Ultraviolet resonance Raman spectra of insulin and alpha-lactalbumin with 218- and 200-nm laser excitation.

Ultraviolet resonance Raman (RR) spectra, with 200- and 218-nm excitation from a H2-shifted quadrupled Nd:YAG laser, are reported for insulin and alpha-lactalbumin in dilute aqueous solution, at pH values known to produce differences in the exposure of the aromatic residues to solvent. At 200 nm, the spectra are dominated by tyrosine bands, whose intensity is lowered somewhat in protein conformations in which tyrosine is exposed to solvent. The expected shift in the relative intensities of the components of the approximately 850-cm-1 tyrosine doublet is difficult to discern because the higher energy component shows much greater resonance enhancement and the lower energy component appears as a weak shoulder. The peptide vibrations, amides I, II, and III, are also enhanced at 200 nm. The infrared active amide II mode is particularly prominent, although it is not observed in Raman spectra with visible excitation. In addition, the amide I band is quite broad in the 200-nm RR spectra, and the peak frequency is lower than that seen in visible excitation Raman spectra and is close to the infrared frequency. It appears that 200-nm excitation produces resonance enhancement of the infrared-active components of both amide I and amide II. Excitation at 218 nm enhances tryptophan modes strongly. The 876-cm-1 band, assigned to a deformation mode of the five-membered ring, shows a measurable upshift upon exposure of tryptophan to solvent, attributable to N-H hydrogen bonding.(ABSTRACT TRUNCATED AT 250 WORDS)