Two-dimensional infrared, two-dimensional Raman, and two-dimensional infrared and Raman heterospectral correlation studies of secondary structure of β-lactoglobulin in buffer solutions

Attenuated total reflection (ATR)/infrared (IR) and Raman spectra were measured at room temperature for beta-lactoglobulin (BLG) in phosphate buffer (pH 6.6) solutions over a concentration range of 1-5 wt %. Two-dimensional (2D) IR and 2D Raman correlation spectra in the amide III region were generated from the concentration-dependent spectral variations of the BLG solutions to investigate band assignments in the region and to explore concentration-induced conformational changes in BLG. The great resolution enhancement yielded by the 2D IR and 2D Raman spectra enabled us to propose very detailed band assignments for the amide III region. Moreover, the basis of the sign of the asynchronous cross-peaks, we revealed the sequence order of the secondary structure changes induced by the protein association; the changes in the random coil structure exposed to water occur first, and then these in other secondary structure elements follow. 2D IR-Raman heterospectral analysis was also attempted for the same IR and Raman data. The heterospectral correlation maps elucidated the correlation between IR and Raman bands in the amide III region, confirming their band assignments.