Fractionation of tryptic gliadin hydrolysates based on proline levels

The central domain (CD) and terminal domains (TDs) of wheat gliadins contain high and low levels of the amino acid proline (Pro), respectively. The CD is rather hydrophilic while the TDs are rather hydrophobic and contain most of the ionisable amino acids, although present only in low levels. Therefore, peptides derived from the CD or TDs may strongly differ in their physico–chemical properties. Trypsin cleaves peptide bonds at lysine and arginine residues which are mainly present in the TDs and was used in the present study. Several fractionation methods were examined to isolate CD and TD related peptides from tryptic hydrolysates. Pro was used as a marker for CD and TD related peptides. Both semi-preparative reversed-phase HPLC separation and fractionation of the water-soluble peptides of the tryptic gliadin hydrolysates by graded ethanol precipitation resulted in peptide fractions with different Pro levels. Whereas the fractions precipitating up to 90% ethanol were Pro rich, a Pro poor fraction consisting of small peptides was soluble in 90% ethanol solution. The water insoluble peptides of the tryptic hydrolysates also showed a low Pro level. Ultrafiltration of the water-soluble peptides using a 5-k membrane resulted in small Pro poor peptides and larger Pro rich peptides.