Abstract

The ultraviolet resonance Raman (UVRR) excitation profiles have been measured for the aromatic amino acids phenylalanine, tryptophan, tyrosine and tyrosinate. Resonance excitation enhances Raman scattering from vibrational modes that distort the ground state configuration towards the configuration of the excited state. The excitation profile maxima are red-shifted with respect to the absorption spectral maxima for each aromatic amino acid. These excitation profiles indicate the excitation required to maximally enhance a particular aromatic amino acid residue in a protein. Individual aromatic amino acids in environmentally distinguishable positions in a protein may have slightly different transition energies, and could therefore be identified and distinguished by proper tuning of the excitation frequency. For example, surface tyrosyl residues in hydrophilic environments will have lower lying excited states due to extensive hydrogen bonding. These residues will be maximally enhanced with longer wavelength excitation than residues buried in hydrophobic pockets within a protein. The frequency of certain vibrational bands, particularly those known to be influenced by substituents on the aromatic ring may also be indicative of the local environment. The relative intensities of other enhanced bands may also contain information concerning specific local environment. For example, the relative intensity of the peaks of the 830/850 cm^-1 Fermi resonance doublet of tyrosine are known from normal Raman studies to be sensitive to hydgrogen bonding. Vibrational substructure is not observed in absorption spectral measurements due to the breadth of the absorption spectral features. The vibrational substructure is amplified in the resonance Raman excitation profiles.

© 1987 Optical Society of America