Electrochemistry and spectroelectrochemistry of bioactive hydroxyquinolines: a mechanistic study.
The oxidation mechanism of selected hydroxyquinoline carboxylic acids such as 8-hydroxyquinoline-7-carboxylic acid (1), the two positional isomers 2-methyl-8-hydroxyquinoline-7-carboxylic acid (3) and 2-methyl-5-hydroxyquinoline-6-carboxylic acid (4), as well as other hydroxyquinolines were studied in aprotic environment using cyclic voltammetry, controlled potential electrolysis, in situ UV-vis and IR spectroelectrochemistry, and HPLC-MS/MS techniques. IR spectroelectrochemistry showed that oxidation unexpectedly proceeds together with protonation of the starting compound. We proved that the nitrogen atom in the heterocycle of hydroxyquinolines is protonated during the apparent 0.7 electron oxidation process. This was rationalized by the autodeprotonation reaction by another two starting molecules of hydroxyquinoline, so that the overall oxidation mechanism involves two electrons and three starting molecules. Both the electrochemical and spectroelectrochemical results showed that the oxidation mechanism is not influenced by the presence of the carboxylic group in the chemical structure of hydroxyquinolines, as results from oxidation of 2,7-dimethyl-5-hydroxyquinoline (6). In the presence of a strong proton acceptor such as pyridine, the oxidation ECEC process involves two electrons and two protons per one molecule of the hydroxyquinoline derivative. The electron transfer efficiency of hydroxyquinolines in biosystems may be related to protonation of biocompounds containing nitrogen bases. Molecular orbital calculations support the experimental findings.
†J. Heyrovský Institute of Physical Chemistry, v.v.i., The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic.
Romana
R
Sokolová
‡Institute of Chemistry, University of Silesia, Szkolna 9, PL-40006 Katowice, Poland.
Jacek E
JE
Nycz
†J. Heyrovský Institute of Physical Chemistry, v.v.i., The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic.
Šárka
Š
Ramešová
†J. Heyrovský Institute of Physical Chemistry, v.v.i., The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic.
Jan
J
Fiedler
§Department of Chemistry and Industrial Chemistry, University of Pisa, Via Moruzzi 3, 56124 Pisa, Italy.
Ilaria
I
Degano
‡Institute of Chemistry, University of Silesia, Szkolna 9, PL-40006 Katowice, Poland.
Marcin
M
Szala
†J. Heyrovský Institute of Physical Chemistry, v.v.i., The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic.
Viliam
V
Kolivoška
†J. Heyrovský Institute of Physical Chemistry, v.v.i., The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic.|∥Department of Inorganic Technology, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 81237 Bratislava, Slovakia.
Miroslav
M
Gál
D002264:Carboxylic Acids / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D055664:Electrochemical Techniques; D004563:Electrochemistry; D004583:Electrons; D006912:Hydroxyquinolines / Q000737:chemistry*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011522:Protons; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D053719:Tandem Mass Spectrometry
D002264:Carboxylic Acids; D006912:Hydroxyquinolines; D011522:Protons