Effects of hydrogen bonding and hydrophobic interactions on the ultraviolet resonance Raman intensities of indole ring vibrations

UV resonance Raman intensities of indole ring vibrations were examined in solutions of different solvent properties to establish the correlation between the intensity and the environment of the indole ring, the main side-chain component of amino acid tryptophan. Each of the Raman bands that are enhanced through resonance with the B-b electronic transition gives the highest intensity when the indole NH site is hydrogen-bonded with a proton acceptor in hydrophobic environments. On the other hand, the Raman bands that gain intensity through resonance with the L-a transition are most enhanced when the indole ring is not hydrogen-bonded. Although these changes in Raman intensity result from small red and blue shifts of the B-b and L-a absorptions, respectively, the Raman intensity changes are much greater than the changes in UV absorption spectra. UV resonance Raman intensities are thus expected to be useful markers of hydrogen bonding and hydrophobic interactions of tryptophan residues in proteins.