Fluorescence Spectral Studies on the Interaction of Alanine and Valine with Resorcinol-Based Acridinedione Dyes in Aqueous Solution: A Comparative Study with Glycine

Interaction of an Aldose Sugar with Photoinduced Electron Transfer (PET) and Non-PET Based Acridinedione Dyes in Water: Hydrogen-bonding Evidences from Fluorescence Spectral Techniques Assisted by Molecular Docking Approach

Fluorescence spectral techniques aided by molecular docking (Mol.Doc) approach were employed in probing the molecular interactions existing between D-glucose and resorcinol based acridinedione (ADR) dyes. ADR dyes has been classified into PET and non-PET dyes based on the substitution in the 9^th position of acridinedione ring structure. Addition of glucose to PET dye (ADR1) resulted in a decrease in the absorbance whereas to that of ADR2 dye (non-PET character in aqueous medium) resulted in a significant increase in the absorbance. The formation of an isosbestic point reveals the existence of a ground state interaction existing between the dye and sugar molecule. Addition of glucose to PET dye resulted in a drastic increase in the fluorescent enhancement (FE) and subsequent addition resulted in a marked decrease in the fluorescent intensity with no apparent shift of emission maximum. Interestingly, neither characteristic shift nor variation in emission intensity was observed in the case of ADR2 dye. Fluorescence lifetime studies of ADR1 dye in the presence of glucose illustrate the existence of multiple distinguishable micro environments of dye. Mol.Doc studies authenticate the co-existence of hydrogen bonding (HB) and hydrophobic interaction wherein the dye and sugar molecule acts as HB donor and acceptor resulting in a stable conformer. These conformers are governed predominantly by HB interactions. The nature of interaction of a simple sugar with ADR dyes are explored in depth by fluorescent techniques in coordination with docking studies is imparted in the present study.

Electrochemical Investigation and Molecular Docking Techniques on the Interaction of Acridinedione Dyes with Water-Soluble Nonfluorophoric Simple Amino Acids

Electrochemical studies of resorcinol-based acridinedione (AD) dyes with nonfluorophoric simple amino acids, glycine, alanine, and valine, were carried out in water. AD probes are classified into photoinduced electron transfer (PET) and non-PET-based dyes, wherein the electrochemical properties and photophysical and photochemical behavior vary significantly based on the nature of substituent groups and the nature of the solute. The oxidation potential of PET dye (ADR1) to that of non-PET-based dye (ADR2) differs significantly such that the addition of amino acids results in a shift of the oxidation peak to a less positive potential and the reduction peak to a lesser negative potential. The extent of shift of oxidation and reduction potential in PET dye is more pronounced than that of non-PET dye on the addition of valine rather than glycine. The variation in the shift is attributed to the presence of an electron-donating moiety (OCH₃) group in the ninth position of ADR1 dye. Consequently, the quenching of fluorescence is observed in ADR2 with non fluorophoric amino acids that are authenticated by the shift of the anodic and cathodic peaks toward a lesser positive potential. Molecular docking (MD) studies of PET and non-PET dye with amino acids portray that neither hydrophobic interactions nor electrostatic or weak interactions such as van der Waals and pi-pi interactions govern the electrochemical nature of dye on the addition of amino acids. Furthermore, the formation of a conventional hydrogen bond between dye and amino acid is established from MD studies. The existence of dye-water-amino acid competitive hydrogen-bonding interactions is presumably well-oriented throughout the aqueous phase as observed through photophysical studies which support our electrochemical investigation.

Interaction of acridinedione dye with a globular protein in the presence of site selective and site specific binding drugs: Photophysical techniques assisted by molecular docking methods

Photophysical investigations and molecular docking studies of photoinduced electron transfer (PET) based fluorophores of acridine family with a globular protein, Bovine Serum Albumin (BSA) bound to non-narcotic drugs like phenylbutazone (PB) and flufenamic acid (FA) were carried out in aqueous solution. PB and FA are site specific and site selective drugs, wherein PB predominantly binds at the site (I) whereas FA selectively orients towards site (II) of BSA. Acridinedione (AD) dyes, both resorcinol and dimedone based are hydrophobic in nature and exhibits a combination of both hydrophobic and hydrogen-bonding interactions that are based on the binding sites in BSA. The extent of displacement of AD from the binding sites of BSA by PB and FA are elucidated and established from variation in the fluorescence lifetime and relative amplitude distribution of free and dye bound in site (I) and site (II). The extent of binding affinity of PB-BSA and FA-BSA in the presence of AD is minimal when compared to other site I and II drugs. This is attributed to AD dye bound to several amino acid residues present in BSA such that the dye prefers multiple binding sites in BSA even in the presence of FA and PB. Further, the dye bound to several amino acid residues of BSA ascertains the combination of hydrogen-bonding, hydrophobic interactions, pi-pi and pi-alkyl interaction apart from the binding through sites (I) and (II) from molecular docking methods. The combination of fluorescence tools with molecular modelling techniques provides an excellent approach in determining the stability of these complexes containing competitive guest molecules in the presence of a fluorescence probe and the binding characteristics of dye in a micro heterogeneous environment.

Combined Spectroscopic Analysis of Terrestrial Analogs from a Simulated Astronaut Mission Using the Laser-Induced Breakdown Spectroscopy (LIBS) Raman Sensor: Implications for Mars

One of the primary objectives of planetary exploration is the search for signs of life (past, present, or future). Formulating an understanding of the geochemical processes on planetary bodies may allow us to define the precursors for biological processes, thus providing insight into the evolution of past life on Earth and other planets, and perhaps a projection into future biological processes. Several techniques have emerged for detecting biomarker signals on an atomic or molecular level, including laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy, laser-induced fluorescence (LIF) spectroscopy, and attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy, each of which addresses complementary aspects of the elemental composition, mineralogy, and organic characterization of a sample. However, given the technical challenges inherent to planetary exploration, having a sound understanding of the data provided from these technologies, and how the inferred insights may be used synergistically is critical for mission success. In this work, we present an in-depth characterization of a set of samples collected during a 28-day Mars analog mission conducted by the Austrian Space Forum in the Dhofar region of Oman. The samples were obtained under high-fidelity spaceflight conditions and by considering the geological context of the test site. The specimens were analyzed using the LIBS-Raman sensor, a prototype instrument for future exploration of Mars. We present the elemental quantification of the samples obtained from LIBS using a previously developed linear mixture model and validated using scanning electron microscopy energy dispersive spectroscopy. Moreover, we provide a full mineral characterization obtained using ultraviolet Raman spectroscopy and LIF, which was verified through ATR FT-IR. Lastly, we present possible discrimination of organics in the samples using LIF and time-resolved LIF. Each of these methods yields accurate results, with low errors in their predictive capabilities of LIBS (median relative error ranging from 4.5% to 16.2%), and degree of richness in subsequent inferences to geochemical and potential biochemical processes of the samples. The existence of such methods of inference and our ability to understand the limitations thereof is crucial for future planetary missions, not only to Mars and Moon but also for future exoplanetary exploration.

A fluorescence approach on the investigation of urea derivatives interaction with a non-PET based acridinedione dye-beta Cyclodextrin (β-CD) complex in water: Hydrogen-bonding interaction or hydrophobic influences or combined effect?

Photophysical studies of resorcinol based acridinedione dyes with beta Cyclodextrin (β-CD) in the presence of urea (U) and tetramethylurea (TMU) were carried out in water. A marked variation in the absorption spectra of dye-β-CD complex was found to be more significant in the case of U rather in TMU. Interestingly, the role of urea on the excited state behavior of dye-β-CD complex is found to be entirely different from that of TMU. The formation of urea-water hydrogen-bonding self assemblies and creation of microspheres of varying environment results in an effective displacement of dye from the hydrophobic nanocavity of β-CD. On the contrary, the dye prefers a more confined hydrophobic micro environment in the presence of TMU. The nature of urea derivative, hydrogen-bonding of urea-water assemblies and hydrophobic influences of methyl moieties in urea molecular framework governs the stability and also the dissociation of dye-β-CD complex. The displacement of dye from the environment of the sugar molecule by urea derivatives is established from fluorescence studies wherein the variation in the spectral behavior of non-PET based dye-β-CD complex is found to be entirely different from that of PET dye. Both hydrogen-bonding along with hydrophobic interactions influences the excited state properties of the both PET and non-PET based acridinedione dyes are elucidated through fluorescence spectral studies. The extent of binding and the microenvironment of the dye in the presence of β-CD and urea are established through molecular docking and fluorescence anisotropy studies.