Tropos, Nevertheless Conformationally Stable Biphenyl Derivatives

Circular dichroism spectrum of (R)-(+)-3,3'-dibromo-1,1'-bi-2-naphthol in albumin: Alterations caused by complexation-Experimental and in silico investigation

This study describes the interaction of human serum albumin (HSA) with the binol derivative (R)-(+)-3,3'-dibromo-1,1'-bi-2-naphthol (R-BrB), which has its optical activity based on the prohibitive energetic barrier for conversion into the enantiomer (S)-(+)-3,3'-dibromo-1,1'-bi-2-naphthol (S-BrB). The objective was to assess the ability of HSA to differentiate axial enantiomers based on their binding efficiency and their impact on the CD spectra. We discovered that both enantiomers were effective ligands, and the CD signal disappeared when equimolar amounts of R-BrB and S-BrB were simultaneously added, indicating no preference for either enantiomer. The complexation resulted in a significant signal increase at 250 nm and a bathochromic effect at 370 nm. Molecular docking simulations were performed, and the lower energy pose of R-BrB was selected for DFT calculations. The theoretical CD spectra of free and complexed R-BrB were obtained and showed alterations corroborating the experimental results. By comparing the difference spectrum (HSA:R-BrB minus HSA) with the spectrum of free RBrB in water or ethyl alcohol, we concluded that the CD signal intensification was due to the increased solubilization of R-BrB upon binding to HSA.

Induced circular dichroism as a tool to monitor the displacement of ligands between albumins

The induction of chirality in a ligand can be a powerful analytical tool for studying protein-ligand interactions. Here, we advanced by applying the technique to monitor the inversion of the induced circular dichroism (ICD) spectrum when ligands move between human and bovine serum albumin proteins (HSA and BSA). ICD experiments were performed using dimers of methyl vanillate (DVT) and vanillin (DVN). The sign and spectra shape were dependent on the albumin type. DVN presented a positive maximum in 312 nm when complexed with HSA and a negative one in BSA. It was possible to induce and follow the time-dependent displacement of the ligand from BSA (2.2 × 10⁶ M^-1) to HSA (6.6 × 10⁵ M^-1) via ICD inversion. The Molecular Mechanics Generalized Born Surface Area approach was used to calculate the binding free energy of the conformers, and a dissociation pathway for each system was proposed using Umbrella Sampling calculations. Four energy minima dihedral angle conformers were identified, and the corresponding CD spectra were calculated using the quantum chemistry approach. Then, weighted spectra for the conformationally accessible conformers were obtained based on each conformer's Boltzmann probability distribution. In conclusion, the methodology described in the manuscript might be helpful in monitoring the movement of ligands between proteins that they bind.

Dynamic Stereochemistry of a Biphenyl‐Bisprolineamide Model Catalyst and its Imidazolidinone Intermediates

In this study, we characterize the dynamic stereochemistry of a biphenyl‐2,2’‐bis(proline amide) catalyst in chloroform and DMSO as representative weakly and strongly hydrogen bonding solvents. Using vibrational circular dichroism (VCD) spectroscopy and density functional theory (DFT) based spectra calculations, we show that the preferred axial stereochemistry of the catalyst is determined by solute‐solvent interactions. Explicitly considering solvation with DMSO molecules is found to be essential to correctly predict the conformational preferences of the catalyst. Furthermore, we investigate the stereochemistry of the corresponding enamines and imidazolidinones that are formed upon reaction with isovaleraldehyde. The enamines are found to rapidly convert to endo‐imidazolidinones and the thermodynamically favored exo‐imidazolidinones are formed only slowly. The present study demonstrates that the stereochemistry of these imidazolidinones can be deduced directly from the VCD spectra analysis without any further detailed analysis of NMR spectra. Hence, we herein exemplify the use of VCD spectroscopy for an in situ characterization of intermediates relevant in asymmetric catalysts.

Methyl divanillate: redox properties and binding affinity with albumin of an antioxidant and potential NADPH oxidase inhibitor

Vanillic acid is a widely used food additive (flavouring agent, JECFA number: 959) with many reported beneficial biological effects. The same is true for its ester derivative (methyl vanillate, JECFA number: 159). Based on the increasing evidence that diapocynin, the dimer of apocynin (NADPH oxidase inhibitor), has some improved pharmacological properties compared to its monomer, here the dimer of methyl vanillate (MV), i.e., methyl divanillate (dimer of methyl vanillate, DMV) was synthesized and studied in the context of its redox properties and binding affinity with human serum albumin (HSA). We found that the antioxidant potency of DMV was significantly increased compared to MV. In this regard, the reduction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical by DMV was 30-fold more effective compared to MV. Ferric ion reduction was 4-fold higher and peroxyl radical reduction was 2.7-fold higher. The interaction with HSA was significantly improved (Stern–Vomer constants, 3.8 × 10⁵ mol⁻¹ L and 2.3 × 10⁴ mol⁻¹ L, for DMV and MV, respectively). The complexation between DMV and HSA was also evidenced by induced circular dichroism (ICD) signal generation in the former due to its fixation in the asymmetric protein pocket. Density-functional calculations (TD-DFT) showed that the ICD spectrum was related to a DMV conformation bearing a dihedral angle of approximately −60°. Similar dihedral angles were obtained in the lowest and most populated DMV cluster poses obtained by molecular docking simulations. The computational studies and experimental displacement studies revealed that DMV binds preferentially at site I. In conclusion, besides being a powerful antioxidant, DMV is also a strong ligand of HSA. This is the first study on the chemical and biophysical properties of DMV, a compound with potential beneficial biological effects.

Methyl divanillate, a derivative of the vanillic acid (flavouring agent, JECFA number: 959) with promising beneficial biological effects.

Reducing Diastereomorphous Bis(phosphane oxide) Atropisomers to One Atropisomerically Pure Diphosphane: A New Ligand and a Novel Ligand-Preparation Design

1,1'-Biphenyl-2,2'-diphosphanes with an achiral bridge spanning C-5 and C-5' form atropisomers that are enantiomers. Accessing them in an atropisomerically pure form requires resolving a racemic mixture thereof or of a bis(phosphane oxide) precursor. 1,1'-Biphenyl-2,2'-diphosphanes with a homochiral bridge spanning C-5 and C-5' form atropisomers that are diastereomers. We synthesized the first compound of this kind 1) atropselectively and 2) under thermodynamic control-seemingly a first-time exploit in diphosphane synthesis. The selectivity-inducing step was a high-temperature reduction of two non-interconverting bis(phosphane oxide) atropisomers (60:40 mixture). It furnished the desired diphosphane atropisomerically pure (and atropconvergently because the yield was 67 %). This diphosphane proved worthwhile in Tsuji-Trost allylations, the Hayashi addition of phenylboronic acid to cyclohexenone, and the asymmetric hydrogenation of methyl acetoacetate (up to 95 % yield and 95 % ee).

Induction of axial chirality in divanillin by interaction with bovine serum albumin

Vanillin is a plant secondary metabolite and has numerous beneficial health applications. Divanillin is the homodimer of vanillin and used as a taste enhancer compound and also a promissory anticancer drug. Here, divanillin was synthesized and studied in the context of its interaction with bovine serum albumin (BSA). We found that divanillin acquires axial chirality when complexed with BSA. This chiroptical property was demonstrated by a strong induced circular dichroism (ICD) signal. In agreement with this finding, the association constant between BSA and divanillin (3.3 x 105 mol-1L) was higher compared to its precursor vanillin (7.3 x 104 mol-1L). The ICD signal was used for evaluation of the association constant, demonstration of the reversibility of the interaction and determination of the binding site, revealing that divanillin has preference for Sudlow's site I in BSA. This property was confirmed by displacement of the fluorescent markers warfarin (site I) and dansyl-L-proline (site II). Molecular docking simulation confirmed the higher affinity of divanillin to site I. The highest scored conformation obtained by docking (dihedral angle 242°) was used for calculation of the circular dichroism spectrum of divanillin using Time-Dependent Density Functional Theory (TDDFT). The theoretical spectrum showed good similarity with the experimental ICD. In summary, we have demonstrated that by interacting with the chiral cavities in BSA, divanillin became a atropos biphenyl, i.e., the free rotation around the single bound that links the aromatic rings was impeded. This phenomenon can be explained considering the interactions of divanillin with amino acid residues in the binding site of the protein. This chiroptical property can be very useful for studying the effects of divanillin in biological systems. Considering the potential pharmacological application of divanillin, these findings will be helpful for researchers interested in the pharmacological properties of this compound.

Activation enthalpies and entropies of the atropisomerization of substituted butyl-bridged biphenyls

Contradictory results for activation enthalpies and entropies are obtained from HPLC, CD and DFT while the atropisomerization energies are similar.

Experimental, DFT and docking simulations of the binding of diapocynin to human serum albumin: induced circular dichroism

Axial chirality in diapocynin induced by human albumin.