Comparison of the Amide I/II Intensity Ratio of Solution and Solid-State Proteins Sampled by Transmission, Attenuated Total Reflectance, and Diffuse Reflectance Spectrometry

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Abstract
The absolute and relative differences in amide I and amide II band intensities of albumin, β-lactoglobulin, and myoglobin as measured by attenuated total reflection infrared (ATR-IR) spectrometry, transmission of aqueous solutions, and KBr disks and diffuse reflectance (DR) spectrometry are compared. The amide I/II intensity ratios of the proteins sampled by ATR, DR, and transmission spectrometry of KBr disks were similar and were significantly different from the intensity ratios of the proteins in solution. The absolute amide II band intensity of dissolved proteins did not vary significantly with changes in pH. The difference in amide I/II intensity ratios between solution and adsorbed proteins was attributed to differences in secondary and possible tertiary structure. The thickness of each protein film was estimated with the use of the absorptivities calculated from the amide II band intensities of the KBr disk spectra. pH had a significant influence on the thickness of the adsorbed films. Differences in film thickness were attributed to a difference in the orientation of the protein molecules at the surface of the germanium internal reflection element.