Spectral properties of sinapine in water environment

In the search for new protein products for human and animal consumption, defatted rapeseed meal has been considered as a potential source of high nutritional value proteins because of its well-balanced amino acid composition. Its use as an animal feed or human food, however, is limited by antinutritive compounds such as sinapine (SNP). SNP is a major phenolic compound of extracted meal flours and creates complexes with albumins. These complexes will be investigated later using spectrophotometric methods. The first step was to establish the spectroscopic properties of SNP in water environments of different pH. The equilibrium of the ionic forms of SNP at pH 5.46-10.58 was analyzed using absorption and fluorescence molecular spectroscopy. Equilibrium constants and pKa of the OH group in SNP were calculated in electronic ground and excited states. The absorption spectrum of SNP with a dissociated hydroxyl group was difficult to obtain due to SNP molecule instability in alkaline solutions. Therefore, this spectrum and molar extinction coefficient were calculated using measured spectra in lower pH. Furthermore, the fluorescence spectra and quantum yield for two ionic forms of SNP as well as pKa⁎ in the excited state were determined.