An experimental and theoretical study of the amino acid side chain Raman bands in proteins

•The Raman spectra of a series of tripeptides have been measured and calculated.•A detailed band assignment has been made for all tripeptides.•Raman frequencies of the tripeptides are blue shifted compared to free amino acids.•Protein Raman spectra are interpreted using the position and band intensities of the tripeptides.•The Raman bands of the tripeptides and those in the proteins show good agreement.

The Raman spectra of a series of tripeptides with the basic formula GlyAAGly where the central amino acid (AA) was tryptophan, tyrosine, phenylalanine, glycine, methionine, histidine, lysine and leucine were measured in H2O. The theoretical Raman spectra obtained using density functional theory (DFT) calculations at the B3LYP/6-311+G(2df,2pd) level of theory allows a precise attribution of the vibrational bands. The experimental results show that there is a blue shift in the frequencies of several bands of the amino acid side chains in tripeptides compared to free amino acids, especially in the case of AAs containing aromatic rings. On the other hand, a very good agreement was found between the Raman bands of AA residues in tripeptides and those measured on three model proteins: bovine serum albumin, β-lactoglobulin and lysozyme. The present analysis contributes to an unambiguous interpretation of the protein Raman spectra that is useful in monitoring the biological reactions involving AA side chains alteration.

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