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PROTONATION AND DEPROTONATION EQUILIBRIA OF LUMICHROME
Author(s) -
Laser N.,
Feitelson J.
Publication year - 1977
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1977.tb09170.x
Subject(s) - chemistry , deprotonation , protonation , photochemistry , absorption spectroscopy , ion , tautomer , absorption (acoustics) , analytical chemistry (journal) , stereochemistry , organic chemistry , physics , quantum mechanics , acoustics
— In order to investigate the possibility of the tautomerization of alloxazine to isolloxazine in its ground state, the parameters affecting the redistribution of charges in the lumichrome molecule were studied. The absorption and emission spectra of lumichrome as a function of pH in the range H 0 = ‐ 6 to pH = 12 were recorded. At extreme pH conditions the spectra of lumichrome are similar to those of the isoalloxazine system. At high acid concentration ( H 0 < ‐ 3.0) the absorption spectrum of lumichrome protonated at N 10 , is practically identical to that of lumiflavin. The fluorescence quantum yield of the two cations is negligible at room temperature. At pH = 10.5 lumichrome is deprotonated at positions N 1 or N 3 , The two monoanions have different excitation spectra. Except for slight differences in the extinction coefficients, the absorption of the anion deprotonated at N 3 is very similar to the lumichrome spectrum. The absorption spectra of the N 1 monoanion and of the di‐anion are similar to the spectrum of lumiflavin except for a blue shift of about 20 nm. Furthermore, the emission spectrum of the N 1 ‐monoanion is identical to that of the isoalloxazine system. These results indicate that the charge distribution in the lumichrome molecule depends on the protonation and deprotonation of the nitrogen atoms at positions 10 and l. Both processes cause a redistribution of charges so that an isoalloxazine ring system is formed.