Acid-base equilibriums of lumichrome and its 1-methyl, 3-methyl, and 1,3-dimethyl derivatives.

Authors:
Ewa Sikorska
Ewa Sikorska
Poznań University of Economics
Poland
Jacek Koput
Jacek Koput
Adam Mickiewicz University
Poznań | Poland
Dr. Igor Khmelinskii, PhD, Prof. Agregado
Dr. Igor Khmelinskii, PhD, Prof. Agregado
University of the Algarve
PhD, Prof. Agregado
physcal chemistry; spectroscopy; climate science
Faro, Algarve | Portugal
Jerzy Karolczak
Jerzy Karolczak
Adam Mickiewicz University
Poland
Mateusz Gierszewski
Mateusz Gierszewski
A. Mickiewicz University
Marek Sikorski
Marek Sikorski
A. Mickiewicz University
Poland

J Phys Chem A 2012 Jul 10;116(28):7474-90. Epub 2012 Jul 10.

Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland.

Lumichrome photophysical properties at different pH were characterized by UV-vis spectroscopy and steady-state and time-resolved fluorescence techniques, in four forms of protonation/deprotonation: neutral form, two monoanions, and dianion. The excited-state lifetimes of these forms of lumichrome were measured and discussed. The results were compared to those obtained for similar forms of alloxazine and/or isoalloxazine, and also to those of 1-methyl- and 3-methyllumichrome and 1,3-dimethyllumichrome. The absorption, emission, and synchronous spectra of lumichrome, 1-methyl- and 3-methyllumichrome, and 1,3-dimethyllumichrome at different pH were measured and used in discussion of fluorescence of neutral and deprotonated forms of lumichrome. The analysis of steady-state and time-resolved spectra and the DFT calculations both predict that the N(1) monoanion and the N(1,3) dianion of lumichrome have predominantly isoalloxazinic structures. Additionally, we confirmed that neutral lumichrome exists in its alloxazinic form only, in both the ground and the excited state. We also confirmed the existence and the alloxazinic structure of a second N(3) monoanion. The estimated values of pK(a) = 8.2 are for the equilibrium between neutral lumichrome and alloxazinic and isoalloxazinic monoanions, with proton dissociation from N(1)-H and N(3)-H groups proceeding at the almost the same pH, while the second value pK(a) = 11.4 refers to the formation of the isoalloxazinic dianion in the ground state.

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July 2012
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