Resonance Raman spectra of poly(L‐lysine), aromatic amino acids, L‐histidine and native and thermally unfolded ribonuclease A

Using excitation radiation of wavelength 248 nm, the peptide linkages of poly( L ‐lysine) give rise to important pre‐resonance enhancements in the Raman spectra of the amide bands, specially of the amide II band located in the 1550–1570 cm −1 shift region. Using the same excitation wavelength, aromatic amino acids and histidine exhibit resonance enhancement, and their resonance Raman spectra are easily obtainable without a fluorescence background contribution. This background is very large, particularly in the case of phenylalanine when longer wavelength excitation is used. The Raman spectra of native and thermally denatured ribonuclease A have been investigated using 248 nm excitation radiation. These spectra show the major contribution of the tyrosine residues of the protein. It seems that the behaviour of the tyrosine doublet is not a completely adequate probe for following the unfolding of the protein.