Modification of proteins during the oxidation of leaf phenols: Reaction of potato virus X with chlorogenoquinone

Potato virus X (PVX) is modified by incubation with chlorogenic acid and polyphenoloxidase. The product made at pH 7 (PVX-Q₁) is grey in colour, retains about 2/3 of its initial infectivity, and contains, on average, 1 molecule of bound chlorogenic acid per protein subunit. The product made at pH 7.8 (PVX-Q₂) is blue, retains at least $\text{1}\text{3}$ of its infectivity, and contains approximately 2 molecules of chlorogenic acid per subunit. Both preparations contain a proportion (18–42%) of cross linked subunits. Brief exposure to trypsin converts subunits of both types of PVX-Q to a form with a slightly lower MW; the reaction goes more extensively with PVX-Q₁ (80% converted) than with PVX-Q₂ (45% converted). Prolonged exposure to trypsin degrades both forms of PVX-Q to free quinic acid and peptides, apparently only one of which contains chlorogenic acid. It is argued that, in PVX-Q₁, predominantly one specific lysine ε-NH₂ has been modified with chlorogenoquinone. The structure of PVX-Q₂ is less clear.