Negative ion electrospray mass spectrometry of polyphenols, catecholamines and their oxidation products

Polyphenols, catecholamines and their oxidation products have a variety of physiological effects and are key components of insect cuticle and other biological structures. Three dihydroxybenzene isomers were easily differentiated using negative ion electrospray mass spectrometry (ESMS) or tandem mass spectrometry (MS/MS). Ions at m/z 107-109 were diagnostic for more complex polyphenols and catecholamines with a dihydroxybenzene moiety. The product ion spectra of other compounds including 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenylalanine (DOPA) and N-acetyldopamine (NADA) had product ions at m/z 121-123 corresponding to methylene homologues of dioxygenated benzene. Oxidation of DOPA using tyrosinase, Ag2O or NaIO4 could be followed using ESMS and several proposed intermediates proceeding from DOPA through a quinone methide intermediate, 5,6-dihydroxyindole and dihydroxyindole quinone were confirmed using negative ion ESMS/MS. Spectra of the NaIO4 oxidation products of 1,2,3-trihydroxybenzene (pyrogallol) were consistent with tetrahydroxybenzene, two non-cyclic tetraoxygenated derivatives and a bicyclic polymerization product, purpurogallin. ESMS can be used to analyse complex polyphenolic conjugates commonly encountered in biological specimens.