Membrane Structure of Substance P. II. Secondary Structure of Substance P, [9-Leucine]substance P,and Shorter Segments in 2,2,2-Trifluoroethanol,Methanol, and on Liposomes Studied by Circular Dichroism

Comparative CD studies with substance P ( 1 ), Leu⁹]substance P (Leu⁹]- 1 ), and their shorter peptide segments supported the membrane structures predicted for substance P and Leu⁹]substance P. They indicated that the C-terminal segments (from residue 3 or 4 onward) can adopt α-helical conformations in hydrophobic environments and on lipid membranes. The N-terminal segment, (residues 1–4) had a poly(proline)-like conformation in aqueous and hydrophobic surroundings. Residues 3 and 4 (Lys-Pro) appeared to belong to both domains and bring about the transition between the two. The estimated free energies of transfer for 1 and Leu⁹]- 1 from their random conformations in H₂O to their partially helical conformations on an aqueous-hydrophobic interface are too small to allow detectable interaction with neutral lipid membranes at low concentrations. The two peptides should, however, interact detectably with anionic membranes because of favourable Boltzmann distribution factors. This prediction was shown to be correct for liposomes prepared from 1,2-dioleoyl-sn-glycero-3-phosphocholine (neutral) and phosphatidylserine (anionic).