Molecular pathways of degradation of wool-related peptides

Abstract

The thermal and alkali-promoted reactions of A-acetyl-A-methylamides of alanine, cystine, glycine, histidine, methionine, phenylalanine, proline, serine, threonine, tryptophan and tyrosine have been studied as models for the reaction of heat and alkali on wool protein. The cystine model, iV,iV-diacetyl-L-cystine- N,N-dimethylamide (DCDMA), has been the principal substrate. The amino acid derivatives were usually heated either as wet solids at 125 “C or in aqueous solution buffered at pH 10-11 at 55“ and 100 “C. A brief photochemical study was conducted with DCDMA. Reactions were monitored by uv spectroscopy and HPLC. Products were isolated by preparative HPLC, characterised by spectroscopy and quantified by analytical HPLC. All the reactions with DCDMA give bright yellow product mixtures. The major components, however, are colourless, and include A,A-diacetyl-thiocystine-A,A- dimethylamide (DTCDMA), diastereoisomers of N,N- diacetyl lanthionine-A,A- dimethylamide (DLDMA) and A-acetyl-A-methylamides of cysteine (NACMA) and dehydroalanine (NADMA). Their formation is best explained by a p- elimination mechanism involving thiocysteinyl and dehydroalanyl derivative intermediates. The product distribution and yields from wet solid samples, however, differ significantly from those obtained with aqueous alkali. DTCDMA is the major product (65%) in wet solid-state conditions, and is formed so readily that it is usually present as a contaminant of the substrate. A-Acetylthio- cysteine-iV-methylamide (NATCMA) is also present amongst thermolysis products, but we could not determine its yield due to intrinsic instability. NADMA is the principal product formed in aqueous alkali (-58%) followed by DLDMA, DTCDMA and NACMA. NADMA is shown to be unstable under acid conditions and decomposes to methylpyruvamide (MPA) and acetamide. Elemental sulphur was isolated from the decomposition of DCDMA under both conditions. Yellow reaction mixtures of DCDMA show three regions of increased absorptions at 300, 385 and 455 nm. Results implicate methylthiopyruvamide as one of the pigments contributing to the yellowing and that polysulphides may also be involved. Of the monopeptides examined, A-acetyl-A-methylamides of histidine, serine, threonine, tryptophan, tyrosine and DCDMA both degrade and yellow in wet solid-state conditions. The derivative of cystine is by far the most reactive monopeptide on treatment with alkali. A-Acetylserine-A-methylamide is the only amino acid derivative which both degrades and yellows in dry solid-state conditions. The protection of the hydroxyl group by benzyl reduces thermal or alkaline yellowing but it remains ineffective against decomposition. Irradiation of DCDMA leads to a drop in pH, decomposition, and the formation of bright yellow solution with A-acetylcysteine-A-methylamide as the principal product. The derivatives of glycine, alanine, serine, and DTCDMA are also observed but in small amounts. Although the occurrence of the minor products is invoked as evidence for the cleavage of C-S bond, the cleavage of S-S bond appears to be predominant. The implications of the results from these model compounds for wool degradation are briefly discussed. The principal conclusion is that the ready formation of the thiocystinyl derivative in the thermal degradation of DCDMA suggests analogous structures within wool proteins play a far more significant role in their chemistry than has been considered hitherto.