Interpretation of IR and Raman spectra of albumin

Object and purpose of work: The subject of the study is bovine serum albumin (BSA). The aim of the work is to give an interpretation of the vibrational spectra of BSA aqueous solution in the region of $\sim$ 1700-600 cm$^{-1}$. Methods: In this regard the experimental measurement of the IR and Raman spectra of BSA and the calculation of vibrational spectra of zwitterionic ion forms 20 amino acids and their dipeptides were carried out. The effect of anharmonicity and intermolecular interaction (IMI) on the vibrational spectra of amino acids was considered. Results: It has been shown that the forms of vibrations of the amino acid side residues forming a polypeptide do not mix with the forms of vibrations of the amide fragment (Amide I, Amide II and Amide III), which allows them to be used for the interpretation of the vibrational IR and Raman spectra of BSA. The comparison of the experimental and calculated spectra of BSA has shown that each of the experimental absorption band of albumin is a superposition of several absorption bands of amino acids side residues, and the influence of IMI between amino acid residues and water molecules leads to a shift of the maximum and the change in the intensity of absorption bands, corresponding to the vibrations of the Amide I, Amide II and Amide III. The calculated energies and vibration frequencies of the bonds involved in the formation of different types of IMI vary within a fairly wide range. If during the formation of a hydrogen bond between the two di-peptides of glycyl-glycine decrease in the frequency of the valence bond vibrational C=O and increase in the Оптика и спектроскопия. Лазерная физика intensity of both the absorption band and the Raman line is observed, then for the valence and deformation vibrational of the polar groups of COO - and N+H3 in case of ion-ion and ion-dipole IMI frequency shift is registered, which is 5-80 cm$^{-1}$, and the intensity varies - by $\sim$3-10 times. It has been shown that the overlap of the absorption bands of amino acid residues with the absorption band of Amide I makes it very sensitive to structural changes, including the manifestation of IMI, whereby the shift in the frequency and intensity of the absorption band of Amide I allows to determine the conformational changes of the protein. Analysis of the intensities of IR and Raman amino acid residues in the region of $\sim$1540 cm$^{-1}$ has shown that IMI leads to a more significant change in the intensity of the absorption band of Amide II in the IR spectrum as compared to the Raman spectrum. In the area of the Amide III vibrations deformation $\delta$(HE) and $\delta$(NH) vibrations of the side chains of several amino acids involved in the formation of IMI are manifested, resulting in the shift of the values of corresponding frequency of deformation vibrations in the Amide III. In the experimental IR spectrum of BSA in the region of $\sim$660 cm$^{-1}$, a wide absorption band of medium intensity is manifested. According to the performed calculation, deformation vibrations of the $\gamma$(OCO$_{-}$) angle of amino acid residues Glu and Asp are manifested in this spectral range, whose participation in the IMI with other amino acid residues and water molecules leads to the shift in the vibration frequency of this deformation vibration and broadening of the corresponding absorption band. Conclusion: Thus, a detailed analysis and interpretation of the vibrational IR and Raman spectra of BSA have enabled one to identify and consider in detail one of the main reasons leading to the frequency shift and change in the intensity of Amide I, Amide II and Amide III, which is the formation of various IMI between amino acids and amino acids and solvent molecules. The interpretation of the vibrational spectra of the zwitterionic forms of 20 standard amino acids in different spectral ranges allows to use it not only to determine the conformational changes of proteins, but also to diagnose the interaction with other molecular compounds, leading, for example, to the formation of complexes.