SynopsisSelf-association of substance P and its C-terminal partial peptide sequences was studied by CD, quasi-electric light scattering, and sedimentation experiments. CD spectra of these peptides are strongly influenced by self-association. They exhibit strong characteristic negative ellipticities, suggesting the formation of a presumably B-type ordered structure.The tendency to form multimers depends on chain length and constitution and has its maximum a t the octapeptide (SP 8). The peptide multimers have a broad distribution of sizes in the range of 30-and 800-nm diameter. Subdivision of this distribution into two size classes gives mean diameters of 60-100 nm (predominating)/200-800 nm for substance P and 30-50 nm/200-800 nm for SP 8 multimers.

CD spectra of substance P (SP) and its C‐terminal partial sequences have been measured in diluted aqueous solution including variation of hydrogen ion concentration. In the far u.v. region there is an overlapping of the amide CD absorption by the CD of the phenylalanine residue aromatic side chains (217–220 nm). This complex CD absorption is reduced during changes from acidic to alkaline pH, especially in those cases where a phenylalanine residue is at the N‐terminus of the peptide chain. These CD changes dependent on pH are due more to charge effects on the aromatic chromophores than to substantial conformational changes. However, solvation effects on the conformational features of SP peptides caused by the deprotonation of the amino groups have to be taken into account. CD and potentiometric titrations indicate that the N‐terminal α‐amino groups of the SP peptides in general are freely accessible to the solvent. Our studies did not give any evidence of the occurrence of ordered structures of SP peptides in diluted aqueous solution.