Comparison of Laurentian Fulvic Acid luminescence with that of the hydroquinone/quinone model system: Evidence from low temperature fluorescence studies and EPR spectroscopy

Abstract

Fluorescence techniques have been used to identify humic substances, (e.g. aquatic fulvic acids from different origins). Synchronous scans have proven adequate for distinguishing detail and differentiating samples. As well, fluorescence has often been used to probe humic interactions with metals and xenobiotic organics. In this study, the fluorescence of a well-characterized material, Laurentian fulvic acid (LFA) was compared with that of a simple hydroquinone/quinone (H 2 Q/Q) model system. Synchronous fluorescence (room T) and laser-excited fluorescence experiments at 10°K were carried out to characterize the fluorophore. The synchronous fluorescence behavior of LFA displayed similarities to that of an equilibrated, aerated, H 2 Q/Q system, but only at higher pH. (Higher pH favours a shift of the redox equilibrium towards quinone.) In contrast, the spectra of LFA suggest the role of "quinone" groups, even at lower pH. A signif- Aquatic Sciencesicant feature of these spectra is a lowest energy band. At low temperature this band was more selectively excited at 470 nm, but vibrationally resolved line narrowed spectra were not observed. Fluorescence lifetimes were very short compared to the laser pulse width of ca. 10 ns. Under the low T condition the spectra of LFA and the model are essentially identical. We suggest that the principal luminophore in LFA is a quinone-hydroquinone type, perhaps a charge transfer complex consistent with the absence of the fluorescence line-narrowing phenomenon. The spectra imply a lowest energy component of LFA emission from a fluorophore very similar to that of the simplest H 2 Q/Q. The significant similarities of the model system to LFA are underscored by striking parallels in the radicals in the two systems seen in well-resolved electron paramagnetic resonance (EPR) spectra that could be obtained at pH = 6.

Microbially-Mediated Fluorescent Organic Matter Transformations in the Deep Ocean. Do the Chemical Precursors Matter?

“…The FIC3 component showed characteristics of tryptophan-like fluorescence and was similar to other previously reported PARAFAC components, which have been associated to tryptophan-like, protein-bound and free amino acids (Yamashita and Tanoue, 2003;Stedmon and Markager, 2005b;Murphy et al, 2008;Kothawala et al, 2012;Kowalczuk et al, 2013).The spectral characteristics of FIC4 have not been traditionally defined (Coble, 1996) but it occupies a region very close to those of the humiclike substances. Other studies that have found this component, mainly in the watershed of tropical rivers (Yamashita et al, 2010a) and its estuaries (Brym et al, 2014), have assigned it to a microbial reduced quinone-like substance (Ariese et al, 2004;Cory and McKnight, 2005).…”

Positional Adaptability in the Design of Mutation-Resistant Nonnucleoside HIV-1 Reverse Transcriptase Inhibitors: A Supramolecular Perspective

“…[24][25][26] The basic feature of any quinhydrone complex is the stable face-to-face, donor-acceptor, and hydrogen-bonding interactions between a hydroquinone and quinone moiety. The most simple, classic example of a quinhydrone is the stable noncovalently interacting dimer complex formed by p-benzoquinone (cyclohexa-2,5-diene-1,4-dione) with 1,4-hydroquinone (benzene-1,4-diol) (Fig.…”

“…This compact and stable complex can be synthesized as dark shining needles, with moderate water solubility, from a 1:1 mixing of the reactants. 26 Quinhydrone complexes will form spontaneously through self-assembly interactions on mixing (solution). A myriad of quinhydrones may synthesized by varying functionalities at the carbon centers (Fig.…”

“…[24][25][26][27][28][29] Notably, however, the main characteristic that we exploit here is that the quinhydrone ''basic structural unit'' or ''complex'' ( Fig. 1) will be the main source of increased torsional, rotational, and translational motion, leading to an NNRTI with increased positional adaptability at an active site.…”

“…22,23 Quinonoid monomers are well known to be able to form unique stable supramolecular dimer assemblies called quinhydrones. [24][25][26] Quinhydrone complexes have similar binding affinities as the monomers that form them, but are relatively more stable and can deliver more flexibility and positional adaptability than the constituent monomers at a particular active site. [24][25][26] They offer an interesting and important new perspective in NNRTI drug design research with implications for other areas of antimicrobial drug design.…”