Stereoselective binding of flurbiprofen enantiomers and their methyl esters to human serum albumin studied by time-resolved phosphorescence

Environmentally sensitive fluorescence of the topical retinoid adapalene

Intrinsic fluorescence of drugs brings valuable information on their localization in the organism and their interaction with key biomolecules. In this work, we investigate the absorption and emission properties of the topical retinoid adapalene in different solvents and biological media. While the UVA/UVB absorption band does not exhibit any significant solvent-dependent behavior, a strong positive solvatochromism is observed for the emission. These results are in line with molecular modeling and simulations that show the presence of two quasi-degenerate states, i.e., a local π-π* and an intermolecular charge-transfer (ICT) state. However, molecular modeling also revealed that, whatever the solvent, at the corresponding equilibrium geometry the lowest and emissive excited state is the local π-π*. Finally, the potential of adapalene to act as a biological probe is demonstrated using albumin, DNA and micelles.

Evaluation of the Interactions between Human Serum Albumin (HSA) and Non-Steroidal Anti-Inflammatory (NSAIDs) Drugs by Multiwavelength Molecular Fluorescence, Structural and Computational Analysis

The interaction between drugs and transport proteins, such as albumins, is a key factor in drug bioavailability. One of the techniques commonly used for the evaluation of the drug-protein complex formation is fluorescence. This work studies the interaction of human serum albumin (HSA) with four non-steroidal anti-inflammatory drugs (NSAIDs)—ibuprofen, flurbiprofen, naproxen, and diflunisal—by monitoring the fluorescence quenching when the drug-albumin complex is formed. Two approaches—the double logarithm Stern-Volmer equation and the STAR program—are used to evaluate the binding parameters. The results are analyzed considering the binding properties, determined by using other complementary techniques and the available structural information of albumin complexes with NSAID-related compounds. Finally, this combined analysis has been synergistically used to interpret the binding of flurbiprofen to HSA.

Can Förster Theory Describe Stereoselective Energy Transfer Dynamics in a Protein-Ligand Complex?

Förster resonance energy transfer (FRET) reactions involving ligands and aromatic amino acids can substantially impact the fluorescence properties of a protein-ligand complex, an impact intimately related to the corresponding binding mode. Structural characterization of such binding events in terms of intermolecular distances can be done through the well-known R^-6 distance-dependent Förster rate expression. However, such an interpretation suffers from uncertainties underlying Förster theory in the description of the electronic coupling that promotes FRET, mostly related to the dipole-dipole orientation factor, dielectric screening effects, and deviations from the ideal dipole approximation. Here, we investigate how Förster approximations impact the prediction of energy transfer dynamics in the complex between flurbiprofen (FBP) and human serum albumin (HSA), as well as a model FBP-Trp dyad, in which recent observation of enantioselective fluorescence quenching has been ascribed to energy transfer from FBP to Trp. To this end, we combine classical molecular dynamics simulations with polarizable quantum mechanics/molecular mechanics calculations that allow overcoming Förster approximations. On the basis of our results, we discuss the potential of structure-based simulations in the characterization of drug-binding events through fluorescence techniques. Overall, we find an excellent agreement between theory and experiment both in terms of enantioselectivity and FRET times, thus strongly supporting the reliability of the binding modes proposed for the (S) and (R) enantiomers of FBP. In particular, we show that the dynamic quenching arises from a small fraction of drug bound to the secondary site of HSA at the interface between subdomains IIA and IIB, whereas the enantioselectivity arises from the larger flexibility of the (S)-FBP enantiomer in the binding pocket.

High enantioselective Novozym 435-catalyzed esterification of (R,S)-flurbiprofen monitored with a chiral stationary phase

Lipases form Candida rugosa and Candida antarctica were tested for their application in the enzymatic kinetic resolution of (R,S)-flurbiprofen by enantioselective esterification. Successful chromatographic separation with well-resolved peaks of (R)- and (S)-flurbiprofen and their esters was achieved in one run on chiral stationary phases by high-performance liquid chromatography (HPLC). In this study screening of enzymes was performed, and Novozym 435 was selected as an optimal catalyst for obtaining products with high enantiopurity. Additionally, the influence of organic solvents (dichloromethane, dichloroethane, dichloropropane, and methyl tert-butyl ether), primary alcohols (methanol, ethanol, n-propanol, and n-butanol), reaction time, and temperature on the enantiomeric ratio and conversion was tested. The high values of enantiomeric ratio (E in the range of 51.3-90.5) of the esterification of (R,S)-flurbiprofen were obtained for all tested alcohols using Novozym 435, which have a great significance in the field of biotechnological synthesis of drugs. The optimal temperature range for the performed reactions was from 37 to 45 °C. As a result of the optimization, (R)-flurbiprofen methyl ester was obtained with a high optical purity, eep = 96.3 %, after 96 h of incubation. The enantiomeric ratio of the reaction was E = 90.5 and conversion was C = 35.7 %.

Pre-denaturing transitions in human serum albumin probed using time-resolved phosphorescence

The investigation of protein dynamics has long been of interest, since protein interactions and functions can be determined by their structure and changes in conformation. Although fluorescence, occurring on the nanosecond timescale, from intrinsic fluorescent amino acids has been extensively used, in order to fully access conformational changes longer timescales are required. Phosphorescence enables processes on the microsecond to second timescale to be accessed. However, at room temperature this emission can be weak and non trivial to measure. It requires the removal of oxygen - a common triplet state quencher and appropriate instrumentation. In this work we make use of a chemical deoxygenator to study room temperature phosphorescence from tryptophan in human serum albumin excited using a pulsed UV light emitting diode. This is extended to monitor the phosphorescence emission upon increasing temperature, allowing pre-denaturing transitions to be observed. Time-resolved data are analysed, both as the sum of exponential decays and using a distribution analysis based on non extensive decay kinetics. These results are compared to a fluorescence study and both the average lifetime and contribution of the different emitting components were found to give more dramatic changes on the phosphorescence timescale.

Stereoselective binding of chiral drugs to plasma proteins

Chiral drugs show distinct biochemical and pharmacological behaviors in the human body. The binding of chiral drugs to plasma proteins usually exhibits stereoselectivity, which has a far-reaching influence on their pharmacological activities and pharmacokinetic profiles. In this review, the stereoselective binding of chiral drugs to human serum albumin (HSA), α1-acid glycoprotein (AGP) and lipoprotein, three most important proteins in human plasma, are detailed. Furthermore, the application of AGP variants and recombinant fragments of HSA for studying enantiomer binding properties is also discussed. Apart from the stereoselectivity of enantiomer-protein binding, enantiomer-enantiomer interactions that may induce allosteric effects are also described. Additionally, the techniques and methods used to determine drug-protein binding parameters are briefly reviewed.