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Bond angle strain in the proline ring as a function of χ2 for Cα-Cβ-Cγ (A), Cβ-Cγ-Cδ (B), and Cγ-Cδ-N (C). In the disfavored region at χ2 ~ 0°, the model values of Cγ-Cδ-N are found significantly below the observed values. This is the key strain that separates the puckers. χ2 frequency distributions for the trans-Pro isomer (D) and the cis-Pro isomer (E). The energy curve in D is due to the CHARMM Cγ-Cδ-N angle term.
