Hydroxycinnamic Acids in Walls of Wheat Aleurone Cells

Abstract

Walls were isolated from sheets of aleurone cells from wheat, freed from associated protein and starch by extraction with phenol-acetic acid-water followed by treatment with porcine pancreatic alpha -amylase, and finally extracted with sodium dodecyl sulphate. Hydroxycinnamic acids (HCA) released from the walls by saponification with 1 M NaOH amounted to 1·84% w/w of wall and comprised ferulic acid (90%), p coumaric acid (10%) and minor amounts (0·006%) of diferulic acids. The isolated walls exhibited an intense blue autofluorescence when irradiated with UV light that was reduced in intensity and became more diffuse after saponification. Acetyl residues (4·9% w/w) were present representing a degree of substitution of two in every nine pentose residues in the wall arabinoxylans. Digestion of the walls with a polysaccharide hydrolase mixture that lacked HCA esterases, released a series of oligomers that contained 50% of the total esterified HCA. These were separated by hydrophobic exclusion chromatography, purified by paper chromatography and two of these were identified by ES-MS and NMR as O -[5- O -feruloyl-α-L-arabinofuranosyl]-(1→3)- O -β-D-xylopyranosyl-(1→4)-β-D-xylopyranose (FAXX) andO -β-D-xylopyranosyl-(1→4)- O -[5- O -feruloyl-α-L-arabinofuranosyl]-(1→3)-O -β-D-xylopyranosyl-(1→4)-β-D-xylopyranose (XFAXX). The extremely low level of esterified diferulic acid indicates that cross-linking between polysaccharide chains by diester-FA is not important, however the esterified FA may play a role in cross-linking polysaccharides to wall proteins.