Unraveling energy transfer and fluorescence quenching dynamics in biomolecular complexes: a comprehensive study of imiquimod-rifampicin interaction

In nature, numerous biomolecules are implicated in charge transfer (CT) and energy transfer (ET) mechanisms crucial for fundamental processes such as photosynthesis. Unveiling these mechanisms is pertinent to multiple disciplines including chemistry, engineering and biochemistry. This article presents a detailed study involving two molecules forming a model system with efficient ET properties. Specifically, their complex exhibits dark quenching phenomena arising from fluorescence resonance energy transfer (FRET) from the donor (imiquimod) to the acceptor (rifampicin). In addition, the energy transfer properties were also elucidated by considering the two forms of rifampicin (RIF), non-ionic and zwitter-ionic in the solution. Supplemented by spectroscopic findings, molecular dynamics simulations and time dependent density functional theory (TD-DFT) calculations conclusively validate the ET properties from imiquimod (IMQ) to RIF forms. Interestingly, these ET processes were found to be associated with pi-pi and hydrogen bond interactions. Their contribution was observed to depend upon the non-ionic and zwitter-ionic form of RIF.

Taking Advantage of a Luminescent ESIPT-Based Zr-MOF for Fluorochromic Detection of Multiple External Stimuli: Acid and Base Vapors, Mechanical Compression, and Temperature

Luminescent materials responsive to external stimuli have captivated great attention owing to their potential implementation in noninvasive photonic sensors. Luminescent metal-organic frameworks (LMOFs), a type of porous crystalline material, have emerged as one of the most promising candidates for these applications. Moreover, LMOFs constructed with organic linkers that undergo excited-state intramolecular proton-transfer (ESIPT) reactions are particularly relevant since changes in the surrounding environment induce modifications in their emission properties. Herein, an ESIPT-based LMOF, UiO-66-(OH)2, has been synthesized, spectroscopically and photodynamically characterized, and tested for detecting multiple external stimuli. First, the spectroscopic and photodynamic characterization of the organic linker (2,5-dihydroxyterephthalic acid (DHT)) and the UiO-66-(OH)2 MOF demonstrates that the emission properties are mainly governed by the enol → keto tautomerization, occurring in the organic linker via the ESIPT reaction. Afterward, the UiO-66-(OH)2 MOF proves for the first time to be a promising candidate to detect vapors of acid (HCl) and base (Et3N) toxic chemicals, changes in the mechanical compression (exercised pressure), and changes in the temperature. These results shed light on the potential of ESIPT-based LMOFs to be implemented in the development of advanced optical materials and luminescent sensors.

Spectroscopy and dynamics of beta-lactoglobulin complexed with rifampicin

In this paper, we report the binding interaction of milk protein, beta-lactoglobulin (BLG), with an antibiotic against tuberculosis, rifampicin (RIF). BLG intrinsic fluorescence from tryptophan (Trp) amino acids was monitored to understand protein-drug interactions. Binding parameters and stoichiometry were estimated with the help of fluorescence spectral changes. Synchronous fluorescence spectroscopy was employed to exclusively monitor the Trp and Tyrosine (Tyr) environment in the presence of RIF. With the help of steady state fluorescence at different temperatures supported by time-resolved fluorescence, we confirmed that the protein forms a static complex with RIF. Thermodynamic parameters, ΔH and ΔS values, showed the involvement of hydrophobic forces between the RIF and BLG. Competitive displacement assay with ANS confirmed the BLG calyx as the binding site for RIF. Energy transfer mechanism from Trp to RIF was attributed to the fluorescence changes in protein upon complexation. The Förster resonance energy transfer (FRET) was used to find distance, energy transfer efficiency and rate of energy transfer between donor (BLG) and acceptor (RIF). Fourier-transform infrared (FTIR) spectroscopy was utilized for estimating changes in the secondary structure of BLG induced by RIF. Molecular docking was used to visualise the binding location of RIF on BLG. Molecular dynamics (MD) simulation studies showed a consistent binding interactions between BLG and RIF during the 100 ns simulation period and this well supported the increased beta sheet content in FTIR. Overall our results establish the potential of intrinsic fluorescence of BLG in combination with biophysical tools to rationalize drug-protein interactions.Communicated by Ramaswamy H. Sarma.

Spectroscopic analysis to identify the binding site for Rifampicin on Bovine Serum Albumin

This article reports the interaction of rifampicin, one of the important antituberculosis drugs, with Bovine Serum Albumin (BSA). Herein, we have monitored the fluorescence properties of tryptophan (Trp) residue in BSA to understand the interactions between protein and rifampicin. Fluorescence intensity of BSA was quenched tremendously upon interacting with the drug. Using steady state and time-resolved spectroscopic tools the static and dynamic nature of quenching have been characterised. Time correlated single photon counting technique confirmed that out of two lifetime components ∼6.2 ns and ∼2.8 ns of BSA, the rifampicin has affected only the shorter lifetime component a lot that was assigned to Trp-213 residue. Hence, it was thought that the drug must have been located near to the amino acid residue. Molecular docking studies have revealed the structural information of drug-protein complex which supported the above conjecture, confirming the nearest tryptophan as Trp-213 to the complexing rifampicin molecule.

Pharmacological Evaluation of Novel Organoiron Dendrimers as Antimicrobial and Anti-Inflammatory Agents

The synthesis of a novel and attractive class of nonsteroidal anti-inflammatory and antimicrobial organoiron dendrimers attached to the well-known drug ibuprofen is achieved. The structures of these dendrimers are established by spectroscopic and analytical techniques. The antimicrobial activity of these dendrimers is investigated and tested against five human pathogenic Gram-positive and Gram-negative bacteria, and minimum inhibitory concentrations are reported. Some of these synthesized dendrimers exhibit higher inhibitory activity against methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecium, and Staphylococcus warneri compare to the reference drugs. As well, the in vitro and in vivo anti-inflammatory activities of these dendrimers are evaluated. The results of in vivo anti-inflammatory activity and histopathology of inflamed paws show that all dendrimers display considerable anti-inflammatory activity; however, second-generation dendrimer (G2-D6) shows the best anti-inflammatory activity, which is more potent than the commercial drug ibuprofen at the same tested dose. Results of the toxicity study reveal that G2-D6 is the safest drug on biological tissues.

Design of Organoiron Dendrimers Containing Paracetamol for Enhanced Antibacterial Efficacy

Paracetamol (acetaminophen) is a common painkiller and antipyretic drug used globally. Attachment of paracetamol to a series of organoiron dendrimers was successfully synthesized. The aim of this study is to combine the benefits of the presence of these redox-active organoiron dendrimers, their antimicrobial activities against some human pathogenic Gram-positive, and the therapeutic characteristics of paracetamol. The antimicrobial activity of these dendrimers was investigated and tested with a minimum inhibitory concentration and this has been reported. Some of these newly synthesized dendrimers exhibited the highest inhibitory activity against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE), and Staphylococcus warneri compared to reference drugs. The results of this study indicate that the antimicrobial efficacy of the dendrimers is dependent on the size of the redox-active organoiron dendrimer and its terminal functionalities. The best result has been recorded for the fourth-generation dendrimer 11, which attached to 48 paracetamol end groups and has 90 units composed of the η⁶-aryl-η⁵-cyclopentadienyliron (II) complex. This dendrimer presented inhibition of 50% of the growth (IC₅₀) of 0.52 μM for MRSA, 1.02 μM for VRE, and 0.73 μM for Staphylococcus warneri. The structures of the dendrimers were characterized by elemental analysis, Fourier transform infrared (FT-IR), nuclear magnetic resonance (¹H-NMR), and ¹³C-NMR spectroscopic techniques. In addition, all synthesized dendrimers displayed good thermal stability in the range of 300–350 °C following the degradation of the cationic iron moieties which occurred around 200 °C.