Acid–Base Equilibriums of Lumichrome and its 1-Methyl, 3-Methyl, and 1,3-Dimethyl Derivatives

Abstract

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.

Study of photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine in dependence of pH using different spectral techniques

“…Based on these spectra and our earlier works, 17,18 we conclude that the two different 5-DAll monoanions are formed almost simultaneously in this pH range (see Fig. 5).…”

“…On the basis of our recent work, we conclude that the behaviour of an alloxazine derivative (lumichrome) 17 is similar to that of 5-deazaalloxazine (9-methyl-5-deazaalloxazine) 18 in aqueous solutions at different pH values; however, the values of the absorption and emission maxima of the corresponding species (cationic, neutral and anionic forms) of these compounds a Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, are shifted relative to each other. The acid-base properties of alloxazines (mainly lumichromes) have been studied quite intensively, 17,[19][20][21] while less information is available on the acid-base equilibria of the derivatives of 5-deazalloxazine.…”

Photophysical properties and photochemistry of a sulfanyl porphyrazine bearing isophthaloxybutyl substituents

“…The SFS study was found to be particularly useful for acid-base equilibria in the ground and excited singlet states of various organic molecules, including lumichrome and quaternary stilbazolium salts [26,27]. Fig.…”

Genome Sequence of Microbacterium sp. Strain TPU 3598, a Lumichrome Producer

“…strain TPU 3598 was isolated from soil as an efficient producer of lumichrome from riboflavin in our previous study (1). Lumichrome is available as a photosensitizer (2, 3) and fluorescent dye (4, 5). The strain exhibited high production of lumichrome by incubation with a medium containing riboflavin, suggesting that the enzyme catalyzing the conversion of riboflavin to lumichrome was inducibly produced.…”