Contribution of Side-Chains to the Infra-Red Spectra of Proteins: the 6.5-µ Band

Abstract

THE strong bands at 3 µ, 6 µ and 6.5 µ which dominate the absorption spectra of proteins in the fundamental region, and which have been observed as well in N-monosubstituted amides and in synthetic polypeptides, have been attributed to vibrations of the peptide links between the amino-acid residues forming the structural backbone of the protein molecule¹. It has been established, however, that, in most proteins, side-chains carrying ionizable groups are joined to the polypeptide backbone, and that at the isoionic point the protein exists in the form of dipolar ions. Of the ionizable groups which have been identified, carboxyls occur in the greatest number and may be expected to contribute significantly to the absorption processes in the spectral region of interest.