Fourier-transform infrared spectroscopy for monitoring proteolytic reactions using dry-films treated with trifluoroacetic acid

Abstract

In this study we explore the potential of using Fourier-transform infrared (FTIR) spectra of trifluoroacetate-protein and peptide complexes for monitoring proteolytic reactions. The idea of treating dry-films of protein hydrolysates with trifluoroacetic acid (TFA) prior to FTIR analysis is based on the unique properties of TFA. By adding a large excess of TFA to protein hydrolysate samples, the possible protonation sites of the proteins and peptides will be saturated. In addition, TFA has a low boiling point when protonated as well as complex-forming abilities. When forming TFA-treated dryfilms of protein hydrolysates, the excess TFA will evaporate and the deprotonated acid (CF 3 coo −) will interact as a counter ion with the positive charges on the sample materials. In the study, spectral changes in TFA-treated dry-films of protein hydrolysates from a pure protein and poultry by-products, were compared to the FTIR fingerprints of untreated dry-films. The results show that time-dependent information related to proteolytic reactions and, consequently, on the characteristics of the protein hydrolysates can be obtained. With additional developments, FTIR on dry-films treated with TFA may be regarded as a potential future tool for the analysis of all types of proteolytic reactions in the laboratory as well as in industry. Fourier-transform infrared (FTIR) spectroscopy is among the well-established methods for the characterisation of proteins and peptides. The repeated amino acid building blocks constructing the backbone of proteins and peptides give rise to multiple distinctive infrared absorption bands (i.e. the amide bands), containing both chemical and structural information. FTIR is therefore frequently used to study and quantify secondary structures of proteins 1-3. At the same time, the protein information in the FTIR spectra has opened the possibility to study a wide range of protein-derived quality features. This includes parameters such as hydration and solvent effects, pH, peptide size distribution and degree of hydrolysis (DH%) 4-9. Currently, peptide size distribution and DH% are measured using laborious and time-consuming techniques. FTIR, on the other hand, represents a rapid alternative which is applicable to an industrial setting. Recent studies have shown that FTIR spectroscopy can be used to predict parameters such as weight-average molecular weight (M w) derived from the peptide size distribution and DH% with high accuracy 10-12. In recent years, enzymatic protein hydrolysis (EPH) has gained significant attention as a versatile processing technology for protein-rich raw materials. For instance, several value-added peptide products have been developed employing EPH on food-processing by-products. In EPH, proteolytic enzymes are used to break down the proteins into smaller fragments, and the resulting protein hydrolysates are very complex, consisting of thousands

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