The Optical Rotatory Dispersion of Aromatic Amino Acids and the Side Chain-dependent Cotton Effects in Proteins*

The optical rotatory dispersion of L-tryptophan, L-tyrosine, L-phenylalanine, and some of their derivatives was investigated in the ultraviolet region as a function of solvent, pH, and temperature. All aromatic transitions accessible to study were found to be optically active, but the rotatory strength was found to be low. The long wave length tyrosine bands at 280 rnp showed a loo-fold lower rotatory strength when compared to the n X* transition of the peptide bond. The greatly increased rotatory strength of aromatic transitions in some proteins must thus be due to vi&al perturbations having their source in the conformation of protein. Investigation of solvent and temperature dependence of amino acid rotation showed that only a change in the charge on the molecule produced measurable increase in the rotatory strength. It is proposed that the enhanced aromatic Cotton effects in some proteins are produced by interaction of the aromatic residue with a neighboring charge or other polarizable group. Since long wave length Cotton effects (250 to 350 rnp) are accompanied by a system of strong Cotton effects at the low wave length transitions of the aromatic residues, the usual estimates of or-helix may be difficult or impossible in protein showing enhanced aromatic rotation.