Secondary structure determination in proteins from deep (192-223-nm) ultraviolet Raman spectroscopy

Raman intensities obtained with UV laser excitation at 223, 218, 204, 200, and 192 nm are reported for the amide I, II, III, and II' bands of random-coil polylysine. The excitation profiles show enhancement via the pi-pi electronic transition, at approximately 190 nm. Enhancement for amide I is weak, however, and most of the intensity can be accounted for by preresonance with a deeper UV transition at approximately 165 nm. The amide II' band dominates the spectrum in D2O, consistent with the suggestion that the main distortion coordinate in the pi-pi excited state is the stretching of the C-N peptide bond. Amide II intensities with 200- and 192-nm excitation are reported for several proteins. The previously reported negative linear correlation with alpha-helix content (due to Raman hypochromism in the alpha-helices) is found not to apply to proteins with high beta-sheet content when the excitation wavelength is 200 nm. Much higher intensities are seen for these proteins and are attributed to a red shift of the pi-pi absorption for the beta-structure. A linear correlation with alpha-helix content is found for excitation of 192 nm, which corresponds to an isosbestic point of the beta-sheet and random-coil absorption bands. Characteristic amide II Raman cross sections are derived for alpha-helical, beta-sheet, and random-coil elements and are used to determine secondary structure for alpha 1- and beta-purothionin, by use of amide II intensities with 200- and 192-nm excitation. The results are in good agreement with a previous determination based on amide I band deconvolution in off-resonance Raman spectra.