Type II β-turn conformation of pivaloyl-L-prolyl-α-aminoisobutyryl-N-methylamide: Theoretical,spectroscopic, and X-ray studies |
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Authors: | B. V. Venkataram Prasad Hemalatha Balaram P. Balaram |
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Abstract: | Pivaloyl-L -Pro-Aib-N-methylamide has been shown to possess one intramolecular hydrogen bond in (CD3)2SO solution, by 1H-nmr methods, suggesting the existence of β-turns, with Pro-Aib as the corner residues. Theoretical conformational analysis suggests that Type II β-turn conformations are about 2 kcal mol?1 more stable than Type III structures. A crystallographic study has established the Type II β-turn in the solid state. The molecule crystallizes in the space group P21 with a = 5.865 Å, b = 11.421 Å, c = 12.966 Å, β = 97.55°, and Z = 2. The structure has been refined to a final R value of 0.061. The Type II β-turn conformation is stabilized by an intramolecular 4 → 1 hydrogen bond between the methylamide NH and the pivaloyl CO group. The conformational angles are ?Pro = ?57.8°, ψPro = 139.3°, ?Aib = 61.4°, and ψAib = 25.1°. The Type II β-turn conformation for Pro-Aib in this peptide is compared with the Type III structures observed for the same segment in larger peptides. |
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