Optical rotatory dispersion of some aromatic amino acids and peptides

Abstract

Optical rotatory dispersions over the range 600−230 mμ have been recorded for the aromatic amino acids d-phenylglycine, l-phenylalanine, l-tyrosine and l-tryptophan and compared with the dispersions of a series of di-, tri- and tetra- peptide amides containing aliphatic and aromatic amino acids, as models for the study of the dispersion properties of proteins. l-Phenylalanine in acid solution does not exhibit a Cotton effect in the region of the B adsorption bands; d-phenylglycine exhibits a weak multiple effect in this region and l-tyrosine shows a very strong effect. The rotatory dispersion curves at long wavelengths of all the peptides can be fitted to a one- or two-term Drude equation, similar to those characterising denatured proteins. All the peptides containing aromatic amino acids show a weak Cotton effect in the 270−250 mμ region; the possibility of detecting an effect in denatured proteins due to aromatic side-chains is discussed and shown to be very small. There is no enhancement of the Cotton effect by lengthening the peptide chain, or in non-polar solvents, and it is suggested that the detection of an effect in native proteins would indicate unusual intermolecular interactions involving the aromatic residue. The aliphatic and aromatic peptides all showed a Cotton effect at about 230 mμ, attributed to the n-π ∗ transition of the amide linkage.