The isolation and identification of compounds causing yellow discolouration of wool

Abstract

Two types of yellow discolouration affect New Zealand wool. Greasy wool has a variable incidence of yellow discolouration within the extraneous grease and suint covering on the surface of the fibres, which can be removed by scouring (scourable diffuse yellow). Bound (or canary) yellow discolouration, in contrast, is bound firmly to the fibre and cannot be removed by scouring. This type of yellowing occurs particularly in cotted fleeces in damp conditions, which favour microbiological activity. It is an important problem because of the reduced price obtained for discoloured wool.

Both forms of yellow discolouration have been extracted by a variety of methods, and then purified using a range of chromatographic techniques. Identification of the purified fractions has required amino acid analysis, ¹³C NMR, ¹H, NMR, FTIR and several variants of mass spectroscopy. Additional information about their properties was derived from functional group staining of TLC plates, UV absorption and fluorescence spectrophotometry.

A major component from the scourable diffuse yellow fraction first isolated was a colourless dicarboxylic compound, 3-amino-3-carboxy-6-hydroxyhexanoic acid, not previously reported. However, two minor components of this fraction were yellow. One is a serine -terpene compound, tentatively identified as 5-(3’, 6’-dihydroxy-3', 4’ –dimethyl-1', 4’ -cyclohexadienyl)-2-amino-(Z)-4-pentenoic acid, and the other, less well defined, is probably derived from fragments of High Glycine-Tyrosine proteins.

Bound yellow discolouration is essentially a mixture of many coloured and fluorescent compounds principally derived from tryptophan, tyrosine, and probably cysteine. The mixtures isolated are unstable and often include fluorescent compounds such as kynurenine, dityrosine and N-formylkynurenine.

A farm survey was carried out to find the incidence of the removable and bound yellow compounds in a wide range of sheep breeds and degrees of wool yellowness. Although the compounds of interest were detectable in the majority of samples, no consistent pattern has been found which would correlate the removable yellow compounds with the yellowness i.e. bound yellow, of the clean wool samples.