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Ring conformations of 4-substituted proline residues. The Cγ-endo conformation is favored strongly when R1 = H and R2 = F (as in flp). The Cγ-exo conformation is favored strongly when R1 = OH (Hyp) or F (Flp) and R2 = H. The Cγ-exo:Cγ-endo ratio is ∼1:2 when R1 = R2 = H (Pro) (DeRider et al. 2002).
