Construction of a polyproline structure with hydrophobic exterior using octahydroindole-2-carboxylic acid.

The proline analogue (2S,3aS,7aS)-octahydroindole-2-carboxylic acid (Oic) has been previously applied as a proline substitute in pharmocologically active peptides and as a structural component of the antihypertensive drug Perindopril. Herein, we describe the formation of an oligoproline structure by an Oic oligomer. A series of Oic oligomers were investigated to show the structural and energetic contribution of appended residues to the structure. NMR investigation of these oligomers revealed an all-trans amide bond structure, and we provide evidence that a cascade-like mechanism is responsible for the all-trans folding cooperativity. X-ray crystallography of the Oic-hexapeptide clearly demonstrates that the all-trans structure of the Oic oligomer is a polyproline II helix. Thus, as a hydrophobic proline analog with a highly stable trans-amide bond, Oic represents an ideal building block for hydrophobic sites of polyproline II structures in biologically relevant contexts.