Overview of molecular forces: Graph of a N-C-O bond angle
Oliver Smart
(c) O.S. Smart 1995, all rights reserved
Back to looking at bond angle interaction

Back to main Molecular Forces index

Back to main PPS course Index

Potential energy curve for the N-C-O Bond Angle


This graph shows the potential energy for a N-C-O bond angle using a harmonic potential with the parameters given in the AMBER potential energy function. Such a term would partly control the angular vibrations of the carbonyl oxygen on the main chain of a polypeptide chain. The bond angle constant K_theta is 80 kcal/(mol.degrees^2) and the equilibrium bond angle (theta_eq) is 122.9 degrees. The dashed line indicates an energy of 0.29 kcal/mol which is equal to 1/2RT at a temperature of 300K. This is the energy that an individual degree of freedom can expect at this temperature and indicates that a this bond could be expect to be undergoing vibrations of the order of 4 degrees at room temperature. If we assumes that the N and C atoms remain mostly static then this would entail motion in this degree of freedom of the order of 0.09 Å for the oxygen atom (as the C-O bond is 1.229 Å long). It is interesting to compare this to a motion of approximately 0.03 Å amplitude to vibrations of the C=O bond (link to graph of this motion). (This is not quite the whole story as there would be another bond angle term for the atoms C_alpha-C-O also restricting the motion).