Conformational studies of peptides: Crystal and molecular structures of L‐3,4‐dehydroproline and its t‐butoxycarbonyl and acetyl amide derivatives

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Abstract
The crystal structures of L‐3,4‐dehydroproline, t‐butoxycarbonyl‐L‐3,4‐dehydroproline amide, and acetyl‐L‐3,4‐dehydroproline amide have been determined. L‐3,4‐Dehydroproline is orthorhombic with a = 16.756, b = 5.870, c = 5.275 Å, and Z = 4; t‐butoxycarbonyl‐L‐3,4‐dehydroproline amide is orthorhombic with a = 6.448, b = 8.602, c = 21.710 Å, and Z = 4; acetyl‐L‐3,4‐dehydroproline amide is monoclinic with a = 4.788, b = 10.880, c = 7.785 Å, β = 105.25°, and Z = 2. The final R value for the L‐3,4‐dehydroproline is 0.046 based on 529 reflections; for t‐butoxycarbonyl‐L‐3,4‐dehydroproline amide, 0.050 based on 792 reflections; and for acetyl‐L‐3,4‐dehydroproline amide, 0.058 based on 632 reflections. The structures clearly establish that the free amino acid exists in the zwitterionic form in the crystalline state. The molecular conformations of the t‐Boc and acetyl derivatives consist of two planes: one involving the primary amide and the other the remaining atoms of the molecule. The acetyl‐L‐3,4‐dehydroproline amide contains a tertiary amide bond in the cis conformation. To the best of our knowledge, this is the first example of a cis bond in an acetyl derivative of an amino acid or peptide. At variance with the previously reported proline amides, which present ϕ and ψ values corresponding to those of a right‐handed α‐helical conformation (conformation A), the t‐Boc and acetyl derivatives both have ϕ and ψ values corresponding to a collagenlike conformation (conformation F).

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