Turn induction by N-aminoproline. Comparison of the Gly-Pro-Gly and Gly psi [CO-NH-N]Pro-Gly sequences.

The folded structure induced by the N-aminoproline residue (the hydrazino analogue of proline, denoted hPro) in the Boc-Gly1-hPro2-Gly3-NHiPr hydrazino tripeptide has been characterized in the solid state by X-ray diffraction, and compared to the usual beta II-turn structure in the Boc-Gly1-Pro2-Gly36-NHiPr cognate tripeptide. It is stabilized by a bifurcated hydrogen bond in which (Gly3)NH interacts with both (Gly1)CO and (hPro2)N alpha. This conformation is retained in CH2Cl2 and CHCl3 solutions, and allows an overall folded conformation of the hydrazino tripeptide in which (iPr)NH is hydrogen-bonded to (Boc)CO. The hPro alpha-hydrazino acid residue appears to promote a local folded structure, and might behave as a beta-turn mimic.