Pulcherosine, an oxidatively coupled trimer of tyrosine in plant cell walls: its role in cross-link formation.

An oxidatively coupled trimer of tyrosine has been isolated from hydrolysates of primary cell walls of a tomato cell culture. UV-absorption, fluorescence and 1H NMR spectra showed that the trimer was pulcherosine, composed of isodityrosine and tyrosine oxidatively coupled via a biphenyl linkage such that the aromatic core is 2,2'-dihydroxy-3-phenoxybiphenyl. Pulcherosine could act as an intermediate in the conversion of isodityrosine to the tetramer, di-isodityrosine. Steric considerations show that the three tyrosine units of pulcherosine could not be near-neighbour residues within a single polypeptide chain. Pulcherosine therefore forms inter-polypeptide cross-links and/or wide intra-polypeptide loops.