyl group at the cis-trans transition state. The projection of the substrate N and C field vectors along the same direction therefore estimates the magnitude of the environment electric field aligned with the peptidyl carbonyl group during the peptide bond rotation.