Naturally occurring quinols and quinones studied as semiquinones by electron spin resonance

A simple procedure using electron spin resonance techniques was applied to detect, identify and quantify quinones and quinols in crude plant extracts. Hydroquinone was determined in Pyrus, plumbagin in Drosera and Ceratostigma, and hydrojuglone in Juglandaceae. Hydrojuglone is found in markedly higher concentrations in Pterocarya and in Juglans than in Carya. Plastoquinol has been observed in 500 of 700 plant extracts studied. Esters of phenolic acids are easily detected and distinguished, e.g. chlorogenic and rosmarinic acids. Esters of homoprotocatechuic and of dihydrocaffeic acid occur widely in the Oleaceae. The limitations of the method are discussed.