Bioactivity and Density Functional Theory Analysis of Conformationally Constrained Helicokinin Peptidomimetics

Peptidomimetics incorporating rigid scaffolds represent a promising strategy for enhancing the metabolic stability and receptor selectivity of neuropeptide analogs. This study describes the synthesis and receptor activation assay of five helicokinin analogs ( 4 , 9 , 12 , 24 , and 28 ) containing a spiro‐oxindole piperidine (SIP) scaffold. Replacement of the Pro‐Trp dipeptide in analog 4 led to a complete loss of activity (EC 50 > 100 µ m ), highlighting its essential role in receptor recognition. In contrast, substitution of the Tyr residue with the SIP scaffold in analog 28 retained full activity (EC 50 = 0.003 µ m ), indicating that the Tyr residue is not essential for receptor binding in this analog series. Other analogs ( 9 , 12 , and 24 ) exhibited intermediate activity, reflecting position‐dependent effects of backbone rigidification. Density functional theory (DFT) calculations at the B3LYP/6‐31G(d,p) level, combined with non‐covalent interaction analysis, revealed that peptide 28 adopts a favorable turn‐like geometry, stabilizing hydrogen bonds and van der Waals interactions, whereas inactive analogs displayed disrupted electronic delocalization and larger HOMO–LUMO energy gaps, as inferred from DFT analysis. These findings demonstrate the utility of spirocyclic scaffold for fine‐tuning bioactive conformations in insect neuropeptide mimetics.