The formation of a 2-keto-5-iminopiperazine protein chromophore cross-link from the alpha-amino nitrogen of residue n, a methionine, to the alpha-carboxyl carbon of residue n+1, a tyrosine, and a dehydration to form a double bond to the alpha-amino nitrogen of residue n+2. This cross-linking is coupled with a dehydrogenation of residue n+1 to form a double bond between the alpha and beta carbons.