Synthesis and Characterization of Model Compounds of the Lysine Tyrosyl Quinone Cofactor of Lysyl Oxidase

4-n-Butylamino-5-ethyl-1,2-benzoquinone (1_ox) has been synthesized as a model compound for the LTQ (lysine tyrosyl quinone) cofactor of lysyl oxidase (LOX). At pH 7, 1_ox has a λ_max at 504 nm and exists as a neutral o-quinone in contrast to a TPQ (2,4,5-trihydroxyphenylalanine quinone) model compound, 4, which is a resonance-stabilized monoanion. Despite these structural differences 1_ox and 4 have the same redox potential (ca. −180 mV vs SCE). The structure of the phenylhydrazine adduct of 1_ox (2) is reported, and 2D NMR spectroscopy has been used to show that the position of nucleophilic addition is at C₁. UV−vis spectroscopic pH titration of phenylhydrazine adducts of 1_ox and 4, 2, and 11, respectively, reveals a similar red shift in λ_max at alkaline pH with the same pK_a (∼11.8). In contrast, the red shift in λ_max at acidic pH conditions yields different pK_a values (2.12 for 2 vs −0.28 for 11), providing a means to distinguish LTQ from TPQ. Reactions between in situ generated 4-ethyl-1,2-benzoquinone and primary amines give a mixture of products, indicating that the protein environment must play an essential role in LTQ biogenesis by directing the nucleophilic addition of the ε-amino group of a lysine residue to the C₄ position of a putative dopaquinone intermediate. Characterization of a 1,6-adduct between an o-quinone and butylamine (3-n-butylamino-5-ethyl-1,2-benzoquinone, 13) confirms the assignment of LTQ as a 1,4-addition product.