Circular dichroism studies of substance P and its C-terminal sequences. CD spectra in aqueous solution and effects of hydrogen ion concentration.

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 alpha-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.