title: Ionization and tautomerism of methyl fluorescein and related dyes.
abstract: The protolytic equilibrium of methyl ether of fluorescein is studied in water, aqueous ethanol, and in other solvents. The constants of the two-step dissociation are determined by spectrophotometry. In water, the fractions of the zwitterionic, quinonoid, and lactonic tautomes are correspondingly 11%, 6%, and 83%, as deduced from the UV-visible spectra. Corresponding study of the ionization of the methyl ether ester of fluorescein, fluorescein ethyl ester, and sulfonefluorescein allows testing the correction of the attribution of the microscopic dissociation constants of methoxy fluorescein. The results of nuclear magnetic resonance and infrared spectroscopy, as well as the X-ray analysis confirm the predomination of the lactonic structure of the molecular species in solid state and in DMSO. Contrary to it, the spectroscopic studies in both hydrogen-donor bond (HDB) and non-HBD solvents confirm that the presence of lactonic monoanion is atypical for the dye under study and, with high probability, also for the mother compound fluorescein.
authors.affiliation: Department of Physical Chemistry, Kharkov V. Karazin National University, Kharkov 61022, Ukraine. Electronic address: mchedlov@univer.kharkov.ua.
authors.forename: Nikolay O
authors.initials: NO
authors.lastname: Mchedlov-Petrossyan
authors.affiliation: Department of Physical Chemistry, Kharkov V. Karazin National University, Kharkov 61022, Ukraine.
authors.forename: Tatyana A
authors.initials: TA
authors.lastname: Cheipesh
authors.affiliation: Department of Physical Chemistry, Kharkov V. Karazin National University, Kharkov 61022, Ukraine.
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Shekhovtsov
authors.affiliation: Institute of Physico-Organic Chemistry and Coal Chemistry, National Academy of Science of Ukraine, Donetsk 83114, Ukraine.
authors.forename: Andrey N
authors.initials: AN
authors.lastname: Redko
authors.affiliation: Institute of Physico-Organic Chemistry and Coal Chemistry, National Academy of Science of Ukraine, Donetsk 83114, Ukraine.
authors.forename: Vladimir I
authors.initials: VI
authors.lastname: Rybachenko
authors.affiliation: Institute for Single Crystals, National Academy of Science of Ukraine, Kharkov 61072, Ukraine.
authors.forename: Irina V
authors.initials: IV
authors.lastname: Omelchenko
authors.affiliation: Institute for Single Crystals, National Academy of Science of Ukraine, Kharkov 61072, Ukraine.
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Shishkin
keywords: Dissociation constants; Fluorescein methyl ether; Molecular and nuclear magnetic resonance spectroscopy; Tautomerism; X-ray structure analysis