Cyclodimers of p‐coumaric and ferulic acids in the cell walls of tropical grasses

Abstract

Cell walls from leaves and stems of three tropical grasses (Setaria anceps cv Nandi (Schumach) Moss), pangola grass Digitara decumbens Stent and spear grass Heteropogon contortus (L) Beauv ex Roemer & Schultes were extracted with alkali, and ether‐soluble fractions were prepared from the acidified solutions. Cyclobutane dimers derived from p‐coumaric acid (CA: 3‐(4‐hydroxypheny) propenoic acid) and ferulic acid (FA: 3‐(4‐hydroxy‐3‐methoxyphenyl) propenoic acid) were found in all plant residues. Structural information on the dimers was obtained by gas chromatography‐mass spectrometry. Stem cell walls differed from those of leaf in having major dimer components which contained residues tentatively identified as coniferyl alcohol (3‐(4‐hydroxy‐3‐methoxyphenyl)‐2‐propen‐l‐ol). Species differences in leaf cell wall cyclodimers were evident. Pangola and setaria grasses had dimers mainly derived from FA, whereas spear grass had over 70% of dimers derived from CA. Di‐esterification of some dimers in the cell walls was confirmed by the release of dimers containing the reduced forms of CA and FA (coumaryl and coniferyl alcohols respectively) when leaf cell walls were extracted with borohydride. Head to tail dimerisation giving derivatives of truxillic acid (t‐2, c‐4‐diphenyl‐r‐l, t‐3‐cyclobutanedicarboxylic acid) predominated, although evidence was obtained for the presence of head‐to‐head coumaric acid dimers (derivatives of truxinic acid, t‐3, t‐4‐diphenyl‐rl, c‐2‐cyclobutanedicarboxylic acid) in setaria stem and spear grass leaf. The results suggested that cyclodimers in grass cell walls occurred in cross‐linking structures of varying complexity between macromolecules, where they would possibly contribute to fibre strength and thus have an adverse efect on the nutritional value of forage.