The Photolysis of Tyrosine and its Possible Relationship to the Yellowing of Wool

Abstract

Tyrosine, in the presence of air is degraded by uv light to 3,4-dihydroxyphenylalanine (DOPA), ammonia and a yellow-brown pigment. The rate of production of the pigment, as measured by the Yellowness Index (YI), is very pH-dependent, production being most rapid in alkaline solution. The degradation of tyrosine is less pH-dependent. When the tyrosine was totally degraded, the pigment could be bleached by further exposure to the irradiation. In an atmosphere of nitrogen, degradation of tyrosine is slower and no pigment is formed. Thus, decomposition may proceed through either of two alternative mechanisms. In that mechanism in which pigment is produced, bleaching simultaneously occurs, but at a slower rate than color production. It is concluded that there is not necessarily a quantitative relationship between amino acid destruction and pigment producion.