CIRCULAR-DICHROISM STUDIES OF SUBSTANCE-P AND ITS C-TERMINAL SEQUENCES - CD SPECTRA IN AQUEOUS-SOLUTION AND EFFECTS OF HYDROGEN-ION CONCENTRATION

Abstract

CD [circular dichroism] spectra of substance P (SP) and its C-terminal partial sequences were measured in diluted aqueous solution including variation of H+ concentration. In the far UV region there was an overlapping of the amide CD absorption by the CD of the Phe residue aromatic side chains (217-220 nm). This complex CD absorption was reduced during changes from acidic to alkaline pH, especially in those cases where a Phe 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 must 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. No evidence of the occurrence of ordered structures of SP peptides in diluted aqueous solution was found.