A study on the interaction of nile blue with Uracils: A spectroscopic and computational approach

Highlights

• Photoinduced interaction was probed by steady state and time resolved measurements.

• The fluorescence quenching depend on position and substituent of uracil molecules.

• Lifetime measurements reveal static quenching mechanism.

• BDE, calculated by B3LYP program impacts the antioxidant activity of uracils.

• The antioxidant activity was studied in-vitro by deoxy ribose degradation assay.

Abstract

The present work focuses the investigation on fluorescence quenching of nile blue (NB) in presence of various substituted uracil molecules. UV–Visible absorption studies signify the possibility of ground state complex formation between NB and uracil molecules. The increase in concentration of quencher molecules greatly influences the emission spectra of NB. The bimolecular quenching rate constant (k_q) were calculated and found to depend on the position and electronic properties of substituent in quencher molecules. Fluorescence quenching experiments were performed at different temperature to calculate the thermodynamic parameters. The fluorescence lifetime measurements show that the quenching process proceeds through static quenching. The mechanism of fluorescence quenching includes the possibility of proton transfer. The bond dissociation enthalpy (BDE) reveals the release of H from the quencher molecules. The quencher molecules possess antioxidant activity and identified using deoxyribose degradation assay. The position of substituent and its electronic property are key features to address the antioxidant activity of uracil molecules.

Introduction

Nile blue (NB) has received considerable attention towards biological interaction over other fluorescent based molecules [[1], [2], [3], [4], [5], [6]], due its superior advantages [7]. Nile blue is an interesting molecule (shown in Scheme 1) and belongs to the class of cationic oxazine dyes. The fascinating molecule employed as promising materials for various optical and photonic devices [[8], [9], [10]]. NB is a potent photodynamic therapy agent owing to its absorption of light in the 600–650 nm range. The cationic dye (NB) show high binding affinity towards DNAs and react efficiently with tumor cells than normal tissues to retard the tumor growth [11,12].

NB emits light in the red region of spectrum and employed as an analytical reagent to determine the antioxidant capacity of commercial wine and fruit juice samples [13]. Q.Y. Chen et al. investigated on the determination of nucleic acid by using NB, as a fluorescence probe [14]. NB possesses extensive biological applications. The interaction between NB and DNA was widely reported [15].

Uracil is an essential integral part of RNA and ensues naturally in pyrimidine base pairs. Uracil derivatives find immense applications in biological and pharmacological activities viz., drug delivery, enzyme synthesis [[16], [17], [18]]. For instance, 6-methyl uracil is an antioxidant and also used in the treatment of hepatitis and pancreatitis [19,20]. The pharmacological activity of many drugs correlates with their antioxidant activity [21,22]. Particularly 5-substituted uracil derivatives are important class of non-steriodial anti-inflammatory agents with antioxidant activity. S.A. Grabovskiy et al. reported the structure activity relationship of uracil derivatives against free radicals [23].

Previously we have reported the investigation on 9-aminoacridine, acriflavine and triphenylamine with various antioxidants and TiO₂ nanoparticles [[24], [25], [26], [27], [28], [29], [30]]. The ability of sensitizers to undergo excited state electron transfer by various pyrimidines was extensively studied [31]. The foremost objective of us to study the interaction of NB with appropriate biological target and in this perspective, probing the interaction of biologically potent NB with uracil derivatives is of outmost important and chemically relevant to pharmacological research. Fluorescence as a detection method provides high sensitivity down to the single molecule, ease of application, and a subnano-second temporal and sub-micrometer spatial resolution. Fluorescence quenching method is very sensitive and provides direct information on the antioxidant activity of the molecules.

We are interested to understand the fluorescence quenching of NB with various uracils to explore the role of H in quencher molecules and their contribution in antioxidant activity. The quencher molecules chosen in the present study was shown in the Scheme 2. Moreover, it is important to understand the transfer of H from aromatic molecules as it could solve many biological problems [32]. To comprehend the behavioural pattern of molecules, we have performed UV–Vis absorption, steady state fluorescence and lifetime measurements. Understanding the mode of fluorescence quenching of NB with various uracils, our next endeavour focused to assess the extent of antioxidant activity of quencher molecules. Further DFT calculation studies support the release of H from quencher molecules. Finally, deoxyribose degradation assay indicates the antioxidant activity of quencher molecules.