title: Acid-promoted tautomeric lactonization and oxidation-reduction of pyrroloquinoline quinone (PQQ).
abstract: Acid-treatment facilitates PQQ detection by electron ionization mass spectroscopy with a molecular ion at M/e 330 and a base ion formed by triple decarboxylation at M/e 198. Other ions found probably arise through acid-catalyzed tautomeric lactonization of PQQ to PQQ-lactone (PQQL) with subsequent oxidation of PQQL and reduction of PQQ. We propose that a carboxyl group, presumably the 9-carboxyl, attacks a double bond in PQQ, reversibly converting the 4,5-orthoquinone into an 4,5-enediol and forming an isomeric lactone, PQQL, of 330 daltons. The masking of carbonyls may explain the low reactivity of PQQ with carbonyl reagents in acid. Acid-promoted tautomeric lactonization with carbonyl-masking is known to occur with fluoresceins, phenolphthalein and other compounds, but has not been recognized before with PQQ. Acid-treated PQQ demonstrates molecular and other ions derived from reduced PQQ (PQQ(2H] or its lactone at M/e 332 with a base ion at M/e 200. There is compelling evidence for a dehydrogenated lactone, PQQ(-2H)L), at M/e 328 with a base ion at M/e 196. We suggest that PQQ, in tautomeric equilibrium with PQQL, oxidizes PQQL to PQQ(-2H)L (328 daltons), with its concurrent reduction to PQQ(2H) (332 daltons). With acidified D2O, PQQ shows deuterated products with ions at M/e values consistent with lactonization and oxidation-reduction. An analytically useful quinoxaline adduct, formed from PQQ and 2,3-diaminonaphthalene (PQQ-DAN) of 452 daltons, also undergoes acid-tautomerization-lactonization and oxidation-reduction similar to PQQ showing molecular ions at M/e 450, 452 and 454 and decarboxylation-derived strong (base) ions at M/e 318, 320 and 322.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Laboratory of Human Biochemistry, Children's Hospital Corp. Boston, MA 02115.
authors.forename: P M
authors.initials: PM
authors.lastname: Gallop
authors.forename: E
authors.initials: E
authors.lastname: Henson
authors.forename: M A
authors.initials: MA
authors.lastname: Paz
authors.forename: S L
authors.initials: SL
authors.lastname: Greenspan
authors.forename: R
authors.initials: R
authors.lastname: Flückiger
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D003067:Coenzymes / Q000378:metabolism*; D006868:Hydrolysis; D007202:Indicators and Reagents; D007536:Isomerism; D007783:Lactones / Q000378:metabolism; D013058:Mass Spectrometry; D010084:Oxidation-Reduction; D045542:PQQ Cofactor; D015363:Quinolones / Q000378:metabolism*; D011810:Quinoxalines / Q000378:metabolism; D014269:Trifluoroacetic Acid
chemical_list: D003067:Coenzymes; D007202:Indicators and Reagents; D007783:Lactones; D015363:Quinolones; D011810:Quinoxalines; D045542:PQQ Cofactor; D014269:Trifluoroacetic Acid