Enantioselective binding sites on bovine serum albumin to dansyl amino acids

Chiral 8-aminoBODIPY-based fluorescent probes with site selectivity for the quantitative detection of HSA in biological samples

Human serum albumin (HSA) is one of the vital proteins in blood serum, and its optimum level is a reflection of the physiological well-being of an individual. Any abnormalities in serum HSA levels could often be a sign of disguised physiological disorders. The importance of fast and accurate determination of serum HSA levels has led to the development of various quantification methods. Among these, fluorescence-based methods employ molecular probes capable of producing selective responses on interaction with HSA. Herein, we report chiral 8-aminoBODIPY-based probes having blue emission for the quantitative detection of HSA in buffer and human blood serum. A pair of 8-aminoBODIPY enantiomers, namely R-PEB and S-PEB, were synthesized. They exhibited a fast 'turn-on' fluorescence response towards HSA, allowing its detection and quantification. In PBS buffer, R-PEB and S-PEB showed very good sensitivity with a limit of detection (LoD) of 25 nM (K_D = 9.84 ± 0.14 μM) and 39 nM (K_D = 18.67 ± 0.21 μM), respectively. The linear relationship observed between the fluorescence intensity of R-PEB/S-PEB and the HSA concentration in serum samples allowed us to generate a reference curve for HSA estimation for practical applications. Examination of unknown serum samples showed a good correlation with the results obtained by the benchmark BCG method. Interestingly, the difference in these probes' dissociation constants and LoD indicated their differential binding to HSA. Considering the availability of multiple ligand binding sites in HSA, their binding preferences were investigated in detail by displacement assays using site-specific drugs. These studies showed the preferential affinity of R-PEB towards site II, which was further substantiated using molecular docking studies. However, these displacement assays could not identify the preferred binding site of S-PEB. Blind docking studies indicated that S-PEB occupied a site closer to FA5. Selective binding of R-PEB to site II and its characteristic photophysical response can be utilized to quickly screen potential site II binding drugs.

Analysis of the Equilibrium Distribution of Ligands in Heterogeneous Media–Approaches and Pitfalls

The equilibrium distribution of small molecules (ligands) between binding agents in heterogeneous media is an important property that determines their activity. Heterogeneous systems containing proteins and lipid membranes are particularly relevant due to their prevalence in biological systems, and their importance to ligand distribution, which, in turn, is crucial to ligand’s availability and biological activity. In this work, we review several approaches and formalisms for the analysis of the equilibrium distribution of ligands in the presence of proteins, lipid membranes, or both. Special attention is given to common pitfalls in the analysis, with the establishment of the validity limits for the distinct approaches. Due to its widespread use, special attention is given to the characterization of ligand binding through the analysis of Stern–Volmer plots of protein fluorescence quenching. Systems of increasing complexity are considered, from proteins with single to multiple binding sites, from ligands interacting with proteins only to biomembranes containing lipid bilayers and membrane proteins. A new formalism is proposed, in which ligand binding is treated as a partition process, while considering the saturation of protein binding sites. This formalism is particularly useful for the characterization of interaction with membrane proteins.

Application of Photoinduced Electron Transfer with Copper Nanoclusters toward Finding Characteristics of Protein Pockets

Proteins possess various domains and subdomain pockets with varying hydrophobicity/hydrophilicity. The local polarities of these domains play a major role in oxidation-reduction-based biological processes. Herein, we have synthesized ultrasmall fluorescent copper nanoclusters (Cu NCs) that are directed to bind to the different domain-specific pockets of the model protein bovine serum albumins (BSA). Potential electron acceptors, methyl viologen (MV) derivatives, were chosen such that they specifically reach the various domains following their hydrophobicity/hydrophilicity. Here, we have used MV²⁺, HMV⁺, and DHMV²⁺, possessing hydrophilic, intermediate, and hydrophobic specificities. Being electron acceptors, these derivatives draw electrons from the Cu NCs through photoinduced electron transfer (PET). The rate of PET varies at the different domains of BSA based on the local environment which has been analyzed. Here, PET is confirmed by steady state as well as time-resolved fluorescence spectroscopy. This study would provide a measurable way to identify the location of the different domains of a protein which is scalable by changing the superficial conditions without unfolding the protein.

Thin Layer Chromatographic Resolution of Some β-adrenolytics and a β2-Agonist Using Bovine Serum Albumin as Chiral Additive in Stationary Phase

Direct enantiomeric resolution of commonly used five racemic β-adrenolytics, namely, bisoprolol, atenolol, propranolol, salbutamol and carvedilol has been achieved by thin layer chromatography using bovine serum albumin (BSA) as chiral additive in stationary phase. Successful resolution of the enantiomers of all racemic β-adrenolytics was achieved by use of different composition of simple organic solvents having no buffer or inorganic ions. The effect of variation in pH, temperature, amount of BSA as the additive, and composition of mobile phase on resolution was systematically studied. Spots were visualized in iodine vapors. Native enantiomers for each of the five analytes were isolated and identified and their elution order was determined. The limit of detection was found to be 0.7, 1.2, 0.84, 1.6 and 0.9 μg (per spot) for each enantiomer of bisoprolol, atenolol, propranolol, salbutamol and carvedilol, respectively.

Enantioselective fluorescent sensors: a tale of BINOL

The development of automated, high-throughput organic synthesis and screening techniques has created an urgent demand for methods that rapidly determine the enantiomeric composition of chiral compounds. Enantioselective fluorescent sensors offer the potential for real-time, high-sensitivity techniques for determining enantiomeric data in high-throughput chiral assays. In this Account, we describe a range of fluorescent sensors derived from 1,1'-bi-2-naphthol (BINOL), a readily available biaryl compound with axial chirality. We show that BINOL can be used to construct structurally diverse, chiral fluorescent sensors to carry out highly enantioselective, sensitive recognition of chiral amino alcohols, α-hydroxycarboxylic acids, and amino acid derivatives. For example, we prepared an (S)-BINOL derivative whose 3,3'-positions are attached to two chiral amino alcohol units, each having two phenyl substituents. This compound shows a fluorescence enhancement of 950-fold in the presence of (S)-mandelic acid but very little change in the presence of (R)-mandelic acid. It also allows the enantiomers of this α-hydroxycarboxylic acid to be visually discriminated by an enantioselective precipitation process. A structurally similar (S)-BINOL-amino alcohol molecule, but with three rather than two phenyl substitutents in each of the two amino alcohol units, was found to exhibit generally enantioselective fluorescence responses toward structurally diverse α-hydroxycarboxylic acids. We further prepared a pseudoenantiomeric analogue of this compound from (R)-H(8)BINOL, which has the opposite chiral configuration at both the biaryl center as well as the pendant amino alcohols. These two compounds have opposite enantioselectivity in the recognition of a chiral substrate, with distinctly different fluorescence emission wavelengths. By mixing them together, we developed a pseudoenantiomeric sensor pair to facilitate chiral assays. Using this pseudoenantiomeric sensor pair allows both the concentration and the enantiomeric composition of a substrate to be determined in a single fluorescence measurement. We synthesized another compound by ligating a terpyridine unit to BINOL and found that coordination of a Cu(II) ion to the terpyridine unit almost completely quenched its fluorescence. Displacement of the Cu(2+) ion from this complex by chiral amino alcohols leads to enantioselective fluorescence enhancement. This BINOL-terpyridine-Cu(II) complex also exhibits enantioselective gel collapsing in the presence of chiral amino alcohols, providing a new visual chiral discrimination method. When a series of light-absorbing conjugated units are attached to the BINOL structure, the resulting multiarmed dendritic molecules show greatly amplified fluorescence responses. Thus, the light harvesting effect of dendrimers can be used to greatly increase the sensitivity of the fluorescent sensors. The progress described here demonstrates that highly enantioselective and sensitive fluorescent sensors can be obtained through a systematic investigation of the structure-property relation between the sensors and the substrates. These sensors show great potential for the development of rapid assays of chiral organic compounds.

Binding behavior of amino acid conjugates of indole-3-acetic acid to immobilized human serum albumin

The affinity of indole-3-acetic acid (IAA), indole-3-propionic acid, indole-3-butyric acid and 24 of their amino acid conjugates to immobilized human serum albumin, as expressed by the retention factor k (determined by HPLC), was dependent on (1) lipophilicity, (2) chirality and (3) functional groups in the amino acid moiety; in some cases conformation plays an additional role. Two lipophilicity-related parameters afforded quantitative correlations with k: retention on a C18 reversed-phase column (experimental approach) and the distance between the hydrophilic and hydrophobic poles of the molecules (in silico approach). Most compounds examined are possible metabolic precursors of IAA, an experimental tumor therapeutic.

Isolated rat kidney perfused with dextran and bovine serum albumin: A stable model for investigating renal drug handling

INTRODUCTION

The rat isolated perfused kidney (IPK) preparation is a very useful model for pharmacokinetic and pharmacologic studies. Bovine serum albumin (BSA) is the oncotic agent used most commonly in IPK models, but the protein is expensive and varies significantly in quality. The present study evaluated the use of dextran to replace a large proportion of BSA as the oncotic agent, to establish a more reliable and economic IPK model for pharmacokinetic studies.

METHODS

The right kidneys of male Sprague-Dawley rats were isolated and perfused in recirculating mode with Krebs-Henseleit pH 7.4 buffer containing amino acids, glucose and 65 g/l BSA (BSA group, n=11) or 6.5 g/l BSA plus 36 g/l dextran (dextran/BSA group, n=6). (14)C-Inulin was added to the perfusate to permit estimation of glomerular filtration rate (GFR). An antiviral guanosine analogue, AM188, was administered to the IPK perfusate to investigate its renal disposition. During the 130-min experimental period, urine was collected in 10-min intervals and perfusate was collected at the midpoint of these intervals.

RESULTS

The kidney functional parameters were generally better and more stable in the dextran/BSA IPKs when compared to the BSA group. At a similar perfusate flow rate, the IPKs in the dextran/BSA group exhibited lower renal artery perfusion pressure, a higher GFR, and more extensive tubular reabsorption of water, glucose, and sodium. These functional parameters were acceptable and stable throughout the whole experimental period in the dextran/BSA group. The renal clearance of AM188 was higher in the dextran/BSA group compared with that in the BSA group.

DISCUSSION

Using a large proportion of dextran and a small proportion of BSA as oncotic agent in perfusate provides an improved IPK preparation. This offers a reliable and economic rat IPK model for pharmacokinetic studies.

Separation of drug enantiomers by liquid chromatography and capillary electrophoresis, using immobilized proteins as chiral selectors

Proteins display interesting chiral discrimination properties owing to multiple possibilities of intermolecular interactions with chiral compounds. This review deals with proteins which have been used as immobilized chiral selectors for the enantioseparation of drugs in liquid chromatography and capillary electrophoresis. The main procedures allowing the immobilization of proteins onto matrices, such as silica and zirconia particles, membranes and capillaries are first presented. Then the factors affecting the enantioseparation of drugs in liquid chromatography, using various protein-based chiral stationary phases (CSPs), are reviewed and discussed. Last, chiral separations already achieved using immobilized protein selectors in affinity capillary electrochromatography (ACEC) are presented and compared in terms of efficiency, stability and reproducibility.

Chiral discrimination of N-carbazole-carbonyl derivatives of alpha-amino acids with a short linear alkyl side chain by bovine serum albumin

Chiral discrimination of racemic carbazole carbonyl (CC)-amino acids with linear alkyl sidechain (C(1)-C(4)) by bovine serum albumin (BSA) was investigated by competitive replacement experiments using dansyl-L-proline and dansyl-D-norvaline as fluorescent probes. It was found that the CC derivatives of the D-forms of alanine (C(1)), amino butyric acid (C(2)), norvaline (C(3)), and norleucine (C(4)) bound to the dansyl-L-proline site much more strongly than their L-forms, whereas the interactions between both enantiomers of these amino acids with dansyl-D-norvaline site were slight.