Infrared Spectra of Globular Proteins in Aqueous Solution

Abstract

Globular proteins exist in their native state in aqueous environments. However, the most detailed structural models for protein conformation are obtained in the solid state by X-ray diffraction of single crystals [1], These solid state structures cannot be assumed to remain invariant upon dissolution in aqueous media [2]. Vibrational spectroscopy is sensitive to conformational changes in biopolymers, and it can be used to obtain structural information in both the solid and solution states. Vibrational spectra are most commonly obtained by infrared absorption and Raman scattering techniques. The information obtained from these methods are highly complementary [3]. Raman spectroscopy has been used to study globular proteins in both the solid and aqueous phases [4], while infrared spectroscopy has been fundamental in determining the secondary conformation of fibrous proteins in their native solid state. Infrared absorption studies of aqueous solutions of globular proteins have been severely limited due to the strong interferring H2O absorption [5], The majority of infrared solution studies have been concerned with the amide I and amide II bands in D2O solutions [5], However, the amide frequencies observed in D2O solution are affected by hydrogen-deuterium exchange in the peptide groups [5], Infrared studies of globular proteins in H2O has been limited to the amide I region of myoglobin, a-lactoglobulin, and a-casein, and the spectra were obtained by extremely difficult and tedious differential techniques [6].